Short Circuits

I never really understood short circuits and blowing fuses with too many things plugged in until I was 61 years old. Yes, I knew that the problem was too much electricity going through the wires, which would then get hot enough to cause a fire, so a fuse is installed with a soft metal that will melt and break the circuit before the rest of the wires heat up too much. Why does it get too much electricity, though? It is easy to see why a fuse would blow if something external introduced too much electricity to the system--- a lightning strike, for example. But what is the problem with too many appliances plugged in? Surely that would soak up more electricity, not introduce more of it. The appliances are consumers, not producers, after all.

But that reasoning is wrong. True, the appliance are users, not producers. But they are also, when turned on, new channels through which the electricity can flow. That's they key.

Start with the idea of how much electricity there is. That is called the charge, and is the basic scientific unit for electricity (measured in "coulombs" a very ugly name). The charge is made up of lots of electrons or of lots of electrons being missing, which makes for an imbalance either way. When you get opposite charges on each end of a wire, electricity starts flowing between them to try to equalize the charge, and we have a "force". We measure the difference in charges as the "voltage", which shows how much charge is affecting the wire circuit. Charge is like water flowing through a pipe, and voltage is like the pressure that moves the water. There can be a lot of water, but it doesn't matter how much if the pressure is low: it's the pressure that matters. But it isn't only the pressure that matters. The pipe is of a limited size, and the flow of charge per second is called the "current", measured in "amperes".

This gets us close to the problem of blowing a fuse. If there's too much current, that's the problem of too much electricity overloading the wire. So now let's think of why there might be too much current. Too much charge and voltage is the obvious answer. That's why lightning could blow the fuse. But household voltage is fixed at 110 volts ( 120, even if labelled as 110), so too much voltage is not the usual problem.

But there's something else that affects the current besides the voltage: the pipe. Remember, it's of limited size. So a narrow pipe would let less water per second through than a wide pipe. The same is true of wires. There, we call the difficulty the charge has in getting through the wire easily the wire's "resistance". A thin wire has more resistance than a thick wire; it's harder for the electricity to flow through quickly in a large amount. It's not just the width, either. I don't know yet if the length matters, actually--- will a long wire have more resistance than a short wire? (It does dissipate more electricity as heat, but that's different, maybe.) But what the wire is made of definitely matters. Copper has low resistance. Plastic has high resistance. Aluminum has pretty low resistance. Silver has the lowest resistance of any metal.

To be continued...