Sunday, May 26, 2002

A grounding in basic electricity

A while back I posted an item about static electricity. The idea was to lay some basic groundwork leading up to a discussion of power generation and distribution for nontechnical people. In what follows I'll assume that the material in that earlier post is familiar.

Static electricity is comparatively simple to analyze - it's about electric charges that are not moving. Start moving those charges around and entirely new phenomena emerge.

A faucet provides a familiar analogy for these phenomena. It provides a connection to a supply of water that is under pressure, in what for our purposes is a limitless quantity. The electrical analogy for this pressure is the potential, or voltage. And just as water flows from where the pressure is higher to where it is lower, electric current flows from higher voltages to lower voltages.

Alright, go find a sink with a working faucet, and open the drain. You want to make sure that that water has some place to go once you turn it on. The electrical analogy for this is a "ground" - either way you are establishing a place where the flow can go without accumulating or "charging up".

Now turn the water on very slowly. Does water come out immediately? No, it takes some time to fill the spout before the water comes out. In an electric circuit, we have an analogous phenomenon called "capacitance" - you have to charge up the wires before you can deliver any power.

Now turn the faucet more wide open. You get more flow. That's because the faucet internals are positioned to ease the flow of water, which is analogous to electric current. Electric current is measured in units of charge passing through per second - the most commonly used unit is the "ampere" or "amp".

Now turn the faucet wide open. You're delivering as much flow as you can. That's because you have minimized the amount of flow resistance in your water "circuit". In an electric circuit the corresponding phenomenon is called "resistance".

The water flowing through the faucet is at a higher pressure (say 40 pounds per square inch, or psi) on the inlet of your faucet. Where it leaves the faucet it is at the same pressure as the rest of the room, or 0 psi. What happened to the pressure? The relevant energy was consumed by fluid flow friction inside your faucet, or its "resistance". In electric circuits the resistance of a device tells you the ratio of how much your voltage will drop across it to the amount of current that is flowing at the time.

Now shut off the faucet as quickly as you can. Depending on how your plumbing is designed, you might have felt a "water hammer" caused by forcing the water moving in your pipes to come to a sudden stop. There is an analogous phenomenon in electric circuits called "inductance".

Hmm, that's strange. Electricity is a bunch of moving charges which are almost massless, and I never see wires jump when I flip off a switch. So where is this inductance crap coming from?

Well, this post is getting king of long, so I guess I'll cut it off here. So you'll just have to sit on pins and needles for the next thrilling installment...

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