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

Just a simple question

  1. Apr 2, 2005 #1
    I have very simple question which I really don't understand:

    If you hang a permanent magnet on the metal bar and leave it there
    it will hang for years there. I simply ask where does the energy
    come from?

    This is my first question on this forum so I would really like if
    someone could answer this question to me.
  2. jcsd
  3. Apr 2, 2005 #2


    User Avatar
    Science Advisor
    Gold Member

    Hi, and welcome.

    If you put a book on a table and leave it there, it will stay for years.

    No work is being done, no energy is required.
  4. Apr 2, 2005 #3
    I was thinking of the following configuration:

    | Iron Bar |
    | Permanent Magnet |


    At first sight, the permanent magnet is not moving. So no work
    is generated.

    From that viewpoint you can say if the chopper is still in the air there
    is no work done. However, in order to float still the propeller must
    rotate and engine is turning propeller. Then work is done by the

    But if we look at the problem more closely we see that there is
    the magnetic field involved. Since there is the magnetic field
    according to 4th Maxwell equation there must be current somewhere.
    Then if the current is flowing, the work must be done somewhere.
    So where does that energy come from? (i.e. what is the fuel)

    P.S. Newbie question. How do I insert equations in the text? :confused:
  5. Apr 2, 2005 #4

    The "current" comes from the motion of electrons orbiting the nuclei of atoms in the magnet. It turns out that when electrons are forced to move in a very small space (like in an atom) they don't lose their energy. They can keep on going forever.

    Think about the iron bar that holds the magnet. Half of its atoms are the top and the other half are the bottom of the bar. So you can think of one bar of metal as being two bars stuck together. In this case it's electric forces holding the two halves together, but you'd be pretty surprised if they came apart!
  6. Apr 2, 2005 #5
    Ok, it's true that the electron orbiting the nuclei produces the magnetic
    field. It's also true that electron spin is producing magnetic field. However,
    by doing so (in permanent magnet case) electrons produce real work. Is
    this then the violation of conservation of energy?

    To me, saying that electron violates the conservation law sounds like
    dogma. Perhaps the conservation laws are dogma?

    You are right, but that's different sort of the problem.
  7. Apr 2, 2005 #6
    I don't see why you think it takes work for a permanent magnet to produce a constant magnetic field. The hanging up in the air thing is irrelevant, and your question reduces to:

    "Where do magnets get their infinite energy?"

    Which shows a misunderstanding of what energy is (the ability to do work).

    Magnetic fields are created by currents, and current is charge/time. In wires, charges cannot move at constant velocity in a straight line (the bump eachother) so currents always dissapate (resistance). But inside of an atom, there is no one to bumb in to, and an object in motion stays in motion without any work needing to be done!
  8. Apr 2, 2005 #7
    Here is why I do think that permanent magnets do work when producing
    constant magnetic field:

    Electrons have both spin and they rotate around the nucleus. Lets concentrate on spin only. This spin represents a tiny current which
    creates magnetic field. In permanent magnetic materials these spins
    are aligned so their orientation is not entirely random so atoms have
    net magnetic field.

    Now this spin generated magnetic field make the electron move sideways
    (Lorentz force). So we have the force and the path thus electron does
    work. That is why I think permanent magnets do work.
  9. Apr 2, 2005 #8

    If that were true, wouldn't the electrons eventually all pile up on one side of the magnet? But they don't; they're bound to individual atoms, which are themselves locked into place in the solid metal. A very high temperature, or a very large external magnetic field can erase the magnetization. But just keeping a stationary piece of iron from falling won't do it.
  10. Apr 2, 2005 #9
    I didn't think that way. What I meant is that the electron spin creates magnetic field. And this magnetic field is acting on all the moving charges in that field (including the electron itself). But if you look at the direction of Lorentz force, it does not make the electron go away from the atom (eventually, it could put it in higher orbit). So these electrons won't pile up, they may just revolve around the nuclei faster.
  11. Apr 2, 2005 #10
    Well....it just doesn't work that way when you're talking about electrons confined to a volume the size of an atom! Their spin and orbital angular momentum take on fixed, discrete values that don't increase or decrease gradually. Either a big enough chunk of energy is absorbed (or emitted) to make the electron jump to a higher (or lower) energy level, which can increase (or decrease) its orbital angular momentum (but interestingly not its spin angular momentum!) or everything stays the way it is. Forever!
  12. Apr 2, 2005 #11
    Okay, imagine the magnetic field of electron A doing work on another electron B as you describe (lets avoid the issue of self interaction).

    Then the energy of electron B goes up by the same amount as electron A's energy goes down. That is:

    [tex] W_{AB} = -W_{BA} [/tex]

    So the net work done by the system is zero, because the electrons are only exchanging energy (not creating it). They are free to do this untill the end of time.
  13. Apr 2, 2005 #12
    so by you saying it does not require work or energy, you mean like a space shuttle can continue forwards through a vacuum forever until a force is met of the correct properties to counteract that movement, as the electron particles are moving through a vacuum, their is no resistance from other particles, unless other particles are travelling on the same shell at different speeds, which as far as i know is not possible, and therefore does not require any form of interaction or conversion of energy
  14. Apr 2, 2005 #13
    Yes you cannot use the macroscopic (or gradual icrease of the speed/energy) model to show that, I agree. But the start of the discussion was whether the magnet hanging from metal bar does work or not. And I think I've illustrated enough to show that work is done since you have the force with component in the direction of the path (F=v x B where v is the speed and B is the magnetic field due to the electron spin) and I don't think you can prove differently. So it may be that something more is going on :confused:
  15. Apr 2, 2005 #14
    Okay. But I was reffering exactly to the issue of self interaction. To clarify
    this; the spin of the electron is exerting the force on itself by the means
    of magnetic field which was created by the spin.

    ... at least that's what theory says o:)
  16. Apr 2, 2005 #15
    What "path"? Electrons in atoms don't move in "paths". They just exist in orbitals which have associated with them discrete values of energy and angular momentum and a blurry indication of where the electron is likely to be located. The notion that work equals force times distance doesn't work on electrons in atoms. The "distance" is too small.
  17. Apr 2, 2005 #16
    Almost any path works so that issue is unimportant. If what you say is
    true than the Lorentz force would be nonexistant at small distances.
  18. Apr 2, 2005 #17


    User Avatar
    Science Advisor
    Gold Member

    Dodig, I'm not quite sure what you're trying to say here.

    Do you understand, or do you genuinely believe that physics as we know it is wrong?
  19. Apr 2, 2005 #18
    No more so than the Coulomb force alone. And in fact using the 1/r Coulomb potential that the 1/r^2 Coulomb force implies (classically) works just fine.
  20. Apr 2, 2005 #19
    I generally think physics is quite right about things. This is just one
    little obstacle that bothers me :bugeye:
  21. Apr 2, 2005 #20


    User Avatar
    Science Advisor
    Gold Member

    Why does it bother you any more than a book resting on a table?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook