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Unit 4 Part 1 - Electricity - Cambridge IGCSE Physics Revision 2025 to 2028

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Summary

Core Theme

This content provides a comprehensive introduction to the fundamental concepts of electricity, covering static electricity, electric fields, and the basics of electric circuits, including current, voltage, resistance, power, and circuit configurations. It also touches upon electrical safety measures.

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Let's get shocked, shall we?

Electricity. Now, unit four is split

into two parts. So, electricity and

magnetism. Today, we're just focusing on

electricity. And overall, this unit is

about charges. So, what's the charge? We

discussed the charge in the structure of

an atom. How like protons and neutrons

and electrons protons electrons have

positive and negative charge. We'll deal

with that again. So we split this into

static electricity portions. So what's a

charge and what's an electric field.

That's a very minor topic. Then we talk

about electric circuits. We'll start by

talking about the properties of a

circuit. So what quantities do we

measure? Voltage and current and

resistance and power and such. And then

we go into series and parallel

connections and multiple components and

so on.

Once we're done with that, we'll just

finish things up by talking about

electrical safety because, you know,

electricity can kill you and we do want

to keep you safe. At least you should

know how to keep yourself

safe. Let's start. Uh before we

continue, the same friendly reminder.

This checklist, this list of topics uh

is from the curriculum. I want you to

use this as a checklist to see if you

understand the topic or not. So for

example, if you say that state that

there are positive and negative charges

and you understand that I'm sure you do,

uh you're fine. If not, you need to

revise that topic again. Okay, let's

electricity. First, when we talk about

electricity, we have to talk about the

electric charge. A charge is a property

of matter that experiences forces near

other charges. What do I mean? This

means that all matter in the universe

has charge. Whether it's a proton,

neutron, electron, matter, me, you,

everything is charged. But there are two

different types of charges. There are

positive charges and there are negative

charges and they're

opposites. Things can either be positive

or negative or neutral. Neutral objects

are neutral because they have both

positive and negative charges

together. So they have a resultant of

zero charge.

charge is measured in a unit called

kulum. Now I want to focus on this for a

second. The charge of one proton is 1.6

10 ^ of -19 kum and the charge of one

electron is 1.610 6 10 the^ of9 kum you

don't have to memorize these please

don't okay but what I mean is each

proton or electron like subatomic

particle isn't one charge isn't one kum

of charge it's actually like a fraction

of a charge cuz later on when I mention

something like hey uh the charge is

going to split it doesn't mean that we

physically break a charge if you have

one kum you actually have millions of

tiny little particles stuck together and

yeah sure these can split and travel

0.1.0 can travel here 0.6 can travel here

here

depends. So later on when we say half an

ampere 2 aair current splits this is why

it can split. Okay.

Okay.

Now the relationship between charges is

very basic and I'm sure you know like

charges repel. So positive and positive

repel. Negative and negative repel.

Opposites attract. Positive and negative attract.

attract.

However, neutral objects can also be

attracted to charged objects whether

it's positive or negative. Why? Because

inside a neutral object, we said you

have both positive and negative

charge. And these rearrange so that the

positive is closer. So attraction is a

bit stronger. And the same thing happens

here. If you have a negatively charged

particle next to something neutral, the

positive and negative rearrange so the

positive is closer and the attraction is

stronger than the

repulsion. Regardless, anything neutral

is attracted to anything

charged. Yeah. When it comes to

materials, we have two types of

materials. Good conductors of

electricity and bad conductors of

electricity, which we also call

insulators. Conductors are good

conductors because they have freemoving

electrons. We sometimes call them

deoized electrons or free moving ions.

So it's electrons that are free to

move. You should know that all metals

are freeing electrons. So copper, iron,

aluminium, you know, constant tan,

nickel, whatever it is. Insulators do

not conduct because they do not have

free moving electrons. Somebody forgot

the letter T here when typing this sheet

aka me well do not have free moving

electrons. Uh things like rubber and

plastic and glass and wood. You should

be familiar with different types of insulators.

However, even though

insulators cannot conduct electricity,

they can be charged like you can make

them become charged by rubbing them with

another insulator. So for example, if you

you

rub a metal rod with a

cloth, a wooden cloth, for

example, a plastic rod, not a metal rod,

a plastic rod, with a wooden

cloth, the friction between the rubbing

of the cloth and the plastic rod causes

some electrons to be transferred from

one object to another. The cloth could

lose electrons and give it to the rod or

vice versa. This depends on the

materials and we're not going to study

which materials lose or

gain. But you do need to know that if an

object loses

electrons, it becomes positively

charged. If an object gains

electrons, it becomes negatively

Right? You cannot do this with

conductors by the way. And this is

something that was removed from the

syllabus in 2023. Uh how do we charge

conductors? And it's a method called

induction, but it's not part of our

curriculum anymore. Okay? So keep that

in mind. Next, what is an electric

field? Now the word field in general

means a region, a space where something

experiences a force. So electric field

is a region where charges experience a force.

force.

Okay. What is the direction of an

electric field and how do we draw it? We

draw the direction of electric field

using straight lines coming out of a

positively charged

point. So it's always out of a point. Or

if it's a sphere, it's always out of the

positively charged sphere. or if the

point is negative, it's drawn into

it. Question is why? Like why do we draw

the field lines like that? That's

because if I zoom in and I tell you,

hey, if you decide to put a positively

charged particle here, what's going to

happen to it? Gets repelled. What about

a positively charged particle here? Gets

repelled. So each of these lines shows

you the direction of the force on a

point. Which means if you put that same

positively charged

particle here next to the negative uh

point charge, it gets

attracted, right? It gets attracted.

You might say that what what about

electrons or like negative

charges? Shouldn't we draw those? Like

we don't have to. We know that positive

and negative are always opposite to each

other. So if you have an negatively

charged particle here, it'll be

attracted or here it'll be repelled.

Which means negative charges always move

Okay. So when I ask you to draw the

field lines, you always draw them going

out of a positive charge or into a

negative charge. Out of a positive or

into a

negative. Now one more feature here. The

space between the lines shows you how

strong a field is. So, if you take a

look at this field

diagram, this is the same field, but now

it's a metal sphere that's possibly

charged. Or maybe it's a plastic sphere.

We don't care. But the closer the lines

are together, the stronger the field is.

And the farther away the lines are from

each other, the weaker the field is,

which makes a lot of sense if you think

about it, because the field has to be

stronger near the charge and weaker far

away from the charge.

So if I put two charges, one here and

one here, and I ask you which one

experiences more force, like they both

get repelled, but which one experiences

more force? B experiences more force cuz

A is further away and the field here is weaker.

weaker.

Finally, if you put a positive charge in

front of a negative charge, a plate in

front of a plate, you end up with an

electric field that is equally

spaced almost at

least. This type of field is called a

uniform field. And we always draw the

field lines from positive to

negative. Right? A uniform field means

the strength of the field is constant

Now, let me draw one more diagram.

Something that sometimes shows

up, not always, but then. Imagine that

you have a positively charged sphere and

a negatively charged sphere. And I ask

you to draw the field line between them.

So, you draw lines from positive to

negative. But because they're spheres,

the field still gets weaker as you move

So out of the

positive into the

negative. So it's as if I try to join

these two fields

together. Sure you can see it now. But

the more interesting diagram is what if

I have a positive charge next to a positive

positive

charge. Sure they should repel. But they

don't just physically repel. Even the

fields repel.

other. Out of the positive, out of the

positive. Do you know what that means?

This means that if I zoom in here and I

put something perfectly in the middle

between them, it doesn't move. If I put

a positive charge exactly in the middle,

it doesn't move because this space is a

melt space. There's no field here. Zero free

free

field. Okay?

Now let's solve a couple of questions.

Figure 8.1 shows a negatively charged

metal sphere. Draw four lines. You just

want four to show its electric field and its

its

direction. Uh let me pull out this line

tool. And let's draw. While I draw, I'll

answer a question. What if I put a

negative charge exactly in the middle in

that space in that null space? Nothing

as well. Nothing

moves. You might say, "But it gets

attracted." Yeah. To which side? You don't

don't

know if it's perfectly in the

middle. Now, because this is a

negatively charged sphere, you draw the

lines going into it. Piece of

advice. If I draw a sphere, do not draw

the field lines going inside the sphere.

You should draw nothing in here.

But if this was just a negatively

charged dot, sometimes called a point

charge, and I ask you for the same

thing. Yeah, you can draw lines exactly through

through

it and it'll look exactly the

same. Okay? So, be careful. If it's a

sphere, nothing inside the sphere. If

it's a point charge, yeah, just go

fine. A plastic rod is rubbed with a

cloth. The rod and cloth becomes charged

as electrons move between them. The rod

becomes negative. Which diagram shows

how the rod becomes negatively charged

and shows the final charge on the

cloth? All right.

right.

Uh now if the rod becomes negative this

means it

has gained electrons. It has gained

electrons which means the cloth becomes

positive. So this gains

electrons and this becomes negative and

this becomes positive. So my choice is

B. Electrons move from the cloth to the

rod and this becomes negative. This

Easy. All right. I have another

question. Do we draw arrows or just Oh

yeah, absolutely. When it comes to field

lines, you must draw

arrows. Otherwise, why did we go over

explaining drawing the lines? Because

the lines show you the direction of the

force. And force is a vector. And fields

are vectors, by the way, cuz they have

direction. You have to show the

direction with

arrows. Lines going into a negative or

out of a positive if it's a positive

charge. Okay.

You mean we draw four lines or do you

need to do more? I mean, I'm sorry, but

you're given instructions. Listen to

them. He said four lines. The examiner

on. All right. Next

up, current electricity. Another giant

checklist because it's full of tiny tiny

little dots. I'm not going to read the

full checklist.

I want to go straight into current and

voltage and

resistance. Before I do, however, before

I talk about current or voltage or

resistance, let's talk about circuits in

general. The purpose of any electric

circuit like this

one is to deliver electricity. A battery

is a source of energy. It has chemical

energy inside. Or maybe this is an

electric socket. So it's just electrical

energy direct. The point is to deliver

the electrical energy from the source of

the energy that you have to my target

that needs that source. In this case, it's

it's

what? It's the light bulb. So electric

charges travel through the circuit to

give that thing energy and it leaves.

But in order to do so, there needs to be

a force acting on it. For the time

being, and I'm going to change my mind

very quickly now. I'm going to assume

that positive charges are moving inside a

circuit. Now, in any circuit symbol

before I move on as well, the long line

of the battery symbol is positive. The

short line is negative. All right?

So when I close a switch, a switch needs

to be closed in order for the circuit to

work. The term open means the circuit's

not going to work. The term closed means

the circuit is now going to work cuz now

it can conduct electricity.

electricity.

Now the moment I don't know why it's I

want to choose the entire thing, please.

Thank you. There we go. The moment you

close the switch, a positive charge gets

pushed out of the positive side of the

battery. zoom moves through because it

gets repelled now because it was pushed

out. It now has energy. It goes through

the light bulb which is a target which

takes the energy away because it has

resistance. It slows it down. Slows down

the charge and takes the energy away.

The charge is still positive but it

leaves without energy and goes back to

the negative side of the battery to

essentially recharge again. Get some

more energy gets pushed out and you end

up with

a sequence and you end up with a

sequence a current that

flows. So when I say what is a current

and I ask you what is a current you say

it is the rate of flow of charge or the

amount of charge that passes through a

point per unit

time. Now charge is measured or sorry

charge is measured in kum. Current is

measured in

ampere where 1

ampere means 1 kum per second. So

there's one charge that goes through the

circuit per

second. The equation that defines

electric current is I = Q / T where I is

current. Now the reason we use I for

current is because I is currently short

for electric current intensity.

So they got the I from intensity. Q is

for charge. I don't know why they chose

Q. So many things in physics use the

letter Q. And T is for time. So current

is just how many charges go through per

second. We can measure current using an

ammeter and a meters must always be

placed in series. Okay.

Now there are two types of currents that

we have in electricity. What we call DC

and AC. DC is short for direct current.

Okay? Not the comic

books. And AC is alternating current.

Well, I I like DC more than Marvel,

though. That's my personal preference,

not the movies, the

heroes. So, what do I mean by DC and AC?

DC is a current that has a fixed

direction which means the source has a

positive and negative side that are

fixed and the value of the current or

the voltage is fixed. So the current

flows from positive to negative and it

has a fixed value that's called DC. AC

however comes from a generator or your

electric sockets which initially come

from a generator in a inside a power station.

station.

The thing is the reason it's called AC

or an alternating current is because the

positive and negative sides of the

terminals are never fixed. This side

could be positive and this side could be

negative. And in the next 0.2 seconds,

so if we have like a 50 Hz AC supply,

this could be positive and this could be

negative. Which means for 0.2 seconds,

the current flows this way. Zoom. And

for the next 0.2 seconds, the current

flows this way. And then zoom and zam.

zoom and zam and backs and back and

forth back and forth back and forth.

