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

Xtracting energy out of a magnet

  1. Feb 11, 2005 #1
    In a dutch forum i'm in discussion with a crackpot about extracting energy out of a magnet. He claims that when a iron object enter the range of a permanent magnet it need energie to become ferromagnetic. Is this really so and if it is where comes the energy from?
  2. jcsd
  3. Feb 11, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    A thing becomes ferromagnetic because that is the lowest energy state under the given conditions. If the paramagnetic state had a lower energy than the ferromagnetic state, then your object would not turn into a ferromagnet spontaneously.

    Your "friend" is wrong.
  4. Feb 11, 2005 #3
    So if i understand it correct: When a ferromagnetic object comes in a magnetic field, the lowest energy state is the stat where all magnetic spins point out the same direction. So you don't need energy, in fact the object gives energie away. So where do tehe energy go to?

    Do you know crackpot's who aren't :smile:
  5. Feb 11, 2005 #4

    Ferromagnetism is the phenomenon where one atom tells its neigbor to align its spin along the same direction. This is an interaction that becomes dominant under a certain transition temperature. So a specimen will be (like gokul said) in its ferromagnetic state (if it has that specific property) rather then in the paramagnetic state. When you rise temperature, this spin spin-interaction will be disturbed and chaos will rise among the spins. Eventually with high enough temperature, all spins will be directed randomly : ie the paramagnetic state.

  6. Feb 11, 2005 #5
    Good question. Ever heard of the Einstein de Haas effect ?

    If you have a ferromagnet and you apply an external B field to align the spins, they will add up to produce a change in angular momentum (you know : J = L+S where S is the spin and L is the angular momentum, L is the TOTAL angular momentum), manifested by the rotation of the magnet. S changes due to the transition from paramagnetic to ferromagnetic (all spins align). But J must be constant and therefore L must change and this means that the specimen will rotate. You can actually observe this rotation.

    The experiment is conducted by haning some ferromagnetic specimen (that is initially in its paramagnetic state, if you will) on a small string and then applying the external magnetic field B.

  7. Feb 11, 2005 #6
    another thing.. you don't want to think that if you put a ferromagnet in a field in one direction, you get some energy, so that if you take it out and put it in a field in the opposite direction, you get some more energy, rinse and repeat for infinite energy. In order to take the magnet out of the field, you have to add energy to it equal to the amount of energy gained by putting it in the field.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook