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Doubt reagarding magnetic fields

  1. Dec 28, 2009 #1
    Is the statement below always true:

    Work done by a magnetic field is zero.

    If it is. How do magnets attract iron?
  2. jcsd
  3. Dec 28, 2009 #2
    A magnetic field can do work by reducing its stored magnetic energy. When you put a magnetic BB in a magnetic field, you reduce to total stored magnetic energy of the system. The stored magnetic energy is

    W = (1/2)∫B·H dVvolume= (1/2μμ0)∫B2 dVvolume

    Because B is continuous (Div·B=0), and the relative permeability μ of the BB is much greater than 1, no magnetic energy is stored in it (or in its dipole field).

    Now recall that the force in the z direction is Fz= ∂W/∂z

    So the magnetic BB is pulled into regions of higher B.
    Bob S
  4. Dec 28, 2009 #3


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    Yes. Griffiths will go as far as to explicitly state this in his textbook. Generally what happens in situations where it looks like only the magnetic field is available to do the work is that while any work is being done, the system is in a state of dynamics. This causes the magnetic field to be time-varying with reference to the charges that are being worked upon. This means that there is also an electric field from the viewpoint of the charges and it is this electric field that does actual work.
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