Video Transcript
Many of us may know that we can
power electrical appliances such as this toaster by pushing the plug attached to it
into the plug socket and flicking the switch into the on position. But how exactly do these plugs
work? Why are they shaped the way they
are? And what exactly do they do?
Well, we can probably guess that
they’re designed to carry electrical current from the socket to the appliance. So, let’s start by imagining a
simplified circuit diagram as an analogy to the circuit that’s formed when our
appliance is plugged into the socket. This circuit diagram consists
firstly of a simple AC source, or alternating current source, which in this case
represents the alternating current that’s generated at a power station. And then, these dotted lines
represent the transmission grid, such as the national grid, that carries power from
the power station all the way to our homes or, for that matter, to any socket that
we can plug our appliance into.
And then, we can imagine that these
two wires here are inside the electrical socket. And then, they connect to these two
wires here, which are inside the plug and the cable that connect to our
appliance. And we’ve drawn these last two
wires together just to help us visualize a little bit that they’re inside a
cable. Of course, that cable is the one
running from the plug to the appliance. And then, we finally modeled our
appliance as a little resistor here.
Now, many appliances do much more
than just act as a resistor. But if we imagine that our
appliance is the toaster that we saw on the opening screen earlier, then the
resistor being the main component of the toaster is perfectly fine. Because remember, when a current
flows through a resistor, the resistor ends up heating up. And that’s exactly what we want in
a toaster. We want it to heat up when a
current flows through it so that we can warm up our bread and make some toast. And so, this whole circuit diagram
is a massive oversimplification of the circuit formed when we plug our toaster into
a socket.
This part relates to the power
generated in a power station. This part is the national grid. This part is the plug socket. This part is the plug and
cable. And this part is the appliance. So, let’s look in a bit more detail
at the part involving the plug and the cable. Let’s imagine that we look at the
plug from this angle here. Let’s imagine this is our eye, and
we’re looking at the back of the plug. This is what we’d see. This is the plug, and this is the
cable coming out of the plug, where the other end of that cable attaches to our
appliance, in this case our toaster.
If we unscrew these three screws,
which can often be found in different places on the back of the plug, we can
actually lift off the back plastic casing from the plug, and we get to look inside
the plug. When we do, this is what we
see. We can see firstly that the cable,
which goes into the base of the plug, is held firmly in place by a piece of plastic
bolted into the plug here and here. We can also see that the cable
splits into three different wires, a blue wire, a green and yellow wire, and a brown
wire. The blue wire is always on the
left, the green and yellow wire is always in the middle, and the brown wire is
always on the right.
Now, these three wires are attached
to the three different pins of the plug that we would be able to see on the other
side of the plug. In other words, if we were looking
at the plug from this angle, if we placed our eyeball here, then what we would see
is the plug itself and the cable coming out if it. And this would be the top pin,
which is slightly longer than the two lower pins.
But anyway, coming back to this
diagram, if we look very closely at each one of the three wires, we see that the
wires themselves aren’t colored blue, green and yellow, and brown, respectively. We see that the wires themselves
are actually these bits here that are made from copper protruding from the colored
parts. And the colored parts themselves
are made from plastic. In other words, we have three
copper wires that form a part of this cable, and each one of those copper wires is
coated in colored plastic.
This colored plastic is there for a
couple of different reasons. Firstly, the colors allow us to
tell which wire is which. And at this point, it’s probably a
good idea to learn the names of the three different wires. The blue wire, always wired to the
left pin of the plug, is known as the neutral wire. The green and yellow wire, always
wired to the top pin, is known as the Earth wire. And finally, the brown wire, always
wired to the right pin, is known as the live wire. Each one of these wires serves a
very specific purpose. And it’s important that we wire
them in the correct configuration, which is the one shown in this diagram.
But coming back to the colored
plastic that coats these wires, we said already that the colors themselves help us
identify which wire we’re dealing with. So, for example, we’ll know that
we’re working with an Earth wire if we see a green and yellow coating around it. But the other function of the
plastic is to act as an electrical insulator. In other words, the wires
themselves, the copper wires, are designed to carry electrical current. And so, if somebody were to
accidentally touch the exposed copper wires, then there’s a high chance that they’d
receive an electric shock. They would be electrocuted.
And so, the plastic coating is
designed to prevent this because plastic is not a good electrical conductor. In fact, it’s a very good
electrical insulator. So, if someone were to accidentally
touch the plastic coating, even if there was a current flowing through the copper
wire itself, then they’d have a much lower chance of getting electrocuted. And in fact, that’s also why the
large cable itself, which is made up of the three wires, also has a plastic coating
around it. To act as further insulation to
reduce the chances of electrocution if somebody accidentally touches this part of
the wire.
