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Why don't magnets violate conservation of energy?

  1. Feb 19, 2012 #1
    One thing that always puzzled me was that to me, magnets seem to violate the law of conservation of energy. They obviously do not, and it is purely my ignorance that is allowing me to think this, but I need some explanation as to why they don't. What I mean by this is the following..

    Say you have two magnets a distance "d" apart. If you slowly inch one magnet closer to the other by amount dx, there is a point where the magnet will pull the other magnet in. Therefore, would it be reasonable to say the magnet does work on the other magnet? Where did the energy come from to pull the other magnet in? It surely did not come from you, since you only gave the first magnet enough work to move it a small distance dx closer to the magnet.

    I have an explanation I patched together and I would like to know how accurate it is, if at all.

    Would it be perhaps that when these two magnets were first magnetized, an amount of energy went into them to align the dipole moments in the atoms. This energy was stored in the magnetic field of the magnet and it is THIS energy that it uses to pull the other magnet in. By this logic, then the magnet would eventually lose its magnetism over time, which is true correct?

    How much of this, if any is correct? If its not, can anyone provide an explanation?

  2. jcsd
  3. Feb 19, 2012 #2
    "If you slowly inch one magnet closer to the other by amount dx, there is a point where the magnet will pull the other magnet in."

    To causes such motion to get it past the forces holding them apart, takes energy.

    Now they have pulled together. How much energy will it take to put it all back in the original position??

    When all is taken into account, ( energy but in by Mr hand, or some other source: it is not a violation of conservation is it???

    It would actually be a cost in energy.

    The problem is the ideas that people promote about magnets is such; they do not account for all energy costs: and therefore believe magnets are some violation, when they of course are not.
  4. Feb 19, 2012 #3
    My confusion about that is, if I am only putting in enough energy to move the magnet a distance of dx (Meaning, all my expended energy is going into movement of the first magnet only) then where would the energy come from to pull the other magnet in?

    Maybe I am just not understanding something.. :\
  5. Feb 19, 2012 #4

    you are only looking at one side of the cycle.

    If you are standing ton the edge of a diving board, the only energy you need to fall a certain distance is to move yourself off the board. Gravity does the rest, yet to put yourself back to that position will cost you energy.

    It is easy to see only part of the situation and think it is a violation of COE. When the whole situation is reviewed COE holds.

    With the magnet it is easy to think a little input will get it to move therefore a violation of COE, yet when it is fully looped back to the start position it is a loss in energy and not a violation of COE.
    Think about this: How did the magnets get to the position of being apart in the first place??
  6. Feb 19, 2012 #5


    Staff: Mentor

    Yes, one magnet does work on the other. A magnetic field has a certain energy density, opposite magnetic fields attract and cancel each other, this reduces the energy and does mechanical work. Separating the magnets requires work and restores the energy of the separated fields.
  7. Feb 19, 2012 #6
    Ohh I see! Okay I understand now!
  8. Mar 10, 2012 #7
    Let me know what you think about these videos


    Last edited: Mar 10, 2012
  9. Mar 10, 2012 #8
  10. Mar 10, 2012 #9


    User Avatar
    Homework Helper

    The magnetic field has a potential energy related to the field strength, position of an object in the field, and the force that the field applies to such an object. If the magnets were in space free of any drag, then as they approach or repel, potential energy is converted into kinetic energy (until there's a collision).
  11. Mar 10, 2012 #10
    This is correct. Aligning dipole moments takes energy ( just like aligning two magnet bar). Permanent magnets have strong anisotropy, i.e when the dipoles are aligned ( by an external field) in their preferred direction, they remains in that direction even after removing the external field. So the energy is stored in the magnet.

    Why would a magnet lose its energy? If you mean by doing work, when you again separate the absorbed material from the magnet, you do work on the magnet again and increase its magneto static energy. In the video posted by above, anytime the boy puts the bullet in, he has to do work as the magnets repel the bullet.
  12. Mar 10, 2012 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I'll offer the following gravity analog. Suppose an object is sitting at the very edge of a 100 meter high cliff. Then you push it a small distance dx, and it falls over the edge. Where does the energy come from, that it can reach the bottom of the cliff with a kinetic energy of roughly 1000 J/kg?

    The answer is that the energy was there all along, in the form of gravitational potential energy.
  13. Mar 10, 2012 #12
    Ok but your analogy is incomplete. For example let's talk about the "wheel invention". If you try to move a wheel in the horizontal direction, you will find is not very difficult to accomplish such thing because you are only dealing with friction forces but avoiding putting energy in vertical direction then not changing gravitational potential energy of the object. If you elevate a wheel to a high cliff somehow avoiding friction forces, you need at most a certain amount of work because gravity. So, when you add that up to work done by wheel when it falls then you should have zero.
    But in videos posted above, the people didn't need to overcome gravity field nor (at least as I see) experienced a force in their hands before putting magnet gun in the rail. So think about it.
    Last edited: Mar 10, 2012
  14. Mar 10, 2012 #13


    Staff: Mentor

    There is no evdience that the force on their hands is negligible. Absence of evidence of force is not evidence of absence of force.
  15. Mar 10, 2012 #14
    Ok I agree, these things not proper physics experiment yet. So I should have no more expectation for now.
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