Assuming that it were possible to do so, if a tunnel from one side of a planet to the other were lined with copper wire, could dropping magnets down the hole be used to generate power?

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    $\begingroup$ Moving the magnets to the original position (to keep the generator working) would take more energy than what is provided by the generator. $\endgroup$ – SJuan76 Aug 10 '18 at 14:04
  • $\begingroup$ With tunnels of only a small fraction of that depth, you could exploit geothermal energy. Not forever but for a very long time, $\endgroup$ – badjohn Aug 10 '18 at 14:55

Not forever. A magnet moving through the coil would generate a current; the current would induce a magnetic field that in turn would slow down the magnet. It's the principle of what is known as 'magnetic braking'. Even if you dug these holes, somehow shielded them from the insane heat and pressure of the nucleus, and vacuum-pumped them so that there's no air resistance, the magnet would fall, lose speed, and arrive at the other end with less energy, so not quite reach the antipodes. It would then keep oscillating, while losing energy, and finally stop in the middle. And all you would have done is harvest the magnet's initial gravitational potential energy.

  • $\begingroup$ The whole point of the exercise was to harvest useful energy. If the system was well designed for that purpose, then the magnet would not "arrive at the other end with less energy:" In fact, it would never make it past the center. $\endgroup$ – Solomon Slow Aug 10 '18 at 16:59
  • $\begingroup$ Maybe it's more efficient to harvest it slowly? It could produce less losses. Either way, I just meant to describe the general behaviour, of course if the damping is high enough it'll just stop the first time. $\endgroup$ – Okarin Aug 10 '18 at 17:02

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