So, we’ve seen that inside an
electrical plug, the large cable going into the base of the plug splits up into
three different smaller wires, the neutral wire, the Earth wire, and the live
wire. We’ve also seen that the plastic
coating around each one of these wires is colored to allow us to identify which wire
we’re dealing with and also act as electrical insulation. But what do each one of these wires
actually do? Well, if we come back to our
simplified circuit diagram that we drew earlier, then we can see that in reality,
for our appliance to function, we just need two wires, one on either side of the
appliance.
We can imagine that as one wire
coming into the appliance this way and one wire coming in this way, in order for the
appliance to function in the first place. But as we’ve seen in this plug,
there are three wires. So, we actually have one extra wire
in this plug. We’ll come back to that in a
second. But first, we can learn that the
two wires that are necessary for the circuit to be able to work at all are the live
wire, which we can label as this one. So, that’s the brown wire. And the neutral wire which is the
other one. In other words, the two wires that
complete the circuit in order for our appliance to be able to work are the brown
live wire and the blue neutral wire.
The live wire is at a voltage of
230 volts, whereas the neutral wire is at a voltage of zero volts, where this
voltage zero volts is defined as being at the same voltage as the Earth. Which might seem a little bit
confusing. But the point of this is to realise
that because the live wire is at 230 volts, and the neutral wire is at zero volts,
this means that the potential difference across our resistor, or whatever our
electrical appliance is, is 230 volts minus zero volts. Or in other words, we can say that
230 volts of potential difference are dropped across our resistor, or appliance.
And that 230 volts of potential
difference comes about due to the power source, which in our simplified circuit
diagram is this AC source here, but in reality would be the power station. Although it’s a little bit more
complicated than this because the power station does not produce electrical power at
230 volts. But rather it’s a transformer found
between the transmission grid and the power socket that converts this electrical
power so that it’s at 230 volts. And so, we can imagine that
conventional current, which is the flow of positive charge, flows from the live wire
through our appliance and then out of our appliance through the neutral wire.
And that’s what we mean when we say
that the live wire is at 230 volts and the neutral wire is at zero volts, resulting
in a potential difference of 230 volts across our appliance. Because electrical current or at
least conventional current, which is the flow of positive charge, flows from a
higher electrical potential to a lower electrical potential. And we arbitrarily define zero
volts to be the same as the electrical potential of the Earth or of the ground. So, we’ve seen what the neutral
wire and the live wire do. Let’s now take a look at the
function of the Earth wire.
Let’s imagine that inside our
cable, which connects to our appliance, in this case Toasty McToastface, ends up
malfunctioning slightly. Let’s imagine that the live wire,
which goes all the way through the plug and through the cable into our appliance,
somehow manages to break free of its plastic coating. And the live wire itself, the
copper wire, ends up touching the casing of our toaster, which happens to be
metallic. In other words, what we’ve got
going on now is that our live wire has broken free of its plastic casing and is no
longer electrically insulated properly.
This could happen if somebody trips
over the cable for example, and pulls free the live wire inside the appliance. And in this case, the live wire has
ended up touching the metal casing of our toaster. Well, if somebody was to now come
along and touch the toaster, then this would be very bad news indeed. Because the metallic casing itself
is a very good conductor. Remember, metal is a good conductor
of electricity. And the person touching the toaster
would actually complete the circuit because current could now flow through the
socket into the live wire through the plug itself into the appliance onto the casing
of the appliance and then through the person’s fingers through their body and then
finally down to the ground.
So, there’s a clear path for
current to flow from the live wire, which is at a voltage of 230 volts, is at a high
electrical potential, down to the earth, which is at zero volts as we said already,
and therefore at a lower electrical potential. In other words, in this process,
the person touching the toaster is getting electrocuted. And these electric shocks can cause
a great deal of damage and can sometimes even be fatal. However, this is where the Earth
wire comes into play.
The Earth wire, in the case of a
fault, provides a much lower resistance route for current to flow down to Earth, as
compared with the route that current would take when flowing through a person down
to the Earth. And this is why the Earth wire is
known as the Earth wire. It’s at the same electrical
potential as the Earth, in other words, at zero volts. And it allows a low resistance path
for current to flow in the case of a malfunction of the live wire so that even if
this person here was to touch our toaster’s metal casing, only a very small amount
of current would flow through them.
In other words, the Earth wire is a
safety feature. And there’s a good reason for
having multiple safety features such as the plastic insulation as well as the Earth
wire when it comes to designing plugs and cables. Because the consequences of not
having these safety features, which is electrocution, can be fatal. And in fact, another safety feature
found in plugs, this object here, is known as a fuse. Now, a fuse is not necessarily
designed to prevent electrocution, but the way it works is quite simple and yet very
clever.
The fuse contains a very thin piece
of copper wire. And it’s important to note that the
fuse actually forms part of the electrical circuit that is formed in order for our
appliance to be working. So, the current that flows through
the live wire also flows through the fuse itself. And if, for some reason, due to
some malfunction that current becomes too large, then the copper wire inside the
fuse itself ends up melting. Because, remember, we mentioned
earlier that when current flows through a resistor, and in the real world even thin
wires have some small resistance, then that resistor ends up heating up.
