Gravity - use it as an energy source?

  1. Gravity -- use it as an energy source?

    If gravity is a force, why can't we use it as an energy source?
  2. jcsd
  3. SteamKing

    SteamKing 8,585
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    We do use gravity as an energy source. Every hydroelectric dam relies on gravity to turn the generators which produce electricity. Waterwheels which drive gristmills also use gravity as their power source.
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  4. ah good point, you are absolutely right now that I think about it. Thanks.
  5. Energy from gravity requires dropping something deeper into a gravity well. In the case of a hydroelectric dam, that something is water which fell as rain, and is now moving toward the center of the planet as best it can.

    Note that gravity is not really the energy source for hydroelectric dams, however. Ultimately, nuclear fusion is, as evaporation from the light of the Sun is what gets the water into the air in the first place.(though one could make the argument that gravity is the source of fusion, but I digress)
  6. So that being said, that gravity is not really the energy source, is there any conceptual possiblity where gravity could be an energy source in a more direct form?
  7. tidal heating, not so much for Earth, but I think Jupiter's Lo moon is heated by the tidal forces from Jupiter.
  8. You only get energy out from gravity by draining gravitational potential energy and getting energy in some other form in return. You end up having used up the potential energy. If you want to get it back, that will require at least as much energy as you harvested in the first place. In this sense, it is like a rechargeable battery or a wind-up toy.

    In classical mechanics, making an object smaller and smaller yields a theoretically infinite amount of energy by reducing the gravitational potential energy of the object with respect to itself toward negative infinity. With general relativity, this possibility is avoided because it would involve creating a black hole.
  9. I see, so gravity is only potential energy because some object is higher than it has potential to fall. Once it falls it looses/drains that potential energy and must be moved back up again in order to create more potential energy and it takes work/energy to move it back up again. So there really is no theroretically possible way just sort of plug into the pull of gravity as a form of energy in perpetuity.
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  10. If you create(or find) a microscopic black hole, you can use it for total conversion of matter into energy. Black holes decay via hawking radiation, and they decay faster the smaller they are. So you feed the black hole at a constant rate to keep it from decaying to nothing, and it radiates hawking radiation. If the black hole is charged, you can even move it around.

    You can also extract energy from the spin of a black hole via the Penrose process.

    But I'm not sure either of those technically qualify for what you're asking.
  11. Hm, that is at least at some level it may be possible. I wonder if there is any other more practicle application though? Like a way of generating power here on earth that we haven't thought of's sounding like the answer to that is probably no.
  12. You'll still eventually run out of matter or spin with the two methods I mentioned. They won't let you generate energy in perpetuity.

    Nothing will, as entropy of a closed system always increases.
  13. So essentially, in the literal sense, there is no such thing as renewable energy.
  14. Correct.
  15. If gravity is not the energy source in an hydroelectric dam, neither fusion is. Somehow you have to drop something into a well whether it's a gravity well or another field. So far the only energy source we know might be the big bang, with the creation of time itself.
    About hawking radiation, I am not sure you can say you convert everything in energy, as the evaporation creates particles, and they have a rest mass.
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