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Lesson Video: Cables and Plugs Physics

In this video, we will learn how to wire a plug, and what the purpose of each wire in the plug is.


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.

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