Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electromagnetic force question

  1. Jan 3, 2012 #1
    If two magnets, equally but oppositely charged, were placed one on top of the other, such that they repel each other, but the weight and shape would not allow the top magnet to slide off, what energy is generated by the relative weight (gravity acting on the mass) of the top plate overcoming the magnetic rejection force.

    I realize there should be some heat. Provided the plates remained motionless, wouldn't there be an eletromagetic field generated, actually similar to a motor? Provided the mass of the two plates was sufficient, why couldn't the plates generate usable electricity (provided that coils were embedded in the plates such that the field potential could be captured). Would that not be a conversion of gravity to electrical energy?

    Would this be possible?
  2. jcsd
  3. Jan 3, 2012 #2
    No energy is generated. There is no heat. There aren't any motionless motors, really. You cannot convert gravity into electricity.

    No, this is not possible.

    When the magnets are far apart there is no potential energy in the magnetic fields due to the proximity of the magnets. When you lift one up, chemical potential energy leaves your muscles and goes into the gravitational field formed by the earth and the magnet. When you lower the magnets together, some of this energy is released from the gravitational field and is transferred to the magnetic field between the two magnets. The original energy came from the sun and made food. You ate it and it went into your muscles. You moved the magnets around and the solar energy became magnetic field energy. The gravity field was only a temporary holding place for the energy while you were moving the magnets around. There was no energy to be extracted from gravity here.
  4. Jan 3, 2012 #3
    If there was a properly aligned coil between the two magnets then compressing the two magnets together would generate some amount of electricity. But as soon as the compression stopped, so that the magnets remained the same distance apart, then no more electricity would be produced.

    This is actually more or less how a microphone works. The sounds make the coils compress around a magnetic which generate a small current that can be amplified and feed to a speaker. Yet once the coil stops moving, no matter how strong the magnet or compressed the field, there is no more power output. Power output depends on the change in field, not the field strength. A weight resulting from a gravitational field is a constant such that there is no variation resulting in a power output like with sound.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook