Saveloy: Q: Does the orbiting of planets etc count as an example of perpetual motion? Would it be possible to use the orbit of a moon or satellite as a source of energy (a short-term one, presumably, as I guess it would cause the satellite to fall into the planet eventually)?
I was taught that no, it's not perpetual motion -- it has to do with the curve of a falling object matching the curve of the planet (falling down and always missing the ground), and that, say, the moon will eventually collide with us, given enough billions of years, since the curve isn't precise. (But the sun is likely to turn into a red giant and fry us all long before that happens.)
However, this NASA site seems to differ. I dunno. Maybe it is just about perpetual.
Anyway, I can't think of a way to use an orbit for power in any way other than tidal. Which gives some power, but not much. I think the problem is scale -- the mechanism needed to harness an orbital effect would have to be big enough that its own gravity would throw things off.
I mean, if you shot a magnetic cannonball into orbit around a copper planet, it might generate a charge, but I have a very strong feeling that the charge would nudge them together or apart as soon as it got strong enough to be useful. I don't know.
Wikipedia has more to offer. Here, from the "Orbital Decay" section:
Some satellites with long conductive tethers can also decay because of electromagnetic drag from the Earth's magnetic field. Basically, the wire cuts the magnetic field, and acts as a generator. The wire moves electrons from the near vacuum on one end to the near-vacuum on the other end. The orbital energy is converted to heat in the wire.
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Enamon: We have all been told that if a spaceship is flying away from Earth at a velocity close to the speed of light then time for the occupants of the spaceship slows down in comparison to time for the occupants of the Earth. However, isn't this a faulty conclusion brought about by the structuring of the sentence? After all, while one can say that it is the spaceship that is moving away from the Earth at close to the speed of light, it can also be said that it is the Earth that is moving away from the spaceship at close to the speed of light. Both statements are true as they mean the same thing.
Well, no. It’s not just the Earth. The planet doesn’t exist in a vacuum. It’s everything else. The solar system. The galaxy. All that. Travel takes place between points.
Enamon: Thus while both masses are moving away from us at half the speed of light they are moving away from each other at the speed of light.
Yes. I think the problem is in motion relative to everything else as well, if you want to do anything useful with it.
Enamon: And finally, what is the velocity at which electrons orbit the nucleus?
Part of me wants to say "light speed" and part of me wants to say "that depends."
Ah. Electricity, the charge, travels really, really fast, (that is, nearly the speed of light, but the actual electrons go slower. How much? It depends: The electron in an H atom has a speed according to those calculations of about 2e6 m/sec. Calculations for the heavier elements are doubtful but for a 1 electron atom of Z=100 the speed could be about 2e8 or 2/3 the speed of light....
If you want to get really precise, the electrons don't orbit at all. But can be described as orbiting.
Figure that one out on your own. |