It's very

confusing. If I plot a graph of how the

voltage or the current changes over

time, it looks like this

positive negative. The negative sign

simply means that the

direction has changed. It's in the opposite

opposite

direction. It looks like a wave sine

wave. Okay.

Now, finally, remember how I said

there's a mistake that was made back

when I described the circuit earlier? I

said a positive charge travels from the

positive side of the battery to the

negative side of the battery. Correct?

Yes. That's physically not true because

earlier in static electricity, we said

conductors are good conductors. Metals

are good conductors because they have

free moving electrons and electrons are

negatively charged.

So this thing of us saying, "Hey, the

currents flowing from positive to

hogwash." But it's stuck. Why? A very

long time ago, scientists did not know

what was free to move inside a metal.

Like they knew charges existed. They

knew they could be positive or negative,

but they didn't even know about protons

and electrons

yet. So they decided that charges need

to flow from positive to negative. Why?

Because it's easy. Come on. like

positive, negative, it's it's nice, high to

low. And then decades passed

by until finally some other, you know,

good scientists discovered, hey guys, uh

what's actually free to move inside a

metal is an electron, not a proton, not

a positive

charge. But all our rules in electricity

are fine. They work just

fine. So what do we do?

What we did was we stuck to the concept

of positive to negative. We just called

it the conventional current which is the

flow from positive to negative. And if

somebody asks what are the actual

particles that flow inside a circuit, we

go ahead and we you know submit and say

oh yeah we're sorry it's actually

electrons that travel from negative to

positive. But we don't call it a current

because we don't want to be confused. We

just call it the flow of electrons or electron

electron

flow. So in all of our circuits, if I

ever discuss a

current, it is always from positive to

negative. We never use negative to

positive unless I specifically ask you about

about electrons.

electrons.

Okay, that's quantity number one. Let's

go to quantity number two. Remember

circuit delivers energy. The flow of

those charges is called current. The

energy that we

deliver is called

voltage. Voltage in general simply means

the amount of energy or work done per unit

unit

voltage is the work done or energy

transferred per unit charge. Now, what

do I mean by

that? Uh, I was just changing the

battery for one of my clocks here at

home. It's a nice small, you know, Energizer

Energizer

battery. I remember a long time ago when

Energizer had these really crazy ads

Anyway, and uh it has a positive and

negative sign and written in very small

1.5V. What is this? If I tell you that a

battery has a voltage of 1.5 volts, this

means that each charge that leaves the

battery carries with it 1 and a2 jewels.

If this is one kum, it carries with it

one and a half jewels. Remember the

charge delivers energy. The charge is

not the energy

itself. This means that each and every

single charge will carry one and a half

jewels with it. This is the capacity of

the battery. This is how much it can

give each charge that goes

through. It has nothing to do with the

maximum amount of energy inside the

battery. Because I can buy another like

I have a very old radio. I like to use

it. All right. uh it gives me like nice

old vibes especially when I don't want

to use the internet. But these batteries

the battery uh the radio uses are huge a big

big

chunky almost like toss at somebody's

head and you know give them a fracture

kind of big called D

batteries or DD I don't remember but

it's also 1.5

volts. What's the difference? They will

both give the same amount of energy per charge.

charge.

The difference is how much it

lasts. So that that changes, sure, but

voltage doesn't specify that. Voltage

just tells you how much energy does each

charge take and

deliver. So when these charges go to the

light bulb, they give the light bulb 1.5 jewles

jewles

each. Okay? Yeah, there are

actually two different terms that we use

for voltage. We have emf which is short

for electromotive force and PD which is

short for potential difference. They

both mean energy per charge. But the

difference is emf is the work done or

the energy transferred by a source in

moving a unit charge around the whole

circuit. is the energy per charge of a

source because batteries give the charge

energy. Potential difference is the work

done per unit charge or the energy per

charge lost through a component because

the light bulb takes the energy away

from the charge. So that's why we need

two different names for the same

quantity because it's either energy

gained by the charge. So we give the

charges energy or we take the energy

away from the charge but they both have

the same symbol as far as we're

concerned. It's just V from now on.

Okay. Sometimes use E for emf but

regardless V is

fine. The unit of measurement is called

volt just V as well. And we measure it

using a voltmeter. A voltmeter is this

device over here. And it must always be

connected in parallel. meaning the wires

have to be placed before and after the

thing that you're trying to measure the voltage

voltage across.

across. Okay.

Okay.

Finally, back when we talked about the

charge, we said that you know this

little dinky little charge moves through

the circuit blah blah blah blah blah and

then it has to slow down. It goes

through the light bulb and that causes

it to give the light bulb energy. The

wires however are so good at conducting

electricity the charges do not lose

energy through

it. So what's the deal? The deal is in

the final property called

resistance. What do I mean by

resistance? Resistance is a property of

matter that all conductors have. They

don't it doesn't have a definition.

You're not going to define it yourself.

But know this everything has resistance.

Some materials that are really good

conductors have very low

resistance. And if the resistance is very

very

low, the current is very high, the

charges can zoom quickly. But something

like a light bulb has high

resistance, which causes the current to

what? To decrease, to move slowly. I

want you to think of resistance like

speed bumps on the road.

If a car is traveling, you know, through

a road, and forgive me, this is a

beautiful car. I know, futuristic. But

if there are speed bumps, it slows down

a bit. The more resistance a component

has, the bigger the speed bump. So, it

essentially crashes into the wall. You

lose a lot of energy to go through that

speed bump. Okay? So, more resistance,

lower current. Less resistance, higher current.

current.

The unit of measurement of resistance is called

called

ohm. And then we have Ohm's

law which is arguably the most important

equation in electricity. R= V / I or my

personal version favorite version of it

or rearrangement of it is V=

IR. All right. Memorize this. It's going

to be very important.

Yeah, when it comes to the relationship

between V and I and R, if you take a

look, I is equal to V / R, you need to

understand something. If you increase

the voltage, the current increases as

well. So if this increases, this

increases. Makes sense? Bigger battery, faster

faster

charges. If you increase or decrease,

sorry, the resistance, the current also increases.

increases.

Because less speed bumps means faster

motion or

movement. So current and resistance are

inversely proportional but current and

voltage are directly

proportional. Voltage and current right

now have no relationship because voltage

is something that you decide. I put in a

battery, a 1.5 volt battery. I decide to

put in a 10 ohm light bulb. So just me

putting in a bigger battery doesn't

change the light bulb. Both of these are

factors that you control and they affect

who the current. Keep that in mind.

Voltage and current resistance don't

affect each other at least in simple

circuits with just one component for

now. Now let's talk about resistance

again. Resistance is affected by a few

things. Temperature for one, but we'll

discuss temperature in the next section.

We're assuming that temperature is

constant. The second would be the length

of a wire and the third would be the

cross-sectional area of a wire. What do

I mean? I mean that if I have a wire of

a certain length L, let's say that's I

don't know 10 cm or so and you double

it, the resistance also

also

doubles. the resistance also doubles

because the longer the wire the

obviously the more the charges have to

go through that slow resistance. Same

thing here about with

area. If this is the cross-sectional

area of your wire, it's very small. You

think about it, it barely allows a

charge to move through. But if you have

something that's twice as large, hey, it

doesn't just let one charge through. It

can let two charges through, three

charges through like it depends on how

thick it is. So the thicker the wire is,

the larger the area, the less resistance

it has. It's inversely

proportional. The smaller the area, the

thinner the wire, the larger the

By the way, I like to write this down as

uh just just as for me when I

practice r is equal to l / a. I know it

should be proportional, but I just like

to do this so I remember that oh yeah, r

and l are directly proportional. R and a

are inverse. Yeah, a trick that is very

often used in

exams is the concept of diameter. What

do I mean?

Let's let's assume for a second here in

this example that's already been solved

that you have a wire of a

cross-sectional area A and you double

the area. What happens to the

resistance? It decreases by a factor of

two. It gets halfed. So if the

resistance was 20, now it's going to be

10 because 20 / 2 is 10. Cool.

But if I repeat the same question again

and I say, "Hey, you have a

wire of area A and diameter D and

resistance of 20

ohms." And then you double the diameter.

Not double the area, you double the

diameter. What's the value of the

resistance? It's not going to be 10.

It's going to be 5 ohms. Wait, how did I get

get

that? Remember that the area of a circle

is pir 2 or pi d^2 over

4. So when you double the diameter, so

when the diameter is doubled,

technically the area isn't

doubled. The area gets

doubled squared because you didn't

change the area directly. You changed

the radius or you changed the diameter.

And that has a square factor in

there, which means the area is not two

times larger, it's four times larger. So

the resistance is not two times smaller,

it's four times

smaller. So 20 /

4 gives you

what? 5

ohms. This is a very evil trick. Like

when an examiner wants to make a

question harder, like if they just want

to raise the difficulty a bit, they use

the word diameter instead of area. Keep

that in

mind. How do I find the resistance of

any component? I might ask you to

describe an experiment to do so. So it's

a very simple experiment. You get your

component, whatever it

is, and you connect it to a supply. Now

this could be a variable supply like the

supply itself can change or you can have

a variable resistor in there to change

the voltage or the current and you have

an emter and a voltmeter and you say oh

yeah to find resistance I need to

measure a uh sorry measure V which is

the voltage measure I which is the

current and I can calculate resistance

using R= V over I perfect measure

voltage measure current calculate the

resistance but you know what in physics

we We don't like to do things once, we

like to do things multiple times. So

what we do is we change the voltage like

I said

sometimes by changing the value of the supply

supply

itself. Changing the value of the supply

itself and

second right and second you can change

it using a variable resistor. Assume for

now that you're changing the voltage

directly like you have a set of

batteries or like a variable battery

that you can click new values into and

you start with 0 volt so you get 0

ampere you can't really calculate

anything there but at 1 volt you get

half an ampere now you get the

resistance which is two and then we

repeat the experiment several times you

change the voltage to two volts that

gives you more current you change it to

3 volts that gives you more current you

change it to four that gives you more

current and so on and you get several

values. Now why do we do this? We do

this number one so we can get an average

because it makes sure my results are

reliable and accurate and second what we

can also do is plot a

graph of current against

voltage with voltage on the x axis

because it's the primary thing I'm

changing and currents on the y axis. If

you plot these points and it's a

resistor like a fixed resistor, you

should get a nice straight line going

through the slope of this line or like

the inverse of the slope of this line

shows you what happens to the

resistance. If the slope is

constant, the

resistance is

Now I want to go over IV graphs of

different components. Meaning we we put

a component in that circuit in this

circuit that you see right now over here

over here and we change the voltage and

we get lots of readings when we plot a

graph. There are three components that

you need to remember. One the fixed

resistor which is just a component with

resistance something has fixed

resistance. If you plot an IV graph, it

ends up being a straight line. Which

Constant. All right. But if you put a

lamp like that first light bulb that we

put up there, a filament lamp, and you

start increasing the voltage, you'll

notice that the current does increase.

Like, let's zoom in a bit. Let's zoom in

a bit. You'll notice that the current

does increase.

But it doesn't stay constant for long.

It actually starts decreasing in

slope. The question is why? Why does it

like drop down, droop down? The slope

decreases. Because the higher the

voltage, the brighter the light bulb

gets. But light bulbs don't just get

brighter, they also get hotter. So the

temperature of the light bulb also increases.

increases.

If a metal heats up, its resistance also

increases. Which means what do you think

should happen to the current? It should

decrease. But remember, you're

increasing the voltage. So the voltage

should increase the current, but the

resistance should decrease the current.

So the slope just ends up decreasing

instead because the hotter it gets, the

higher the

resistance and therefore the lower the current

current gets.

gets.

Okay? If I ask you to sketch this, you

should sketch this from memory. If I ask

you to explain why this line slopes this

way, oh yeah, it's because as the

voltage increases, the light bulb gets

brighter. And as it gets brighter, the

temperature increases. And as the

temperature increases, you know, the current

drops. Then finally, we have a

diode. I'm kind of spoiling like the

next section, but a diode is a component

that allows the current to only flow in

one direction. So current always flows

in the direction of the arrow of the diode.

But if the current tries to flow in the

opposite direction, the diode stops

it. So it's like a one-way gate. Current

goes right but can't go

left. So if I plot an IV graph for a

diode, you'll notice that just a little

bit for a small voltage, it doesn't

conduct because diodes are not made of

metals. They're made of what we call semiconductors.

semiconductors.

And it's a specific type of

semiconductor that needs a little bit of

extra voltage to start

conducting. This voltage is around 0.5

to 0.7. It depends on the material. It's

a tiny thing. But the moment the voltage

crosses that value, it shoots up like

boom. It has almost no resistance.

Almost zero. Which is why it doesn't

even have a slope. No, no, no. It's

almost vertical. But we do give it some

slope so we can see what's going

on. The the important feature here is

what? Why is this line horizontal on the other

other

side? Now we didn't focus on that in the

previous graphs because they were all

symmetrical. But the left side of the

graph shows you negative values of

voltage and negative values of current.

Negative simply means we

changed direction.

We changed

direction. All right. And obviously in a

diode, if you change the direction, it

does not let the current through. So the

zero. That's basically it. Nothing

more. Okay? You need to memorize these

all three. You should sketch them right

away and identify which component the

graph is trying to, you know, show you.

final quantity something that we call

power. Now this is honestly from unit

one. So very old quantity not not not

nothing new it's just old. So what is

power? It's the energy per unit time. All

All

right. But here's the thing though in a

circuit in a

circuit a source transfers chemical

energy into electrical energy. Right?

But then in a component like a light

bulb for example, it takes the

electrical energy and turns it into heat

or light. A resistor for example just

gives out heat. A light bulb primarily

gives out light. A speaker gives out

sound and so on. So please note that in

any cell or battery for example, you

change chemical

energy to

electrical. But in a

resistor you change electrical energy to

heat. In a light bulb you change

electrical energy to

light. Now in electricity because we

only measure voltage and current. The

equation to calculate power is not P= E

/T. It's actually P= IV or V * I.

Voltage time current.

Okay. But what if I want

energy? What if I want energy? Now

energy is power times

time back from unit one. And power in

electricity is IV or

VI time. Which means energy in

electricity can be calculated using VIT or

IBT.

Okay. I have a very bad pun. I'm holding back.

pun? Oh, well, I'll hold back. I'll hold back.

back. Yes.

Yes.

Uh, why is it that when you're feeling

down or tired, doctors always want to

give you

vitamins? Because vit gives you

energy. Anyway, I'm sorry. Don't log off now.

now.

Okay, let's keep going. Let's keep

going. I'm

sorry. Now, we have other versions of

this power equations. And we have other

versions. P= I 2 R. People are dying

now. And P= V ^2 / R. Now, where did

these come

from? P= I^2 R is actually P= VI. And I

replaced the V with

IIR. So you end up with I I R which is I^2

I^2

R. P= V ^2 / R is actually P= VI again.

But I = V / R. Back from Ohm's law, it's

the same

thing. So I replaced the I with V / R.

And I end up with VV / R, which is V ^2

/ R.

These formulas are useful if I give you

I and R and ask you to get P right away.

But honestly, if you forget them, that's

fine because if you haven't noticed,

both of them use the same two formulas

P= VI and V= IIR. If you memorize these

two, if you memorize these

two, you can solve any formula involving

I and R or I and V and V and I and P and

R and whatever. Okay. However, this

version of the formula I^2 R will be

very useful later in magnetism when we

talk about

power lost or dissipated as heat in a

transformer or when we transmit electricity.

electricity.

Now, let's talk about something new.

Essentially, we haven't seen this a lot

before because it wasn't there for a

long time in the syllabus. What is a

kilowatt hour?

Now you're like, "What? I've never seen

that." A kilowatt hour is a unit of

energy that we use at

home. It's the amount of energy consumed

by 1,000 watts of power in 1 hour. What

do I mean by

that? Uh, let's say I open up my

application to pay my, you know,

electricity bill this month and I pull

it up. I was like, "Oh, what do you mean

I need to pay £1,000?" It's like it's

not like God. But let's say I have to pay

pay

£1,000. What? How does the electricity

company measure the electrical energy

that I

use? It has that electricity meter,

right? Like right next to your door,

somewhere near your

apartment. Uh but an electricity meter

doesn't measure energy in jewels because

jewels is actually a very small unit of energy.

energy.

We measure it in kilowatt hours. Meaning

how much energy would 1,00 watts of

power like a kettle for example. My

kettle here at home has a power rating

of 1,000 watts. My vacuum cleaner is

1,500 watts. All right. Every device has

a certain wattage. My charger, my phone

charger is like 20 watts, sometimes 30

watts. Depends on if it's a high-speed

charger or not.

And then I just multiply it by 1 hour

because isn't energy power times

time? Kilowatt is power. Hour is time.

So you end up with kilowatt

hours because the values of energy that

we consume at home are super duper

large. We can't use jewels. Okay. So

what is a kilowatt hour? It is how much

energy is used by 1 kilowatt or 1000

watt in 1 hour.

How do I get the value in jewels? Like

if I tell you uh this month I used up 37

kilowatt hours for example that's that's

nothing but let's say I used 37 kilowatt

hours. How do I change that to jewels?

Remember energy is power times time.

This is 37 * kilo means times 1,000

kilo times what's 1 hour in

60 this gives my me the answer in what

in jewles let me work it out so 37 * 1 *

60 * 60 it's a giant value so standard

form 1.33

3 10 ^ of

8. The 3,600 in this uh revision sheet

is the

60. This is extremely new to the

syllabus. We've never used this before.

So, and even in 2023 up to 2024 now like

this year is 2025. Uh we haven't seen it

much yet like maybe

once. Okay. So, we're going to see that

now. Okay.

Now, let's solve a few questions. Define potential

potential

difference. Potential difference or

voltage or emf, it doesn't matter. It's

the same thing. It's the amount of

energy transferred or work done per unit

charge. But add one little extra bit. If

it's emf, it's by a

source to drive a charge per charge by a

source through the whole circuit. If

it's potential difference, it's work

done per unit charge through a component

component. All right? Sometimes these

definitions want you to mention the

energy change that has happened. By the

way, it honestly depends on how many

marks the question is. But also wants

you to mention the energy change. So

here's the question. Potential

difference is the energy change from

what? And you don't have to be specific.

general. It's the energy change from

electrical to something else like

thermal or light or something. So keep

in mind if I ask you to define potential

difference and I ask you to also include

in terms of energy explain it in terms

of energy you have to mention the energy

Okay. Next state the equation which

E equ= W / Q or you can say work per

unit charge. Just write it down as an

equation. Next emf of a battery is 9

volts. The battery is in a circuit.

Calculate the work done by the battery

when it moves a charge of 30 kum. So

this one's easy. Work is energy or sorry

emf time cube. to 9 volts time 30

An electric heater

ohms. The power dissipated meaning lost.

So power dissipated means power lost in

the resistance wire is

what? Wait, hate it when notification

just pops up. Okay. The power dissipate

in resistance wire is 2,500 W. Calculate

the current in the resistance wire. So

how do I calculate current? Look at what

you have. Do not automatically go for V

equals IR. I have power and I have

resistance and I want

current. Short way is to use P = I 2

R. And then I have to square

root my

answer. So square<unk> 2500 over

26 which gives you

9 81. Hey almost like the value of G,

right? Gravity does. Or if you forget

this version you can say oh yeah P

equals uh VI. So I equals P over V. But

I don't have V. So first you get V= IIR separately

separately

right what do we have power and

resistance so what can I

get say P= I V or I= P over V you have

power but you don't have this so you say

V equ=

IIR now you don't have I M you don't

have I but you can say that I * 26 is

your V and plug that in here. So it's

2500 over I *

26 which gives you I^2 = 2500 over

26. You can do

that or you can memorize all the

equations and choose the easiest one. So

So 9.81

Next the resistance of a wire the

resistance wire of a heater has a length

of 1.2 m. So this is the length and area

oh thank god he used the area of 7.9 10

^7. By the way this is a challenging

question. Okay. A new heater is designed

using wire of the same material with a

length of 1.8. 8. Ooh. The length what?

Increased. And a cross-sectional area of

5.8. Oh, and the area what?

Decreased. Calculate the resistance of this

wire.

Uh, well, what was the resistance of the

old wire? 26. So, the first wire was 26 ohms.

ohms.

So what's the new resistance? Well,

first remember

resistance is proportional to length and

inversely proportional to

area. If the length increases,

resistance should increase by how much?

Again, this is a challenging question.

By how much? So first I want to see how

much did the length increase

by? The increase in length is 1.8 over

1.2. It's a

ratio 1.8 / 1.2 which gives me 1.5

times. So the length is 1.5 times

larger. So the resistance will also be

1.5 times

area? How much did the area decrease

by? What's the ratio? What's the change

area.

It's 5.8 since that's the new

area. I just want to see how many times

smaller. So this is

0.734 times smaller.

So what's the value of the new

resistance? Okay, the old resistance was

26. The length increased 1.5 times. So

it increases by 1.5 times. This is

because of the

length. Then the area decreased by

0.73, right? Decreased by 0.73. So we

divide not times by

0.73. That's your area.

Look at this. I take the old resistance,

multiply it by whatever the length has

changed by and divide it by whatever the

area has changed by. You just do the

opposite. Everything will fix

itself. So 26 * 1.5 divided by

734. This gives me a

This is not an easy

question. It's actually quite

challenging because you need to m see

how much it gets multiplied by increased

Okay? We decrease the area,

right? But because resistance and area

are inversely proportional, whatever I

do, it'll be the opposite. You know what

you could have done earlier? You could

have just did the opposite ratio. So

instead of dividing, if you want to

ratio. But the reason I did this is to

keep you know my thought process the

same. What's the change in area? It's

0.7 times smaller. So how does the

resistance change? Is it multiplied by

0.7 times? No, it's inverse. So it's

Next, a 2500 W heater is used in a

country where electricity costs $0.3 per

kilowatt hour. Calculate the cost of

Okay. Uh, how do I get the cost? Well,

he said it costs $0.3 per kilowatt hour,

which means I need the number of

kilowatt hours, but I have power and

time. So, what do I do first? Step one,

let's get the energy, which is power

times time. So,

2500* 2 days. I can't use days. It has

to be in, you know, seconds, right?

or if you think about it in hours

because I want the power to be in

kilowatt and the time to be in hours. So

look at how I'm changing this. This is

the power. This is the

time. I don't want it in watts and days.

I want this in kilowatt. So this is

going to be 2.5

kilowatt times 2 days is how many

240 kilowatt hours. Now we have the

energy in kilowatt hours. Number two,

you need to get the

cost. If 1 kilowatt hour is $0.3,

then 120 kilowatt hours is how many dollars?

36. You pay

$36 because you kept your heater on

continuously for two days. Who does

Next. A wire has a uniform

cross-sectional area. Which statement is

correct? Okay, let's read. The

resistance of the wire is directly

proportional to its cross-sectional area. What? No, it's inversely

area. What? No, it's inversely proportional to the area. So,

proportional to the area. So, no. The resistance of wire is directly

no. The resistance of wire is directly proportional to cross-section area. Oh,

proportional to cross-section area. Oh, yeah. That's the same thing. The

yeah. That's the same thing. The resistance of a wire is directly

resistance of a wire is directly proportional to the length. Yes. And

proportional to the length. Yes. And inversely proportional to the area. Yes,

inversely proportional to the area. Yes, it was probably C. Well, let's read the

it was probably C. Well, let's read the last one. The resistance is directly

last one. The resistance is directly proportional to length and inversely

proportional to length and inversely proportional to

proportional to diameter.

diameter. Nuhuh. But you may say, but but it's

Nuhuh. But you may say, but but it's inversely proportional to area. Yeah,

inversely proportional to area. Yeah, but area is equal to pi d² over 4. If he

but area is equal to pi d² over 4. If he said to its diameter squared, he would

said to its diameter squared, he would be

be right. But it's not inversely

right. But it's not inversely proportional to diameter inversely

proportional to diameter inversely proportional diameter squared. So C is

proportional diameter squared. So C is still the correct answer.

Next, the diagram shows the current voltage graph for a metal wire. What can

voltage graph for a metal wire. What can be deduced from the graph? Like what can

be deduced from the graph? Like what can we find out from the

we find out from the graph? As the voltage increases, the

graph? As the voltage increases, the wait a second before before I keep

wait a second before before I keep going. Isn't this a straight

going. Isn't this a straight line? Straight line means it's a fixed

resistor. This means the resistance is constant before I even start. And the

constant before I even start. And the temperature is constant. Nothing is

temperature is constant. Nothing is changing because the voltage and current

changing because the voltage and current are increasing uniformly with each

are increasing uniformly with each other. So as the voltage increases the

other. So as the voltage increases the temperature increases. No. As the

temperature increases. No. As the voltage increases, the temperature

voltage increases, the temperature decreases. No. As the voltage increases,

decreases. No. As the voltage increases, the resistance increases. No. As the

the resistance increases. No. As the voltage increases, the resistance

voltage increases, the resistance remains constant. Yes, that's what I

remains constant. Yes, that's what I want. I didn't even need the term as the

want. I didn't even need the term as the voltage increase, resistance is

voltage increase, resistance is constant. Moving on.

