Think about a water pipe with water flowing through it. What would happen if you changed the size of the pipe? Water would have more difficulty flowing through a small pipe than it would a large one. Similarly, electrons can move easier through some substances (conductors) than others (insulators). The degree to which the material resists the flow of electrons is called the resistance of the substance and is measured in ohms (abbreviated as a the Greek letter Ω). The amount of resistance a wire has depends on four things:
- The length of the wire. The longer a wire is, the greater its resistance, as the electric current has to pass through more media.
- How thick it is. The thinner the wire, the greater its resistance as well, as it is harder for the electrical current to move along.
- What it is made of. As we have seen, the composition of the wire (what it is made of), also determines resistance.
- Its temperature. The warmer a wire is, the greater its resistance.
Watch this video to see an interactive look at resistance in a wire.
OK, so let's look at a couple of things that affect resistance here. As you can see, we'll see the flow of electrons down here, which is a product of how much resistance actually exists. So our formula for resistance in an electrical circuit is rho, which is the conductivity of the wire, the length of the wire, which L, and capital A, which is the area of the wire.
So what we see here is as we increase rho, we're actually going to be increasing the resistance in the circuit. So I'll put that back down here. If we increase the length, we're going to see that resistance also increases. And then when we increase area, we're going to see that resistance actually decreases. So the larger the object, the less resistant. And as you can see, the tube actually gets bigger down here.
So if we were to play with some combinations, you'll see that we have an area that's fairly small. We have a high resistance. But then if we actually increase the length, we're actually going to see even more resistance. And then if we increase the conductivity of the wire, we're actually going to even have more resistance.
Whereas if we lower it, the conductivity is really small, resistance goes down, the length goes down, and we're actually going to have a very small R there as well. And then if we increase the area, we're going to have even smaller. And this should make sense because it's very easy for the electrons to flow through this area very easily.
Wires can be made of different metals. Aluminum and copper can be good conductors as both have low resistance. However, sometimes metals with a high resistance are used for special purposes. Nichrome is a metal that is also a conductor, but it has higher resistance than aluminum or copper. When a large electrical current goes through nichrome, it gets very hot. Toasters and electric heaters use wire coils made of nichrome or a metal with similar properties inside them in order to create heat. Tungsten, which is used as the filament in many light bulbs, is another metal that is a conductor but has a high resistance. When current passes through a tungsten wire, it gets so hot that it glows white-hot.
Question
How much resistance does a superconductor have?
Some materials are called superconductors because they lose most of their resistance at low temperatures. When you send a current into it, that current flows straight through it as if it wasn't there. Better yet, you get persistent currents. If you start a current flowing in a loop of superconductor, it will keep flowing forever, unlike a normal conductor, where losses to heat and so on will cause the current to dissipate pretty quickly unless you do something to keep it going. Mercury is a good conductor at ordinary temperatures, but it becomes a superconductor at -270°C. If scientists could find a way to make a superconductor at normal temperatures, the cost of moving electricity from a power plant to your house would decrease dramatically.