And if a very large current flows
through our fuse, then our thin copper wire heats up a lot, eventually so much so
that the wire melts and breaks, thus effectively breaking the circuit. And so, no current flows in that
circuit at all now. This prevents very large currents
from flowing into our appliance. And therefore, prevents the
appliance itself from heating up so much that it catches fire. In other words, the fuse itself
melts way before the current becomes large enough to be dangerous to our
appliance. And once we figure out why the
current is so large, we can simply replace the fuse itself with a brand new one. And we’re ready to go once
more.
Now, as well as all the safety
features that we’ve seen so far, plugs and cables are filled with lots more. An example of this is the fact that
the top pin, the pin connected to the Earth wire, is much longer than the two lower
pins, the pins connected to the neutral and live wires. The reason for this is because when
we stick our plug into a socket, the Earth wire pin is the one that connects
first. In other words, before the two pins
that actually carry current to and from our appliance connect, the connection to the
Earth wire is already established.
And this way we can ensure that if
there is something wrong with our wiring, such as for example the live wire touching
the metallic casing of our toaster, then the Earth wire is already connected and
ready to take current away in this direction towards the Earth. And there are lots of different
safety features just like this found in the design of plugs and cables to help
ensure that they’re as safe as possible. But anyway so, now that we’ve seen
all of this, let’s take a look at an example question.
Electrical wires are often made of
a copper core covered in a thin plastic coating. Which of the following are reasons
why copper is chosen as the material for the core of the wire? a) It is a good
thermal conductor. b) It is very cheap. c) It is ductile. d) It is very light. E) It is a good electrical
conductor.
So, this question is focusing on
why exactly we choose copper as the material to make most of our electrical wiring
from. We’ve been given five different
potential reasons, and we need to work out which of these are actual reasons as to
why we use copper. So, let’s go through them one by
one.
Firstly, starting with a), it is a
good thermal conductor. Now, a good thermal conductor
easily allows heat to flow through it. And it’s true that copper is a
pretty good thermal conductor. However, this is not a reason why
we’d select copper. Because when we choose a wire
material, we’re interested in how it conducts electricity, not so much in how it
conducts heat. Strictly speaking then, good
thermal conductivity is not a factor in our choice because the fact that it’s a good
thermal conductor is not really relevant to electrical wires. And so, option a) is not an answer
to our question.
Moving on to option b) then, it is
very cheap. Now, copper is kind of cheap, but
there are other substances that are much cheaper. For example, if we think about
metals, then aluminium is cheaper than copper. And so, it’s not really true that
copper is very cheap. By the way, the reason that we
don’t use aluminium is because aluminium is not as good of an electrical
conductor. Copper is much better at carrying
an electrical current than aluminium is.
Anyway, moving on to option c)
then, it is ductile. Now, this word here ductile refers
to a material, in this case copper, that can be drawn out into a thin wire. In other words, any material that
we can take from being, let’s say, a slab and apply pressure to it in certain
directions to draw it out into a thin long piece of wire without breaking, of
course, is known as a ductile material. And this is a very useful property
of copper because if we want to make long thin wires, then we need a material that
can be turned into long thin wires. And therefore, option c) is one of
the answers that we’re looking for. The fact that copper is ductile is
one of the reasons that we use it to make electrical wiring.
Moving on to option d) then, it is
very light. Now, this is not really true,
either. In fact, for the same amount of
stuff, for the same amount of substance, if we compare copper and aluminium once
again. So, say for example, we take two
cubes of the exact same volume, one is copper, and the other is aluminium, then the
aluminium cube is lighter. Or in other words, we can say that
the density of aluminium, the amount of mass found in every unit of volume, is lower
than the density of copper. And so, it’s not true that copper
is very light.
Therefore, moving on to the final
option, option e) says that copper is a good electrical conductor, which means that
it’s good at carrying an electrical current. And this definitely is true. And it is a reason that we use
copper in order to make electrical wires because we need something that can easily
carry an electrical current. And therefore, at this point, we’ve
gone through all of the options and found our answers. The reasons that we use copper to
make electrical wires is because, firstly it is ductile, and secondly it’s a good
electrical conductor.
So, having gone through an example
question, let’s summarize what we’ve talked about in this lesson. We saw firstly that electrical
plugs contain three different wires. The blue one connected to the left
pin of the plug when seen from behind is known as the neutral wire. The green and yellow wire is known
as the Earth wire. And the brown wire is known as the
live wire. We also learned about the specific
functions of each one of these wires. And finally, we also saw that there
are various safety features built into the design of plugs and cables, for example
plastic insulation around every single copper wire, the fuse, the Earth wire, and
many more.