Yeah. Next question. A battery is connected to a circuit. It's switched on

connected to a circuit. It's switched on for 1 minute. During that time, there is

for 1 minute. During that time, there is a current of 0.4 ampere. So, hold on. I

a current of 0.4 ampere. So, hold on. I have the time and I have the current.

have the time and I have the current. The battery supplies a total of 48

The battery supplies a total of 48 jewels. I have the energy. Which row

jewels. I have the energy. Which row gives the charge and the emf of the

gives the charge and the emf of the battery? So I want charge which is Q and

battery? So I want charge which is Q and emf which is V just

emf which is V just voltage. Now when it comes to Q, what do

voltage. Now when it comes to Q, what do I have? I have time and I have current.

I have? I have time and I have current. Oh, that's all I need. Current is charge

Oh, that's all I need. Current is charge over time, which means charge is current

over time, which means charge is current times

times time. Current is 0.4 and the time is 1

time. Current is 0.4 and the time is 1 minute. I can't use minutes, right?

minute. I can't use minutes, right? Because the term charge or current is

Because the term charge or current is ampere which is charge per second. So

ampere which is charge per second. So it's time 60 not*

it's time 60 not* 1. So 4 * 60 this gives me 24 kum. So

1. So 4 * 60 this gives me 24 kum. So the answer is either C or

the answer is either C or D. The emf of the battery. Now here you

D. The emf of the battery. Now here you have a few options. You have energy. So

have a few options. You have energy. So option one is to use the definition of

option one is to use the definition of voltage which is energy per charge or

voltage which is energy per charge or work per unit charge. And since we

work per unit charge. And since we already got the charge from the previous

already got the charge from the previous step, you can say it's 48 over 24 which

step, you can say it's 48 over 24 which gives you 2

gives you 2 volts.

volts. Or you have energy and current and you

Or you have energy and current and you want voltage and you have time. So you

want voltage and you have time. So you can use E equals

can use E equals VIT which means V is equal to E over I

VIT which means V is equal to E over I T which is 48 over uh 0.4 * 60 again

T which is 48 over uh 0.4 * 60 again it's the same answer

it's the same answer too. So the answer

too. So the answer is both options are

is both options are okay. Very

okay. Very nice. Next. Oh yeah. Time for the good

nice. Next. Oh yeah. Time for the good stuff.

stuff. Let's talk about

Let's talk about circuits. But these were circuits. Yeah.

circuits. But these were circuits. Yeah. Yeah. Yeah. Let's talk about bigger

Yeah. Yeah. Let's talk about bigger circuits. Let's first talk about all of

circuits. Let's first talk about all of these components. Now, you don't have to

these components. Now, you don't have to memorize them perfectly 100%. It's good

memorize them perfectly 100%. It's good that you remember what they look like,

that you remember what they look like, but you don't have to memorize them all.

but you don't have to memorize them all. Okay?

Okay? Okay. So, as I was saying, you don't

Okay. So, as I was saying, you don't need to memorize the components. Not all

need to memorize the components. Not all of them, not 100%. But it's good to know

of them, not 100%. But it's good to know most of the basic ones. There's a lot

most of the basic ones. There's a lot more than these, lots more, but at least

more than these, lots more, but at least these are some of the basic ones that

these are some of the basic ones that you'll see very

you'll see very frequently. So starting from the top

frequently. So starting from the top left hand, the cell and the battery are

left hand, the cell and the battery are basically the same thing. A battery is

basically the same thing. A battery is just multiple cells. Long line is

just multiple cells. Long line is positive, short line is negative,

positive, short line is negative, positive and negative. Switch is always

positive and negative. Switch is always drawn open because a closed switch is

drawn open because a closed switch is just a line. Power supply. This could

just a line. Power supply. This could mean anything. This could be like a

mean anything. This could be like a battery. It could be a battery pack full

battery. It could be a battery pack full of batteries that gives you a voltage.

of batteries that gives you a voltage. It could be your computer's ATX power

It could be your computer's ATX power supply that controls and gives you

supply that controls and gives you different voltages. Regardless, it's

different voltages. Regardless, it's just two circles. And either it could be

just two circles. And either it could be variable with an arrow through it or

variable with an arrow through it or not. It doesn't matter. An AC power

not. It doesn't matter. An AC power supply means a socket. It's the same

supply means a socket. It's the same symbol as a power supply with with an AC

symbol as a power supply with with an AC wave in the middle. Voltters and emters.

wave in the middle. Voltters and emters. You know the drill. You know what a

You know the drill. You know what a diode is. A light emmitting diode is a

diode is. A light emmitting diode is a diode that emits light. So we just draw

diode that emits light. So we just draw the diode and two light bulbs, sorry,

the diode and two light bulbs, sorry, two light rays out of it. A light bulb

two light rays out of it. A light bulb is a circle with an X. A fixed resistor

is a circle with an X. A fixed resistor is a rectangle. A variable resistor is a

is a rectangle. A variable resistor is a rectangle with an arrow. A potential

rectangle with an arrow. A potential divider is a resistor with an arrow

divider is a resistor with an arrow that's coming from the outside. We'll

that's coming from the outside. We'll see the difference near the end of the

see the difference near the end of the session. A thermister is a resistor that

session. A thermister is a resistor that changes based on temperature. So this is

changes based on temperature. So this is basically a thermometer that's placed in

basically a thermometer that's placed in it. An LDR is a resistor. It changes

it. An LDR is a resistor. It changes based on light. So you have light rays

based on light. So you have light rays coming in to hit it. A heater is like a

coming in to hit it. A heater is like a resistor but with more resistance and

resistor but with more resistance and more heat. So we just draw a few lines

more heat. So we just draw a few lines in the

in the middle. A buzzer is something that

middle. A buzzer is something that produces sound. All right. A bell also

produces sound. All right. A bell also produces

produces sound. They both basically have the same

sound. They both basically have the same symbol. It's like a half circle. doesn't

symbol. It's like a half circle. doesn't matter which side is

matter which side is up. And the fuse is a safety feature

up. And the fuse is a safety feature that we'll discuss. It's like a resistor

that we'll discuss. It's like a resistor in terms of the symbol, but there's a

in terms of the symbol, but there's a line in the middle to represent the wire

line in the middle to represent the wire inside. We will see these often.

inside. We will see these often. Okay, let's describe some of these in

Okay, let's describe some of these in more detail. First, a variable resistor.

more detail. First, a variable resistor. It's resistance resistor whose

It's resistance resistor whose resistance can change. You change the

resistance can change. You change the resistance of a variable resistor by

resistance of a variable resistor by changing the length. Now the way it

changing the length. Now the way it works and if we zoom in here, this is

works and if we zoom in here, this is the structure of it. You have a long

the structure of it. You have a long resistor

resistor wire that is connected to one side of

wire that is connected to one side of the battery, but the other side is not.

the battery, but the other side is not. The wire itself is connected to what we

The wire itself is connected to what we call a sliding contact. It's like a clip

call a sliding contact. It's like a clip that clips on and you can move it back

that clips on and you can move it back and forth and slide it back and forth.

and forth and slide it back and forth. This length is the effective length that

This length is the effective length that affects the resistance. The longer this

affects the resistance. The longer this length becomes by moving the slider to

length becomes by moving the slider to the right, the greater the resistance.

the right, the greater the resistance. The smaller this length, the lower the

The smaller this length, the lower the resistance. So if you move it to the

resistance. So if you move it to the left, the resistance decreases. If you

left, the resistance decreases. If you move it to the right, the resistance

move it to the right, the resistance increases. Okay? Now, the reason for

increases. Okay? Now, the reason for that, if you're curious, is because as

that, if you're curious, is because as the current moves from positive to

the current moves from positive to negative, it only passes through this

negative, it only passes through this section of the wire and then it goes

section of the wire and then it goes out.

out. So if this entire wire was 100 ohms and

So if this entire wire was 100 ohms and it passes only through half of the

it passes only through half of the length, then it's actually passed

length, then it's actually passed through how many ohms? 50. You can

through how many ohms? 50. You can always get the value of the resistance

always get the value of the resistance by checking the ratio of the lengths.

by checking the ratio of the lengths. Like are you through half of it, a

Like are you through half of it, a quarter of it, a tenth of it, 3/4 and so

quarter of it, a tenth of it, 3/4 and so on. Is this concept used for light

on. Is this concept used for light switches where you control the

switches where you control the brightness? Yes, absolutely. But we'll

brightness? Yes, absolutely. But we'll see that light switch control thing in

see that light switch control thing in the end cuz that's actually a potential

the end cuz that's actually a potential divider. But it does use a variable

divider. But it does use a variable resistor in it. Okay. Now the purpose of

resistor in it. Okay. Now the purpose of it is to control the

it is to control the current. Next, what's a thermister? From

current. Next, what's a thermister? From its name, it's a

its name, it's a thermal therm resistor. Depends on heat.

thermal therm resistor. Depends on heat. If the temperature of a

If the temperature of a thermister

thermister increases, its resistance decreases. It

increases, its resistance decreases. It doesn't increase like the light bulb

doesn't increase like the light bulb because thermisters are made of what we

because thermisters are made of what we call

call semiconductors. They're not metallic

semiconductors. They're not metallic conductors. All right? Metals when heat

conductors. All right? Metals when heat when they heat up, sure, their

when they heat up, sure, their resistance does increase. That's true.

resistance does increase. That's true. But non-metallic conductors which are

But non-metallic conductors which are called semiconductors are basically

called semiconductors are basically silicon and doped with boron and

silicon and doped with boron and nitrite they're non-metallic conductors

nitrite they're non-metallic conductors when they gain energy usually in the

when they gain energy usually in the form of heat in this case they get more

form of heat in this case they get more freemoving electrons they become better

freemoving electrons they become better conductors which means they resist

conductors which means they resist less and when they cool down they lose

less and when they cool down they lose that energy they lose those freemoving

that energy they lose those freemoving electrons they go back to their atoms

electrons they go back to their atoms So the resistance

So the resistance increases. So the temperature and

increases. So the temperature and resistance are inversely proportional.

resistance are inversely proportional. So be careful. If it's a metal, when the

So be careful. If it's a metal, when the temperature increases, the resistance

temperature increases, the resistance increases. When it's a non-metal, a

increases. When it's a non-metal, a thermister is the only nonmetal we'll

thermister is the only nonmetal we'll care about for now. When the temperature

care about for now. When the temperature increases, the resistance

increases, the resistance decreases. Its

decreases. Its cousin or its brother from another

cousin or its brother from another mother is the LDR.

mother is the LDR. It basically is a resistor that's

It basically is a resistor that's affected by light. Basically, when the

affected by light. Basically, when the light shining on that resistor

light shining on that resistor increases, its resistance decreases as

increases, its resistance decreases as well. It's inversely

proportional. And when the light on it decreases, meaning you put it in the

decreases, meaning you put it in the dark, its resistance

dark, its resistance increases for the exact same

increases for the exact same reasons as

reasons as what? as a

what? as a thermist. It's made of a non-metallic

thermist. It's made of a non-metallic conductor. So when it gains energy,

conductor. So when it gains energy, doesn't get hot, gains energy from the

doesn't get hot, gains energy from the light, sunlight, light bulb, doesn't

light, sunlight, light bulb, doesn't matter. It gets more freeoving electrons

matter. It gets more freeoving electrons and it becomes a better conductor. Its

and it becomes a better conductor. Its resistance

resistance decreases. Yeah. If I plot a graph of

decreases. Yeah. If I plot a graph of how the resistance changes with

how the resistance changes with temperature or light, this is how we

temperature or light, this is how we sketch it. This is how we sketch

sketch it. This is how we sketch anything that's inversely proportional.

anything that's inversely proportional. Not a straight line going down, but a

Not a straight line going down, but a curve with a negative slope. Okay, a

curve. Finally, the diode. What is a diode? We described this earlier. It is

diode? We described this earlier. It is a component that only allows the current

a component that only allows the current to flow in one

to flow in one direction. Meaning, if you take a look

direction. Meaning, if you take a look at this here, the current flows from

at this here, the current flows from positive to negative. If the current is

positive to negative. If the current is going to flow in the direction of the

going to flow in the direction of the diode, it passes through lights up the

diode, it passes through lights up the circuit as if it's not there. It has no

circuit as if it's not there. It has no resistance. But if you connect the diode

resistance. But if you connect the diode in reverse and if the current is going

in reverse and if the current is going to flow in the opposite direction, it

to flow in the opposite direction, it doesn't even leave the supply again.

doesn't even leave the supply again. Again, cuz some some some seasons think

Again, cuz some some some seasons think this, look at look at this. It does not

this, look at look at this. It does not go through the circuit. The charges

go through the circuit. The charges don't go through the circuit. meet a

don't go through the circuit. meet a diode and go like there's a diode in

diode and go like there's a diode in reverse. We need to stop now. No, no,

reverse. We need to stop now. No, no, there's no current in the first place.

there's no current in the first place. There's no current in the first place.

