Dear Straight Dope:
Cecil's column on lightning rods sparked (if you'll pardon the expression) a different thought: How do they "ground" electrical outlets on the space station? Or in any spacecraft, for that matter? Where do all those unattached electrons go? Do they whiz on over to the Van Allen Belt? Does the vacuum of space have the unlimited potential of earth? Or does every space shuttle have a big static electricity spark whenever it lands? Does this have implications for colonies on the moon and Mars?
Whence cometh this silly notion that every electrical circuit must have a ground? Speaking as a scientist, I blame electrical engineers. Science is pure and holy. It’s when engineers put science into practice that things get messed up.
There is no need to have a ground in a circuit–that is to say, an earth ground, at zero volts. It’s possible to have a full circuit where the lowest potential is a thousand volts, or a trillion, or any amount you care to name. In fact, the notion of “zero potential” is a bit arbitrary in the first place, since all that ever matters is potential difference. If one lead in your circuit is at 1,000 volts and the other is at 1,120 volts, you’ve got a difference in potential of 120 volts. Your electric lights will work just fine.
There are practical reasons for wanting to ground things, though. There are a lot of items in everyday life that we don’t want to have carrying large currents–ourselves, for instance. Since current will flow (or at least attempt to flow) whenever you have a potential difference, the simple solution is to make sure that the things we are regularly in contact with, like metal appliances and the earth, are all at the same potential. The easiest way to do this is to connect all your appliances to the earth using a ground wire, which is literally connected to the ground. In your house this is accomplished by connecting your electric service panel to the incoming water supply pipe (which runs through the ground), or to a metal rod buried in the earth.
Now, on the space station, folks aren’t touching the earth much. All that matters there is that everything they do touch is at the same potential, and this is easily accomplished by attaching all the “grounds” to each other–"grounding to the frame," as electronics technicians say. Charges rarely enter or leave the space station or the shuttle, and then only in small amounts in some specialized experiments, so there’s no huge zap when the shuttle lands. (There might be some charge picked up while passing through the atmosphere, but that’s another matter.)
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