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Wonder about F=qvB

  1. Oct 18, 2009 #1


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    In applying F=qvB, must the charge move with velocity V? Or the V is only relative velocity so that even the charge is stationary but the magnetic field is moving the same result (force) can be achieve?
  2. jcsd
  3. Oct 18, 2009 #2


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    Think about the difference between a motor and a generator. How would you describe the forces involved?
  4. Oct 18, 2009 #3
    yes, all velocities are relative. Relative motion produces the force....
  5. Oct 18, 2009 #4
    In any reference frame a charge q has a velocity v. The force due to the magnetic filed is qvxB. This is a part of the Newton equations. The magnetic filed B can be space-time dependent or constant, whatever. But if the charge velocity is equal to zero, no magnetic force is possible: this term equals zero. Only electric qE, elastic kx, etc., may still act. To keep a charge at rest, all forces should cancel in the Newton equations. Otherwise it will move under qE, for example.
    Last edited: Oct 18, 2009
  6. Oct 18, 2009 #5


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    What if we have a reference frame that the charge is at rest, but a magnet is move close to it? Will it experience a force and be moved?
  7. Oct 18, 2009 #6
    Yes, it will. A moving magnet, apart from magnetic field creates an electrical field E determined with the magnet velocity, so the total force will be non-zero.

    The value of electrical filed E can be calculated from the Lorentz transformations for fields. In the non relativistic approximation it will be EVxB where V is the magnet velocity.
    Last edited: Oct 18, 2009
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