There's no current in the first place. Zero

Zero current. Diode stops. Now a diode has

current. Diode stops. Now a diode has primarily one function to convert AC to

primarily one function to convert AC to DC and there are two ways to do it. The

DC and there are two ways to do it. The lazy way which is called the halfwave

lazy way which is called the halfwave rectifier

rectifier method. Okay, which basically means you

method. Okay, which basically means you take

take the normal AC that you're used to

the normal AC that you're used to alternating current and you pass it

alternating current and you pass it through one diode. When the current

through one diode. When the current flows this way, it goes through.

flows this way, it goes through. Yay. And when the current tries to go in

Yay. And when the current tries to go in the opposite direction, it doesn't

the opposite direction, it doesn't because that would make it reverse that

because that would make it reverse that reverse compared to the diode. So, it

reverse compared to the diode. So, it doesn't go through zero.

But this is a very lazy way of doing it because you've essentially gotten a pair

because you've essentially gotten a pair of scissors and you've cut out half of

of scissors and you've cut out half of your energy. You only allowed one side

your energy. You only allowed one side to go through but not the other. The

to go through but not the other. The negative doesn't go

negative doesn't go through. So how do we fix

through. So how do we fix this? If we want to purely convert AC to

this? If we want to purely convert AC to DC, we use what is called a full wave

DC, we use what is called a full wave rectifier or a bridge.

rectifier or a bridge. It is essentially four diodes connected

It is essentially four diodes connected in a diamond

in a diamond pattern. Two of them look

pattern. Two of them look up and they're parallel to each other

up and they're parallel to each other and two of them look down when they're

and two of them look down when they're also parallel to each other. Two of them

also parallel to each other. Two of them look up and two of them look down. You

look up and two of them look down. You do not have to memorize this by the way,

do not have to memorize this by the way, but you do know have to you do have to

but you do know have to you do have to know if a diode is drawn correctly or a

know if a diode is drawn correctly or a bridge is drawn correctly or not.

bridge is drawn correctly or not. Remember what I'm saying. Two of them

Remember what I'm saying. Two of them are parallel to each other looking down.

are parallel to each other looking down. Two of them are looking a up. I don't

Two of them are looking a up. I don't care which is up and down though. Like

care which is up and down though. Like these could be the ones up. So these

these could be the ones up. So these have to be the ones

have to be the ones down. But if I draw the diodes like

down. But if I draw the diodes like this, like each one is in a different

this, like each one is in a different direction. this is

direction. this is wrong. So that's the first thing you

wrong. So that's the first thing you need to identify. The second is knowing

need to identify. The second is knowing how to trace the current through a

how to trace the current through a diode. Like knowing how to draw how the

diode. Like knowing how to draw how the currents flowing through the diodes. You

currents flowing through the diodes. You don't have to, like I said, memorize the

don't have to, like I said, memorize the diagram, but you have to look at how it

diagram, but you have to look at how it goes through. So I'm going to draw two

goes through. So I'm going to draw two currents. One in red when the top side

currents. One in red when the top side of the supply is positive and the bottom

of the supply is positive and the bottom is

is negative, and one in blue

negative, and one in blue which is when the top is negative and

which is when the top is negative and the bottom is positive like when the

the bottom is positive like when the current flips and look how it reaches

current flips and look how it reaches the other side like the other

the other side like the other terminal. So let's start with the red.

terminal. So let's start with the red. Current goes up, goes right, goes down,

Current goes up, goes right, goes down, reaches a

reaches a junction. It cannot go left. So it goes

junction. It cannot go left. So it goes right, reaches the next junction. It

right, reaches the next junction. It cannot go left. So it goes right. So

cannot go left. So it goes right. So this side is positive and this side is

this side is positive and this side is negative. And the current goes through.

negative. And the current goes through. No

No problem. If you want to see the current

problem. If you want to see the current going back, this is how it goes

back. Okay. How about the blue

How about the blue side? Let's follow the blue line. Now

side? Let's follow the blue line. Now the blue means the bottom terminal is

the blue means the bottom terminal is positive. So it goes down, goes to the

positive. So it goes down, goes to the right, reaches a junction, left or

right, reaches a junction, left or right. Left is blocked off. So it goes

right. Left is blocked off. So it goes right through D2. Then it meets the next

right through D2. Then it meets the next junction. Left or right? Left is

junction. Left or right? Left is completely blocked off. So it goes

completely blocked off. So it goes right. Oh, this is still

right. Oh, this is still positive. So what reaches the

positive. So what reaches the output? This is negative. So flows this

output? This is negative. So flows this way and on the way

back when it reaches the negative it flips. You essentially take the negative

flips. You essentially take the negative version of this graph and you flip it.

version of this graph and you flip it. That's all you do. This is called a full

That's all you do. This is called a full wave

wave rectifier. So again, if I tell you you

rectifier. So again, if I tell you you only use one diode, it's a jump and a

only use one diode, it's a jump and a zero. A jump and a zero. If you use a

zero. A jump and a zero. If you use a full wave rectifier, four dodes, it's

full wave rectifier, four dodes, it's jump. No negatives. No

jump. No negatives. No use. Finally, what is an LED or a light

use. Finally, what is an LED or a light emmitting

emmitting diode? It's just a diode that shines

diode? It's just a diode that shines bright, gives out light when a current

bright, gives out light when a current goes through it. That's it. So, it's

goes through it. That's it. So, it's basically a diode, but it's got the

basically a diode, but it's got the extra function of glowing or like

extra function of glowing or like emitting light.

Let's get into the highlight of this particular

particular chapter. Series and parallel

chapter. Series and parallel connections. This is the part that

connections. This is the part that confuses some people and gives them

confuses some people and gives them trouble. But you need to remember

trouble. But you need to remember something. Whatever I say in series will

something. Whatever I say in series will be the opposite of what happens in

be the opposite of what happens in parallel. All right? And then you need

parallel. All right? And then you need to memorize exactly what happens to the

to memorize exactly what happens to the current.

current. the resistance and the

the resistance and the voltage. The current, the resistance and

voltage. The current, the resistance and the

the voltage. Sometimes it increases,

voltage. Sometimes it increases, sometimes it decreases, sometimes it

sometimes it decreases, sometimes it splits, sometimes it

splits, sometimes it doesn't. Sometimes it's the same,

doesn't. Sometimes it's the same, sometimes it's not. Like you need to

sometimes it's not. Like you need to memorize how each method of connecting

memorize how each method of connecting components together changes these three

components together changes these three factors. So starting off with series

factors. So starting off with series connections. A series connection means

connections. A series connection means you connect the two resistors along the

you connect the two resistors along the same

same wire. So if I were to connect them to

wire. So if I were to connect them to let's say a 10V supply and there's a

let's say a 10V supply and there's a current I going through it. It's number

current I going through it. It's number one the current is the same. You have

one the current is the same. You have the same current going through both

the same current going through both because it's only one wire. That's the

because it's only one wire. That's the current. Second, when you connect

current. Second, when you connect resistors in series, their combined

resistors in series, their combined resistance

resistance increases and you calculate it using R1

increases and you calculate it using R1 plus R2 plus R3 plus you just add

plus R2 plus R3 plus you just add them. So if this was

them. So if this was uh 10 ohms and 10

uh 10 ohms and 10 ohms, the total would be 20 ohms.

Finally, but most importantly, what happens to the

happens to the voltage? The voltage of the supply in

voltage? The voltage of the supply in series

series gets split. Now, the voltage of the

gets split. Now, the voltage of the supply doesn't change, but the voltage

supply doesn't change, but the voltage that you try to deliver gets split. The

that you try to deliver gets split. The potential difference

potential difference splits. If I were to calculate the

splits. If I were to calculate the voltage across the first resistor, I

voltage across the first resistor, I would first need to get the current. So

would first need to get the current. So I'd say I = V / R which is 10 / 20.

I'd say I = V / R which is 10 / 20. That's the total voltage and the total

That's the total voltage and the total resistance which is 0.5 amp. And then if

resistance which is 0.5 amp. And then if I ask you to get the voltage only across

I ask you to get the voltage only across the first resistor

the first resistor V1 you would say I

V1 you would say I R1 which is 0.5 that's the

R1 which is 0.5 that's the current time 10 which is 5. This means

current time 10 which is 5. This means that this resistor only gets 5 volts

that this resistor only gets 5 volts from the supply which means the other

from the supply which means the other one also gets 5 volts from the supply.

one also gets 5 volts from the supply. The voltage the total voltage that you

The voltage the total voltage that you have gets split gets

have gets split gets divided. It's not always equal. It

divided. It's not always equal. It depends on the ratio of the resistors.

depends on the ratio of the resistors. For example, if this wasn't 10 and 10,

For example, if this wasn't 10 and 10, what if this was a Let me delete all of

what if this was a Let me delete all of this. What if this was a

this. What if this was a uh let's just use the same supply, a 10

uh let's just use the same supply, a 10 ohm resistor and uh 90 ohm resistor.

ohm resistor and uh 90 ohm resistor. Going for the extreme, the first

Going for the extreme, the first resistor will only get one volt and the

resistor will only get one volt and the second resistor only gets nine gets the

second resistor only gets nine gets the rest because the voltage gets split

rest because the voltage gets split according to the ratio of the

according to the ratio of the resistors. One will increase, the other

resistors. One will increase, the other has to decrease.

has to decrease. so that the total voltage stays the

so that the total voltage stays the same. It's 10

same. It's 10 volts. Okay, that's if they're in

volts. Okay, that's if they're in series. What if the resistors are in

series. What if the resistors are in parallel? Well, everything I said gets

parallel? Well, everything I said gets flipped over. Huh? In series, I said the

flipped over. Huh? In series, I said the current is the same. Like it doesn't

current is the same. Like it doesn't change, right?

change, right? But in

But in parallel uh you

parallel uh you have

have two branches for the current to go

two branches for the current to go through. It's not just one loop. It's

through. It's not just one loop. It's two loops actually. So the current that

two loops actually. So the current that flows through the circuit

flows through the circuit splits. Does that mean it gets smaller?

splits. Does that mean it gets smaller? On the contrary, the combined resistance

On the contrary, the combined resistance of resistors in parallel doesn't

of resistors in parallel doesn't increase. It actually decreases.

increase. It actually decreases. Because if you think about it,

Because if you think about it, connecting resistors in parallel on top

connecting resistors in parallel on top of each other is like making a wire

of each other is like making a wire thicker. You allow more charges to go

thicker. You allow more charges to go through at the same time. So connecting

through at the same time. So connecting resistors in

resistors in parallel gives the charges more space to

parallel gives the charges more space to go through which increases the current.

go through which increases the current. So the current gets very large and the

So the current gets very large and the resistance

resistance A

A decreases. How do we calculate it? Using

decreases. How do we calculate it? Using this

this rule R equals product over sum or R2 R1

rule R equals product over sum or R2 R1 * R2 over R1 +

* R2 over R1 + R2. So for them example, if this was 10

R2. So for them example, if this was 10 ohms and 10 ohms, their combined

ohms and 10 ohms, their combined resistance wouldn't be 20.

resistance wouldn't be 20. It would be 10 * 10 over 10 + 10 which

It would be 10 * 10 over 10 + 10 which gives me

gives me 5. The combined resistance of resistors

5. The combined resistance of resistors in parallel

in parallel decreases and it has to be lower than

decreases and it has to be lower than the smallest resistor. So if I have 10

the smallest resistor. So if I have 10 and 10 has to be less than 10. If I have

and 10 has to be less than 10. If I have 10 and 20 still less than 10. If I have

10 and 20 still less than 10. If I have 10 ohms and 2 ohms has to be less than 2

10 ohms and 2 ohms has to be less than 2 ohms. The resistance just decreases

ohms. The resistance just decreases drops hard.

Finally, this is also voltage is arguably very important. Why? Because in

arguably very important. Why? Because in series, if you remember, the voltage

series, if you remember, the voltage gets split. In parallel, it doesn't. In

gets split. In parallel, it doesn't. In parallel, all the components get the

parallel, all the components get the same voltage.

same voltage. This means that this 10V

This means that this 10V supply can deliver 10 volts completely

supply can deliver 10 volts completely to the first resistor and 10 volts

to the first resistor and 10 volts completely to the

completely to the second. Everything in parallel gets the

second. Everything in parallel gets the same voltage. And that's because each

same voltage. And that's because each like resistor gets its own charge. Like

like resistor gets its own charge. Like this is a charge carrying 10 volts goes

this is a charge carrying 10 volts goes through just this. Here's another charge

through just this. Here's another charge carrying another 10 volts. It only goes

carrying another 10 volts. It only goes through

through R2. In series, the same poor charge has

R2. In series, the same poor charge has to do what? If you had a charge carrying

to do what? If you had a charge carrying 10 volts, oh, it has to go through both

10 volts, oh, it has to go through both R1 and R2. So, it's forced to split its

R1 and R2. So, it's forced to split its voltage. But in parallel, each one gets

voltage. But in parallel, each one gets its own

its own voltage because it gets its own charge,

voltage because it gets its own charge, full

full voltage. Very good.

