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Homework Help: Magnetic force on a charge carrying wire

  1. Feb 18, 2008 #1
    [SOLVED] Magnetic force on a charge carrying wire

    1. The problem statement, all variables and given/known data

    A current carrying wire is in a magnetic field as shown below. The magnet is on a scale. Given this setup, does the scale read more or less weight? Explain using the Right Hand Rule and Newton's Third Law. Note: The apparatus depicted is a magnet sitting on the mass pan of a balance.

    2. Relevant equations

    The RH rule is a bit long to explain here.. basically, the current is flowing W, and the magnetic force is N to S.

    Newton's 3rd: For every action, there is an equal and opposite reaction.

    3. The attempt at a solution

    The wire is carrying current, but the current is not given. Current must be positive, so therefore it has a charge of >0. Positive current exerts a force upward based on the drawing, so the magnet will weigh less. I don't know how to explain this in terms of the RH rule, though. My best guess:

    With my thumb pointing in the direction of the current and fingers wrapped around the wire, the magnetic force flows N to S. My open palm would face left? right? up? down?, which corresponds to the force the magnet will feel. Because of Newton's Third Law, the weight will _____.

    Attached Files:

  2. jcsd
  3. Feb 18, 2008 #2

    Doc Al

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    Staff: Mentor

    Last edited by a moderator: Apr 23, 2017
  4. Feb 18, 2008 #3
    So if the magnetic force on the wire is upward, then the magnet itself will push downward, resulting in a weight gain.


    Thanks for the links, also. Much more informative than my prof's lecture notes.
  5. Feb 18, 2008 #4

    Doc Al

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    Staff: Mentor

    Right. If the magnet pushes the wire up, the wire pushes the magnet down--increasing its apparent weight.
  6. Feb 18, 2008 #5
    Awesome. Thanks for the help!
  7. Oct 6, 2009 #6
    Re: [SOLVED] Magnetic force on a charge carrying wire

    Hmm... What if a wire is attached to the N surface of a very broad magnet? Now if the current is switched on, and the whole setup is free to rotate on a wheel, what will happen? Will the BIL force continue to rotate the wire with the magnet for as long as the current flows?
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