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Stable Magnet Energy What's their source?

  1. Sep 30, 2009 #1
    I've been trying to find the answer to a question i have online but haven't had much luck.

    Where did Stable or Pertinent magnets get their energy from?

    I've read plenty of people saying "magnets don't acquire or release energy" but that doesn't make sense to me. Let's use this scenario to explain why:

    Take 2 magnets.
    Glue them to individual corks and float them in water.
    Aim their North Poles at each other and give them a push toward each other.

    One of two things happen:
    1 - the corks get so close and push each other apart - thus receiving energy from the magnets to change the direction of the cork and make it float in a different direction.

    2 - the corks get so close and get caught in each others magnetic field causing one of the corks to spin around so that the polls will line up.

    An object in motion tends to stay in motion and an object at rest tends to stay at rest. This is true until an outside force changes the object's rest, direction, speed, etc...
    Simple newton physics.

    So... if those corks with magnets do not continue on course until they simply bump off each other or the water friction slows them down, then an outside force (energy) is acting on them.

    The mere fact that the corks will suddenly change course to repel each other or spin on their axis suddenly is proof that a force (energy) is acting upon them.

    if these (stable? permanent?) magnets hold energy then where did the energy come from? The atoms that compose them?

    if all the energy is released (for example - as one magnet pushes another away it is exerting energy. Therefore it is releasing energy to perform the task and holding less within the magnet itself) If all of the energy is released, and the energy comes from the atoms, then what happens to the atoms when all magnetic energy is expelled?
     
  2. jcsd
  3. Sep 30, 2009 #2
    Magnetic fields do no work on charged particles. However, they do wind up doing work on magnetic dipoles. Any introductory physics text on the subject should have the following equations:

    Energy density in a magnetic field: [tex] u_B = \frac{B^2}{2\mu_0}[/tex]
    Torque exerted on a magnetic dipole: [tex] \vec{\tau} = \vec{\mu} \times \vec{B}[/tex]
    Potential energy of a dipole in a field: [tex] U = -\vec{\mu} \cdot \vec{B}[/tex]

    The fact that you can write down a potential involving the magnetic field should be enough to convince that the magnetic field does work in this situation. Of course, you cannot write down a potential for a charged particle, so the magnetic field does no work in that case.

    For a magnetic material, the magnetic state is the ground state of the material. When you take two permanent magnets and put their north poles together, the field from one magnet is trying to disrupt the internal structure of the other magnet by flipping it around, and when you let them go, they will repel or maybe rotate. The source of this energy is you pushing the magnets together. You have to exert work in order to put the magnets in that state, and then there is energy stored in the magnetic field which is released when the magnets push away from each other. (This is not a source of infinite energy, btw. In order to pull two magnets apart that are stuck together, you have to do work against the magnetic field.)
     
  4. Sep 30, 2009 #3

    Dale

    Staff: Mentor

    WaxyChicken, in particular notice the first equation that kanato posted. It says that the magnetic field itself stores energy and that the energy density at any point in space is proportional to the square of the magnetic field there. So there is energy in the magnetic field.

    Now, when you take two magnets and arrange them so that their fields partially cancel each other out (North to South) then the energy in the field is decreased resulting in a net force can be used to do work. On the other hand, when you arrange them so that their fields add constructively (North to North) then the energy in the field is increased and it requires work to be done on the magnets.
     
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