Now, obviously, parallel has a few advantages over series. What do I mean?

advantages over series. What do I mean? At home, most of your light bulbs and

At home, most of your light bulbs and switches and sockets are all connected

switches and sockets are all connected in parallel for three reasons. Number

in parallel for three reasons. Number one, everything in parallel gets the

one, everything in parallel gets the full voltage. So, at home, if my voltage

full voltage. So, at home, if my voltage is 220 volts, this light bulbs gets 220.

is 220 volts, this light bulbs gets 220. This light bulb gets 220. This light

This light bulb gets 220. This light bulb also gets

220. Everything lights up, right? Sure, you end up paying more

you end up paying more money. Capitalism,

money. Capitalism, but that sounded

Scottish. Now, uh, everything gets the same

uh, everything gets the same voltage, but here's the another

voltage, but here's the another advantage. You can switch them on and

advantage. You can switch them on and off separately. or independently,

off separately. or independently, meaning you can close the switch here

meaning you can close the switch here and open this switch and close this

and open this switch and close this switch and you have two light bulbs on,

switch and you have two light bulbs on, one light bulb off. It's up to

one light bulb off. It's up to you. And then if one of them breaks, the

you. And then if one of them breaks, the others still work. That's the advantage

others still work. That's the advantage of parallel, like having them on

of parallel, like having them on separate branches. They don't affect

separate branches. They don't affect each other. If everything in your house

each other. If everything in your house was connected in

was connected in series, if everything in your house is

series, if everything in your house is connected in

connected in series, if one light bulb breaks,

series, if one light bulb breaks, everything goes off.

everything goes off. everything turns

everything turns off.

off. Okay. However, there is something else

Okay. However, there is something else that we do connect in series is very

that we do connect in series is very important. Cells or batteries. You can

important. Cells or batteries. You can have more than one cell in series. By

have more than one cell in series. By the way, like if I put a 6V cell in this

the way, like if I put a 6V cell in this example next to a 3V cell, the voltage

example next to a 3V cell, the voltage that you deliver is not six or three.

that you deliver is not six or three. It's nine. They get added up.

It's nine. They get added up. So when you connect cells in series, you

So when you connect cells in series, you just add their voltages

together. To anybody observant, you look to the right side

observant, you look to the right side over here and say, uh, why is this four

over here and say, uh, why is this four volts not not like eight or

volts not not like eight or something? Why is it four not

eight? Because look at this first cell. It's positive and negative are pointing

It's positive and negative are pointing to the left. So the current is going to

to the left. So the current is going to flow to the left. The second cells

flow to the left. The second cells positive and negative point to the

positive and negative point to the right which means it's in the opposite

right which means it's in the opposite direction. These are in the same

direction. These are in the same direction. These both try to push the

direction. These both try to push the current in the same

current in the same direction. So what do they do? They

direction. So what do they do? They cancel out. Instead of adding them, you

cancel out. Instead of adding them, you subtract

subtract them. So if the cells are in the same

them. So if the cells are in the same direction, you add them. If the cells

direction, you add them. If the cells are in opposite direction, you

are in opposite direction, you subtract.

Yeah. Can you put an AC supply next to a battery? I mean, you

battery? I mean, you can. It's not a good idea,

can. It's not a good idea, though. Finally, let's talk about the

though. Finally, let's talk about the potential divider. A potential divider

potential divider. A potential divider is just a very fancy way of saying two

is just a very fancy way of saying two resistors in series. But the function

resistors in series. But the function here is to actually split the voltage.

here is to actually split the voltage. Like look at this example. If this was a

Like look at this example. If this was a 10 ohm and this is a 30 ohm resistor,

10 ohm and this is a 30 ohm resistor, the voltage will split with a ratio of 1

the voltage will split with a ratio of 1 to 3. So 2.5 volt to 7.5 volt. Do you

to 3. So 2.5 volt to 7.5 volt. Do you know what that means? This means if you

know what that means? This means if you connect another light bulb only parallel

connect another light bulb only parallel to the first resistor, it gets 2.5

to the first resistor, it gets 2.5 volts. You got you get to choose what

volts. You got you get to choose what the voltage is across it. No. No, no,

the voltage is across it. No. No, no, no. But what if I connect another light

no. But what if I connect another light bulb here? This will get what? 7 1/2

bulb here? This will get what? 7 1/2 volts. Because the voltage was split

volts. Because the voltage was split first according to the ratio of the

first according to the ratio of the resistors and then you ended up

resistors and then you ended up connecting this component not parallel

connecting this component not parallel to the entire circuit but parallel only

to the entire circuit but parallel only to the first resistor. So it only gets

to the first resistor. So it only gets the voltage of the first resistor.

the voltage of the first resistor. If you take a look at the second one,

If you take a look at the second one, you connect it parallel to the second

you connect it parallel to the second resistor. So it only gets the voltage of

resistor. So it only gets the voltage of the second

the second resistor. So if I were to turn this

resistor. So if I were to turn this circuit on, the light bulb on the right

circuit on, the light bulb on the right would be bright and the one on the left

would be bright and the one on the left would be

dim. A very similar circuit to this is what we call a variable potential

what we call a variable potential divider.

divider. This is a much cleaner version because

This is a much cleaner version because if I wanted to change the voltage like

if I wanted to change the voltage like increase and decrease the voltage

increase and decrease the voltage directly I don't need a potential

directly I don't need a potential divider with two resistors and then I

divider with two resistors and then I have to split the voltage and then if I

have to split the voltage and then if I want to change the ratio I have to

want to change the ratio I have to change the ratio of the resistors.

change the ratio of the resistors. I can just connect a variable

I can just connect a variable resistor completely to the supply which

resistor completely to the supply which means this entire wire gets the full 10

means this entire wire gets the full 10 volts. But then you connect your slider

volts. But then you connect your slider or sliding contact in parallel to the

or sliding contact in parallel to the light bulb that you

light bulb that you want. So if the sliding contact is in

want. So if the sliding contact is in the middle, it's not parallel to the

the middle, it's not parallel to the full wire. It's parallel to what? Half

full wire. It's parallel to what? Half of it. So it doesn't get 10 volts.

of it. So it doesn't get 10 volts. The entire wire gets 10. Sure. But if

The entire wire gets 10. Sure. But if you're only connected to half of it, you

you're only connected to half of it, you get what? 5

volts. What if you move this parallel here to the very end? You get the full

here to the very end? You get the full 10 volts. Cuz you're now parallel to the

10 volts. Cuz you're now parallel to the entire wire. What if you move it to the

entire wire. What if you move it to the left? You get

left? You get nothing. Because you're parallel to

nothing. Because you're parallel to nothing. Look, you're parallel to an

nothing. Look, you're parallel to an empty wire. Like it's nothing.

empty wire. Like it's nothing. This is called the variable potential

This is called the variable potential divider. And whoever asked me earlier,

divider. And whoever asked me earlier, uh, is this how light bulbs or like

uh, is this how light bulbs or like variable light bulbs that you have at

variable light bulbs that you have at home work? This is how they

home work? This is how they work. A variable potential divider is so

work. A variable potential divider is so much cleaner than just putting a

much cleaner than just putting a variable resistor in series with a light

variable resistor in series with a light bulb. Put it in parallel instead. This

bulb. Put it in parallel instead. This allows you to change the voltage

allows you to change the voltage directly from zero to full to 10 volts.

directly from zero to full to 10 volts. zero being zero being the slider near

zero being zero being the slider near the side that's connected to the battery

the side that's connected to the battery and the maximum being the other side the

and the maximum being the other side the other

end. Okay. So the purpose of it isn't to control current now it's to control what

control current now it's to control what voltage across

voltage across component. As one final rule, a variable

component. As one final rule, a variable potential divider can follow this rule.

potential divider can follow this rule. V_sub_1 over V2= R1 / R2. Meaning the

V_sub_1 over V2= R1 / R2. Meaning the ratio of the voltage is the same as the

ratio of the voltage is the same as the ratio of the resistors. We sometimes use

ratio of the resistors. We sometimes use this when solving questions. It does

this when solving questions. It does help. Do I always need it? Not really,

help. Do I always need it? Not really, but it does

but it does help. Okay, let's solve a few questions.

Starting off with this March question over

here. A circuit consists of a DC supply, a lamp and a

a lamp and a thermister. Draw a circuit diagram of

thermister. Draw a circuit diagram of these components connected in series. So

these components connected in series. So let's go. A DC supply. You can either

let's go. A DC supply. You can either draw just a supply and just write DC on

draw just a supply and just write DC on it or like positive and negative. Or you

it or like positive and negative. Or you can draw a battery or a cell. That's

can draw a battery or a cell. That's fine. A lamp, so light bulb, and a

fine. A lamp, so light bulb, and a thermister are all in series. Do I need

thermister are all in series. Do I need anything else?

anything else? Nope. That's

Explain what happens in the circuit you have drawn. When the temperature of the

have drawn. When the temperature of the thermister increases. Ooh. Ooh. Okay.

thermister increases. Ooh. Ooh. Okay. When the temperature increases, the

When the temperature increases, the resistance of the

resistance of the thermister

thermister decreases. And if the resistance of the

decreases. And if the resistance of the thermister

thermister decreases, the current

decreases, the current will

will increase. But there's a light bulb in

increase. But there's a light bulb in there. So the light bulb becomes what?

there. So the light bulb becomes what? Bright.

Bright. Look at the way I sequenced my

Look at the way I sequenced my logic. I started off with what happens

logic. I started off with what happens when the temperature increases. When the

when the temperature increases. When the temperature increases, the

temperature increases, the resistance decreases of the thermister.

resistance decreases of the thermister. Like I I'll be even of the

theister. But that causes what? That causes the resistance of the

what? That causes the resistance of the circuit to decrease. the total

circuit to decrease. the total resistance increases and the total

resistance increases and the total current

current increases, right? So, the current

increases, right? So, the current increases. That's honestly enough.

increases. That's honestly enough. Honestly, that's enough. But you know

Honestly, that's enough. But you know what? I just want to push

what? I just want to push through. So, the light

bulb is bright or glow bright. It's only two marks. So, honestly, I'm just

two marks. So, honestly, I'm just looking for resistance decreases. So the

looking for resistance decreases. So the current

current increases. That's that's genuinely what

increases. That's that's genuinely what I'm looking for. But I wanted to express

I'm looking for. But I wanted to express exactly what's happening inside because

exactly what's happening inside because if it was worth more marks, that's

if it was worth more marks, that's great. We end up getting more

great. We end up getting more marks. Next question. Ooh, I see an AC

marks. Next question. Ooh, I see an AC supply. Oh, what's

supply. Oh, what's this? Huh? Is this a single diode or a

this? Huh? Is this a single diode or a full wave rectifier? Like without even

full wave rectifier? Like without even reading the question, is this a single

reading the question, is this a single diode or a full wave

diode or a full wave rectifier? Just one diode. Like look at

rectifier? Just one diode. Like look at this. He has the AC supply connected to

this. He has the AC supply connected to one diode. And he's like, "What's the

one diode. And he's like, "What's the voltage across this resistor? If it was

voltage across this resistor? If it was the full wave rectifier, it would be a

the full wave rectifier, it would be a bunny hop going up, down, up, down, even

bunny hop going up, down, up, down, even if it's negative." But it's not. It's

if it's negative." But it's not. It's just one diode. So Cuts off half of the

just one diode. So Cuts off half of the voltage.

7.1 shows a circuit that contains a battery, a switch, and a voltmeter and

battery, a switch, and a voltmeter and three 40 ohm resistors. Oh, this is

three 40 ohm resistors. Oh, this is important. You didn't draw the values.

important. You didn't draw the values. You just mentioned them here. They're

You just mentioned them here. They're all 40 ohm

all 40 ohm resistors. R1 and R2 and R3.

Describe the switch is opened and R1 R2 form a potential divider. Explain what

form a potential divider. Explain what is meant by the term potential divider.

is meant by the term potential divider. So what is a potential divider? Now the

So what is a potential divider? Now the function of a potential divider is to do

function of a potential divider is to do what? Is to split the voltage or divide

what? Is to split the voltage or divide the voltage between

the voltage between two resistors.

two resistors. Right? Proportional to the ratio

Right? Proportional to the ratio depending on the ratio. That is what a

depending on the ratio. That is what a potential divider is and that's the

potential divider is and that's the function of it at the same time. So a

function of it at the same time. So a potential divider is just what a

potential divider is just what a circuit that

circuit that divides the

divides the voltage across two resistors in

voltage across two resistors in series according to their ratio or

series according to their ratio or according to the value of the resistance

according to the value of the resistance or proportional to their values.

Next up, remember all of this. All of this and the switch is still what? Open,

this and the switch is still what? Open, which means this is open, which means R3

which means this is open, which means R3 is not part of the circuit

is not part of the circuit yet cuz the current's going to flow here

yet cuz the current's going to flow here through R1 and R2 and then come back.

through R1 and R2 and then come back. It's not going through R3

It's not going through R3 yet. The reading on the voltmeter is

yet. The reading on the voltmeter is 7.5, which means this is 7.5 volts.

7.5, which means this is 7.5 volts. What is the emf of the

battery? Actually, that's easy. Didn't he say they're all 40 ohm

easy. Didn't he say they're all 40 ohm resistors? This is 40. This is 40. This

resistors? This is 40. This is 40. This is

is 40. But this doesn't

40. But this doesn't count. If I have two resistors that are

count. If I have two resistors that are equal in

equal in value in

value in series, doesn't the voltage split

series, doesn't the voltage split equally between them?

equally between them? Doesn't the voltage split equally

Doesn't the voltage split equally between

between them? So if this gets 7.5, this also

them? So if this gets 7.5, this also gets

gets what?

what? 7.5. Which means the total voltage is

7.5. Which means the total voltage is what? 15. 7.5 + 7.5 is

15. I could tell all of that just because he said, "Hey, they're all 40

because he said, "Hey, they're all 40 ohm resistors," which means they're all

ohm resistors," which means they're all the same resistance.

Okay, which means the voltage splits equally. So if the first one got 7 and

equally. So if the first one got 7 and 1/2, the second one gets 7 and a half.

1/2, the second one gets 7 and a half. If the first one got like 200 volts, the

If the first one got like 200 volts, the second should also get 200 volts.

second should also get 200 volts. However, now the switch is

However, now the switch is closed. Oh boy. What's the resistance of

closed. Oh boy. What's the resistance of the complete circuit?

the complete circuit? Yeah, I'm going to redraw the circuit

Yeah, I'm going to redraw the circuit here, but instead of vertically because

here, but instead of vertically because sometimes mentally in your head it's

sometimes mentally in your head it's hard to imagine, I'll draw it

hard to imagine, I'll draw it horizontally. Like I'm just going to

horizontally. Like I'm just going to rotate this 90°. Here's

rotate this 90°. Here's R1 and then there is

R1 and then there is R2 and then like R3 is above

R2 and then like R3 is above that. Which means what? Which means you

that. Which means what? Which means you have two resistors in parallel first

have two resistors in parallel first which are then in series with this dude

which are then in series with this dude over here.

over here. So step one I need to get these two

So step one I need to get these two together first in parallel and after I

together first in parallel and after I get their combined resistance I add it

get their combined resistance I add it to the series

one. So let's go the parallel version first. It's product

parallel version first. It's product over sum. R1 * R2 over R1 + R2, which is

over sum. R1 * R2 over R1 + R2, which is 40 *

40 * 40 over 40 + 40. This will give me 20.

40 over 40 + 40. This will give me 20. And this is something you can memorize,

And this is something you can memorize, by the way. If you have two resistors in

by the way. If you have two resistors in parallel and they have the same value,

parallel and they have the same value, their combined resistance is half of

their combined resistance is half of them. So 40 and 40 give you 20. 30 and

them. So 40 and 40 give you 20. 30 and 30 give you 15. Five and five gives you

30 give you 15. Five and five gives you two and a half. But then I need to add

two and a half. But then I need to add it to the series resistor. So the 20

it to the series resistor. So the 20 ohms, the combined 40 and 40 gives you

ohms, the combined 40 and 40 gives you 20 plus the 40 for that separate

20 plus the 40 for that separate resistor. This gives you a total of 60

ohms. Knowing what to sequence and when is

important. Calculate the reading on the voltmeter when the switch is now closed.

voltmeter when the switch is now closed. Ooh. Oh, wait, wait, wait. Hold on, hold

Ooh. Oh, wait, wait, wait. Hold on, hold on.

Let me erase this. Or maybe I didn't need to. Yeah, I

this. Or maybe I didn't need to. Yeah, I didn't need

didn't need to. Didn't we just say that these two

to. Didn't we just say that these two together are worth 20

together are worth 20 ohms? And now it's technically in series

ohms? And now it's technically in series with this, which is 40 ohms.

with this, which is 40 ohms. Will the 15 volts of the supply now

Will the 15 volts of the supply now split

split equally? No, it

equally? No, it won't. R1, which is 40, will get more

won't. R1, which is 40, will get more than

than R2. R1, which is the 40 ohms, will get

R2. R1, which is the 40 ohms, will get more than R2 and R3 because they're

more than R2 and R3 because they're together. They're 20 ohms. So, when he

together. They're 20 ohms. So, when he asks you for the reading on the

asks you for the reading on the voltmeter, he means what's the voltage

voltmeter, he means what's the voltage across this?

across this? Now that this is 40 and 20, it's going

Now that this is 40 and 20, it's going to be

to be what? 10 volts. And you can use two

what? 10 volts. And you can use two methods to get this. You can either use

methods to get this. You can either use Ohm's law, which is V= IR. Let's try

Ohm's law, which is V= IR. Let's try that one out first. Or you can use the

that one out first. Or you can use the ratio example. Remember how I used

ratio example. Remember how I used ratios? R1 / R2, V1 over V2. Let's try

ratios? R1 / R2, V1 over V2. Let's try this out. Let's try Ohm's law

this out. Let's try Ohm's law first. Step one, I need the current. So

first. Step one, I need the current. So I= V / R. The total voltage is 15 volts.

I= V / R. The total voltage is 15 volts. So the total resistance is what? 60. Oh

So the total resistance is what? 60. Oh uh wait I didn't I didn't write this.

uh wait I didn't I didn't write this. Okay

Okay 60. This gives me a current

60. This gives me a current of

4.25. Then you use Ohm's law again but to get the voltage of V1 which is the

to get the voltage of V1 which is the voltage across R1. So it's 0.25 25 * 40

voltage across R1. So it's 0.25 25 * 40 which gives you what?

which gives you what? 10.

Yay. Or you can use a ratio example. But

Or you can use a ratio example. But ratio of what exactly? I don't have V2.

ratio of what exactly? I don't have V2. Like I don't know what this voltage is,

Like I don't know what this voltage is, but I do have the total voltage. So

but I do have the total voltage. So here's another version of this ratio

here's another version of this ratio example thing. We can say V1 over V

example thing. We can say V1 over V total equals R1 / R total. It still

total equals R1 / R total. It still works if you compare it to the total,

works if you compare it to the total, not just to the one next to it. So V1

not just to the one next to it. So V1 over what's the total voltage? 15 equals

over what's the total voltage? 15 equals R1, which is the 40

R1, which is the 40 ohms that you want over the total

ohms that you want over the total resistance, which is 60. So V1 is equal

resistance, which is 60. So V1 is equal to 40 / 60 * 15. This gives you 10.

pick your poison. Doesn't matter. They're both

They're both okay. They're both

okay. They're both okay. All

okay. All right, next

right, next question. Oh, wait. Do you want to take

question. Oh, wait. Do you want to take a picture of

a picture of this? Why did it

disappear? The app's being weird. Okay, never mind. You can go back and pause

never mind. You can go back and pause and get the picture there.

Next question. I see three cells in series. I see R1 and R2 and a diode in

series. I see R1 and R2 and a diode in R3 and a

R3 and a voltmeter. The three cells have zero

voltmeter. The three cells have zero resistance. R1, R2, R3 are identical,

resistance. R1, R2, R3 are identical, meaning they're the same. The reading on

meaning they're the same. The reading on the voltmeter is six. The voltmeter

the voltmeter is six. The voltmeter reading is parallel to all three, which

reading is parallel to all three, which means the total voltage or EMF here is 6

means the total voltage or EMF here is 6 volts.

When the diode is conducting, it has zero resistance and zero potential

zero resistance and zero potential difference across it. He's just saying

difference across it. He's just saying the diode doesn't affect anything yet.

the diode doesn't affect anything yet. Determine the emf of one cell. If three

Determine the emf of one cell. If three cells have 6

cells have 6 volts, this means one cell has what? Two

volts, this means one cell has what? Two volts. Cuz 2 + 2 + 2 gives you

volts. Cuz 2 + 2 + 2 gives you six. Then determine the ratio of the

six. Then determine the ratio of the potential difference across R2 to the

potential difference across R2 to the potential difference across R3. That's a

potential difference across R3. That's a slightly challenging question. It's

slightly challenging question. It's actually

actually hard, but it's based on what we just did

hard, but it's based on what we just did earlier. Remember, if this is R1 and

earlier. Remember, if this is R1 and this is

this is R2 and this is now R3, which is in

R2 and this is now R3, which is in series to it, just like the previous 40

series to it, just like the previous 40 40 when he says, hey, what is the ratio

40 when he says, hey, what is the ratio of the voltage of R2 to R3? Uh, sorry,

of the voltage of R2 to R3? Uh, sorry, this is R2. I don't know why I wrote

this is R2. I don't know why I wrote that as R3 and this is R3.

How is the voltage going to get split? Now you might say I don't have values.

Now you might say I don't have values. That's fine. Assume

That's fine. Assume anything. Assume

anything. Assume anything. If this is 10 ohms and 10 ohms

anything. If this is 10 ohms and 10 ohms and 10 ohms, what's the combined

and 10 ohms, what's the combined resistance of R1 to

resistance of R1 to R2? R1 and R2.

R2? R1 and R2. Five. Which means this is technically 5

Five. Which means this is technically 5 to

to 10. And what does he want? He wants the

10. And what does he want? He wants the ratio of the voltage. The ratio of the

ratio of the voltage. The ratio of the voltage is the ratio of the current. So

voltage is the ratio of the current. So 5 to 10 is 1:2. So you can say it's

1:2 or 0.5. That's fine too. It's not an easy question to get. It needs you to

easy question to get. It needs you to have seen questions like this

have seen questions like this before. And the fact that you I can just

before. And the fact that you I can just get, you know, two resistors that are

get, you know, two resistors that are identical. It's half the voltage. So

identical. It's half the voltage. So this gets half, this gets one, and then

this gets half, this gets one, and then it splits. What's the ratio

1:2? Next, all the cells are

Next, all the cells are reversed. Meaning the current now wants

reversed. Meaning the current now wants to

to flow this

flow this way. Wait, if the current wants to flow

way. Wait, if the current wants to flow this way, it cannot flow through R1

this way, it cannot flow through R1 anymore because the diode stops it. So

anymore because the diode stops it. So the currents only flowing through R3 and

the currents only flowing through R3 and R2 which are now technically in what? In

R2 which are now technically in what? In series. R1 doesn't exist now because the

series. R1 doesn't exist now because the current can't go through it. The current

current can't go through it. The current can't go through

can't go through it. Right? Hey, wait a second. If if R1

it. Right? Hey, wait a second. If if R1 goes

goes away, what happens to the total

resistance? It was in parallel. Now it's only in

only in series. Remember when we added things in

series. Remember when we added things in parallel the resistance would decrease.

parallel the resistance would decrease. If you remove the parallel portion the

If you remove the parallel portion the resistance will

resistance will what? Decrease. Sorry increase. The

what? Decrease. Sorry increase. The resistance

resistance increases. You know because you lost

increases. You know because you lost that parallel connection. It's now just

that parallel connection. It's now just series. So the

series. So the current

current decreases. That's very sad.

Then finally, determine the new ratio of the voltage of R2 to

the voltage of R2 to R3. Now, this one should be easy. If you

R3. Now, this one should be easy. If you only have

R2 and R3 in parallel, in series, sorry. And

R3 in parallel, in series, sorry. And they're both the same resistance. The

they're both the same resistance. The voltage will be split

voltage will be split what? equally which as a ratio means

what? equally which as a ratio means it's what? 1

it's what? 1 one. It splits the voltage equally. So

one. It splits the voltage equally. So what's the new ratio? 1

Okay. This was a tough question by the way. Like using ratios and concepts.

way. Like using ratios and concepts. It's a tough question and I picked it on

It's a tough question and I picked it on purpose.

purpose. The final part of this unit or at least

The final part of this unit or at least this section of electricity and we're

this section of electricity and we're done. Let's talk about safety. Now,

done. Let's talk about safety. Now, obviously, obviously, electricity can

obviously, obviously, electricity can kill us. How? Well, it can give us an

kill us. How? Well, it can give us an electric shock. That's bad. And it can

electric shock. That's bad. And it can set us on fire. Not not us directly, but

set us on fire. Not not us directly, but can set everything else on fire.

can set everything else on fire. Like, okay. Can set everything else on

Like, okay. Can set everything else on fire.

fire. So in order to keep ourselves safe, we

So in order to keep ourselves safe, we need a few basic things at home like

need a few basic things at home like insulating wires, using a fuse and so

insulating wires, using a fuse and so on. So let's first take a look at what's

on. So let's first take a look at what's the electricity at home

the electricity at home like an electric

like an electric socket or electricity at home is called

socket or electricity at home is called main's electricity or a main circuit. It

main's electricity or a main circuit. It consists of two main wires and a third

consists of two main wires and a third safety feature. The two main wires are

safety feature. The two main wires are the live

the live wire and the neutral wire. There are no

wire and the neutral wire. There are no positive and negative

positive and negative wires. Why? Because electricity at home

wires. Why? Because electricity at home is always set to 220 volts

is always set to 220 volts AC. Electricity at home is 220 sometimes

AC. Electricity at home is 220 sometimes 240 like it fluctuates a little bit but

240 like it fluctuates a little bit but it's AC which means it doesn't have a

it's AC which means it doesn't have a positive or a negative.

positive or a negative. The live wire is the super high voltage

The live wire is the super high voltage wire that's always fluctuating and the

wire that's always fluctuating and the neutral wire is just there to complete

neutral wire is just there to complete the circuit. This is usually at zero

the circuit. This is usually at zero volts. This means that if I get a piece

volts. This means that if I get a piece of metal and I shove it into just one of

of metal and I shove it into just one of these openings in the socket, if I shove

these openings in the socket, if I shove it into the live wire, I'm a cooked

it into the live wire, I'm a cooked goose. And if I shove it into the

goose. And if I shove it into the neutral wire, I'm aok.

neutral wire, I'm aok. Okay. You and your luck. I don't know

Okay. You and your luck. I don't know which one is live and neutral. We never

which one is live and neutral. We never label them. Electricians and designers

label them. Electricians and designers never label these. Depends on who

never label these. Depends on who connects the wires in the walls. Now the

connects the wires in the walls. Now the earth wire is actually a safety feature.

earth wire is actually a safety feature. Not all the country, not all countries

Not all the country, not all countries have this. This is a safety feature that

have this. This is a safety feature that protects you from getting electrically

protects you from getting electrically shocked if a metal case becomes live.

shocked if a metal case becomes live. But I'll discuss this over here like uh

But I'll discuss this over here like uh we'll talk about the earth wire down

we'll talk about the earth wire down here.

So, as I said, what are the hazards? You can get electrically shocked or you can

can get electrically shocked or you can cause an electric fire. What causes

cause an electric fire. What causes these things? Insulation if it's

these things? Insulation if it's damaged. If you overheat the cables, cuz

damaged. If you overheat the cables, cuz that damages the insulation cuz wires

that damages the insulation cuz wires get too hot, it could melt the

get too hot, it could melt the insulation and it could catch on fire.

insulation and it could catch on fire. Damp conditions means if it's wet,

Damp conditions means if it's wet, obviously you don't charge your phone in

obviously you don't charge your phone in the bathroom. If you do, something's

the bathroom. If you do, something's wrong with you.

wrong with you. uh too much current because too much

uh too much current because too much current will cause the cables and the

current will cause the cables and the sockets to overheat which causes what?

sockets to overheat which causes what? Electric

Electric fires.

fires. Okay. So, what safety features do we

Okay. So, what safety features do we have at home to protect us from these

have at home to protect us from these two main problems? Too much current is

two main problems? Too much current is the reason why things overheat or things

the reason why things overheat or things burn and like you know sockets get

burn and like you know sockets get damaged and everything

damaged and everything else. And to protect you from that, we

else. And to protect you from that, we have a

have a fuse. A fuse is just a very thin wire,

fuse. A fuse is just a very thin wire, by the way. Very thin

by the way. Very thin wire that's always placed on the live

wire that's always placed on the live side of a

side of a circuit. This wire is designed to

circuit. This wire is designed to melt if the current gets too

melt if the current gets too high. So let's say for example, you have

high. So let's say for example, you have a 5 ampere lamp. It uses 5 ampers

a 5 ampere lamp. It uses 5 ampers normally and you close the switch and it

normally and you close the switch and it draws in 5 amp. That's great. It's going

draws in 5 amp. That's great. It's going to work fine. But if the current

to work fine. But if the current increases it'll burn like the lamp will

increases it'll burn like the lamp will burn out. Replace the lamp with any

burn out. Replace the lamp with any device you want. Okay. So what we do is

device you want. Okay. So what we do is we put a fuse over here. A fuse has a

we put a fuse over here. A fuse has a rating. Let's say 6 amp.

rating. Let's say 6 amp. Meaning if the current reaches 6 ampers,

Meaning if the current reaches 6 ampers, it will melt. And when I say melt, I

it will melt. And when I say melt, I don't want you to imagine it slowly

don't want you to imagine it slowly melting away. No, no, no. It just pops

melting away. No, no, no. It just pops right away. Just snaps, breaks

right away. Just snaps, breaks immediately. Okay. So, when you close

immediately. Okay. So, when you close the switch and the current that's

the switch and the current that's flowing through is less than 6 amp, it's

flowing through is less than 6 amp, it's like the normal five, no problem. Fuses

like the normal five, no problem. Fuses like a wire, no problem. But if you

like a wire, no problem. But if you close the switch and for some reason,

close the switch and for some reason, maybe there's an electric surge, maybe

maybe there's an electric surge, maybe somebody's fooled around with the

somebody's fooled around with the switches at home or the wires, whatever

switches at home or the wires, whatever it is, and the current increases for any

it is, and the current increases for any reason, it goes through what first? The

reason, it goes through what first? The fuse and it melts the fuse. This like

fuse and it melts the fuse. This like pops

pops it. So now there is no current that

it. So now there is no current that reaches the light bulb. So the light

reaches the light bulb. So the light bulb turns off, but you kept it

bulb turns off, but you kept it safe. So what we do is if that happens,

safe. So what we do is if that happens, we remove the old fuse, we throw it

we remove the old fuse, we throw it away, we plug in a new fuse, and then we

away, we plug in a new fuse, and then we flip things on again, switch things on

flip things on again, switch things on again. How do I choose a correct rating

again. How do I choose a correct rating for a fuse? You always go for like one

for a fuse? You always go for like one to three ampairs higher than what the

to three ampairs higher than what the device needs. So, for example, if I tell

device needs. So, for example, if I tell you I have an air conditioner that needs

you I have an air conditioner that needs 30 amps to work, well, I had better put

30 amps to work, well, I had better put a

a what? What fuse should I

what? What fuse should I use? Maybe a 32 or a 33

right? Similar to a fuse is a circuit breaker or a trip switch. This is just

breaker or a trip switch. This is just like the fuse. It protects the circuit

like the fuse. It protects the circuit if the current gets too high. If it gets

if the current gets too high. If it gets too high for any reason. But it doesn't

too high for any reason. But it doesn't melt. This time it uses an

melt. This time it uses an electromagnet. We have an electromagnet

electromagnet. We have an electromagnet over here next to what we call an

over here next to what we call an iron switch cuz iron is a magnetic

iron switch cuz iron is a magnetic substance. As long as the current is

substance. As long as the current is low, the magnet doesn't turn on because

low, the magnet doesn't turn on because it's connected to the live wire as well.

it's connected to the live wire as well. But if the current gets too high, the

But if the current gets too high, the current goes through the magnet first.

current goes through the magnet first. It turns on, it magnetizes, and it

It turns on, it magnetizes, and it attracts the

attracts the switch plop, opening it, and therefore

switch plop, opening it, and therefore your circuit doesn't work or your entire

your circuit doesn't work or your entire house doesn't turn

house doesn't turn on. It's exactly the same function as a

on. It's exactly the same function as a fuse. It stops the current if it's gets

fuse. It stops the current if it's gets too high. But this time by opening what?

too high. But this time by opening what? A

A switch using a

magnet. We always choose a rating that's 1 to 3 amps higher than what you need.

1 to 3 amps higher than what you need. Okay? And we also always connect it to

Okay? And we also always connect it to the live wire because if you connect it

the live wire because if you connect it to the neutral wire, this is you cutting

to the neutral wire, this is you cutting out the current on the way out of the

out the current on the way out of the circuit, not on the way in. So the

circuit, not on the way in. So the current's already gone in, burnt your

current's already gone in, burnt your house down, and as the current's going

house down, and as the current's going out, it's like, hey, you want to stop me

out, it's like, hey, you want to stop me now? Sure, go ahead. Your house is

now? Sure, go ahead. Your house is already on fire,

already on fire, though. Okay, that's the circuit

though. Okay, that's the circuit breaker. Now, final question. What's the

breaker. Now, final question. What's the earth wire?

earth wire? The earth wire or sometimes we call it

The earth wire or sometimes we call it the ground wire is a safety feature

the ground wire is a safety feature that's not in all devices and not in all

that's not in all devices and not in all countries either because it's only

countries either because it's only useful if the device you're talking

useful if the device you're talking about has a metal

about has a metal case. So think of a washing machine or a

case. So think of a washing machine or a computer, a PC or a fridge, anything

computer, a PC or a fridge, anything with metal as its outer case.

with metal as its outer case. The wires inside the washing machine,

The wires inside the washing machine, for example, have live and neutral

for example, have live and neutral connections. And if as you connect them,

connections. And if as you connect them, one of the live wires accidentally

one of the live wires accidentally touches the case and

touches the case and you touch the case as

you touch the case as well, the currents going to flow in not

well, the currents going to flow in not to the washing machine, but to you. So,

to the washing machine, but to you. So, you're a nice target. And you light

up. Now, I don't want you to light up. Okay, that's bad. So, what we do is we

Okay, that's bad. So, what we do is we connect a new wire to the metal case

connect a new wire to the metal case that's not part of the

that's not part of the circuit. And we connect it to the ground

circuit. And we connect it to the ground or the earth. When I say ground or

or the earth. When I say ground or earth, I actually mean a big metal rod

earth, I actually mean a big metal rod inside a building. It has to be a

inside a building. It has to be a conductor. We just call it earththing

conductor. We just call it earththing because it's not part of the circuit.

because it's not part of the circuit. So instead of the current going through

So instead of the current going through you that's not a good idea. The current

you that's not a good idea. The current goes through what? The earth instead.

goes through what? The earth instead. Cuz which has a lower resistance? You as

Cuz which has a lower resistance? You as a human being made of flesh and blood

a human being made of flesh and blood and bones and muscles and everything

and bones and muscles and everything else. You've got some resistance by the

else. You've got some resistance by the way. But the earth wire which is made of

way. But the earth wire which is made of copper often or even aluminium has very

copper often or even aluminium has very low resistance. The current's going to

low resistance. The current's going to prefer that over you. Not even a current

prefer that over you. Not even a current wants you.

But that's good otherwise I don't want any

tiger. Now you might be thinking but on its own this means I'm losing

its own this means I'm losing electricity all the time. Well because

electricity all the time. Well because the earth wire has very low resistance

the earth wire has very low resistance it causes the current to spike increase

it causes the current to spike increase a bit increase a lot which causes the

a bit increase a lot which causes the fuse to trip and it's gone.

So, what does it do? What does an earthwire do? It protects you from

earthwire do? It protects you from getting electrically shocked if a metal

getting electrically shocked if a metal case is

case is live. Okay? If something doesn't have a

live. Okay? If something doesn't have a metal case, you don't need a live

metal case, you don't need a live wire. And if you have that earth wire

wire. And if you have that earth wire and a fuse, the fuse will

and a fuse, the fuse will glow.

glow. Okay, let's solve the last couple of

Okay, let's solve the last couple of questions here.

questions here. So, which diagram shows a symbol for a

So, which diagram shows a symbol for a fuse? Yay. D,

fuse? Yay. D, obviously.

obviously. And an electric kettle has a metal

And an electric kettle has a metal casing. The cable for the kettle

casing. The cable for the kettle contains a wire that is connected to the

contains a wire that is connected to the earth of the plug. Which danger does

earth of the plug. Which danger does this guard

this guard against? From the cable getting too hot?

against? From the cable getting too hot? No. From the casing of the kettle

No. From the casing of the kettle becoming live? Absolutely. Because if

becoming live? Absolutely. Because if the casing is live, I get shocked. I've

the casing is live, I get shocked. I've gotten shocked before. It's not a nice

gotten shocked before. It's not a nice feeling at all. The casing of the kettle

feeling at all. The casing of the kettle becoming wet.

becoming wet. No, it's supposed to be wet. The casing

No, it's supposed to be wet. The casing of the kettle overheating. No, that's

of the kettle overheating. No, that's not the issue. The issue is you getting

not the issue. The issue is you getting electrically shocked. So,

electrically shocked. So, be and you'll notice there are no more

be and you'll notice there are no more questions. This last bit, this last

questions. This last bit, this last chapter essentially doesn't show up a

chapter essentially doesn't show up a lot. It's mostly a core part of the

lot. It's mostly a core part of the syllabus, not an extended part. So, it

syllabus, not an extended part. So, it barely shows up, but you know what?

barely shows up, but you know what? shows up in a multiple choice question

shows up in a multiple choice question or two or like maybe a one or two mark

or two or like maybe a one or two mark question paper four it could. So it's

question paper four it could. So it's important to

important to remember and that ladies and gentlemen

remember and that ladies and gentlemen is unit for at least the first half

is unit for at least the first half electricity. X time is magnetus.

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