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Magnetic field homework

  1. Feb 27, 2006 #1
    In this diagram, there is a cross section of a wire that is carrying a positive current out of the plane towards the viewer. Ignoring the earth's magnetic field, show the direction in which a compass would point if placed at location A. Then show the direction in which the compass would point at two other random points.

    OK so for this question I believe I need to use the right hand rule, which states that you point the fingers of your right hand in the direction of the velocity and curl the fingers in the direction of the magnetic field moving through the smallest angle. Your thumb will now point the direction of the magnetic Force exerted on a positive charge.

    using this method I find that the direction in which the compass would point would be to the right. am i on the right path? please help. thanks!
  2. jcsd
  3. Feb 27, 2006 #2


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    Staff: Mentor

    A current in a wire induces circular (equipotential) 'lines' of magnetic field. One has to determine, for a current in or out of the page whether the lines are in the clockwise or counter-clockwise direction, and then which way a magnet/compass (N-S) aligns with the magnetic field lines.
  4. Feb 28, 2006 #3
    ok. that reasoning makes sense but how do i determine whether its moving in the clockwise or counterclockwise direction?
  5. Feb 28, 2006 #4
    The famed "right hand rule."

    Hold your hand flat, with your thumb out as far as it will go.
    Point your thumb in the direction of the current.
    Curl your fingers toward your palm.

    The magnetic field will be going in the direction your fingers are moving.

    The right hand rule works for more than just a current-carrying wire. Check out this page:
  6. Feb 28, 2006 #5
  7. Feb 28, 2006 #6
    wow it's so much easier to visualize now with the right hand rule. thanks. so using the right hand rule, the direction of the magnetic field is perpendicular to the wire and is moving left in the counterclockwise position. I also now that the direction of a magnetic field at any location is the direction in which the north pole of a compass needle points at that location. Thanks for the help!
    Last edited: Feb 28, 2006
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