I have an interesting idea for an energy generator and I want to see what you think.

Basically, you take a small planet and drill from one side to the other so it looks like a sphere intersected with a tube going straight through it.

You then place coils along the inside of the tube which are then connected to batteries.

After that, you drop magnets down the shaft and as they fall through the coils they create electricity.

The magnet would eventually settle in the center of the planet, but before it does so it would travel (hopefully) many times from one end of the planet to the other, in a spring-like motion

So what do you think? How feasible is it?

I should have phrased it better. I didn't mean how feasible it would be to drill the hole, or to keep the planet from collapsing in under such high gravity. I just wanted to know if you could get any decent amount of energy from the dropping itself.

  • $\begingroup$ In theory your proposal would work so long as you have no resistance in your circuit. But without resistance you are producing no active power. If you have resistance the oscillation will eventually die out because the coil exert a velocity dependent force on the magnet. $\endgroup$
    – Azad
    May 15, 2015 at 18:38

2 Answers 2


You appear to be using energy to move all the "stuff" out of a hole, and then filling in the hole again with "special magnetic stuff" to extract the same energy as electricity, with inevitable losses...

Perhaps you could practice by pumping water from Lake Ontario up into Lake Erie, and then selling the electricity you get when the water flows through the Robert Moses/Adam Beck Hydro Electric plants. See how that works out...

  • $\begingroup$ That said, if there is a way to ablate out a borehole or something, this could be a way to convert chemical energy into electrical energy. Obviously, that's not super practical, either. $\endgroup$ May 15, 2015 at 18:59

It is absolutely ridiculous.

The deepest drilling on earth, the Kola Superdeep Borehole, is $12.262\,\mathrm{km}$ deep, the earths diameter is about $13000\,\mathrm{km}$, that is the deepest hole on earth is one thousandth of earth's diameter! Furthermore, the inner of the earth is hot, your coils would melt, your magnets would not be magnetic (as they have a Curie temperature not above $1000\,\mathrm{K}$). And drilling the hole would require enormous amounts of energy (as you have to move the material out of the shaft).

I would go with drilling a kilometre or so and then build a geothermal power plant.


The maximal energy you could get out of it per magnet, is the potential energy of the magnet with respect to centre of the earth (assuming ideal conversion, conductance of the wire, ...).

This is easily calculated (assuming a one kilogram magnet): $$ E = \frac{\gamma m_M m_E}{r_E} \approx 63 MJ. $$ (Yes, this formula is correct for the potential at the surface of a sphere). This is on the same range as the energy obtained by burning one kilogram of gasoline ($H_i \approx 40\,\text{MJ}$).

I repeat: I would go with drilling a kilometre or so and then build a geothermal power plant.

  • $\begingroup$ The "Addendum" was pretty much what I was asking for. Thanks! $\endgroup$ May 15, 2015 at 19:27

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