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Homework Help: Direction of magentic field with two wires?

  1. Aug 11, 2010 #1
    direction of magentic field with two wires??

    1. The problem statement, all variables and given/known data
    Two long, straight wires cross each other at right angles. Find the direction and magnitude of the magnetic field at point P, which is in the same plane as the two wires

    2. Relevant equations
    Bnet = B1 - B2
    B= U0I/2pir

    3. The attempt at a solution
    I've figured out how to get the magnitude, my question is about the direction. Which way would the magnetic field point? I have two rods, rod1 is in the positive Y-Direction and carries a current of 6.00A and rod2 carries a current in the positive X-Direction.

    7A rod has a magnetic field that is coming out of the page, and 6A rod has a magnetic field going into the page. Point P is at (4.00, 3.00)m

    Would the magnetic field at Point P be coming out of the page because the rod with the larger current triumphs?
  2. jcsd
  3. Aug 11, 2010 #2
    Re: direction of magentic field with two wires??

    Bnet could also be B1 + B2 depending on the quadrant you're in. Is there any more problem info? Given the B1-B2 case, the direction of the field will be based on which term is larger, simple as that. The larger term could be larger either due to distance or current.
  4. Aug 12, 2010 #3
    Re: direction of magentic field with two wires??

    It would be in quadrant 1. So if I come across this problem on a test then I can assume that whichever B is larger then that's the direction the magnetic filed will take at a point?
  5. Aug 12, 2010 #4
    Re: direction of magentic field with two wires??

    Even in quadrant 1, the field will depend on the direction of each of the currents.

    Yes the field with the larger magnitude at point P will give you the direction of the magnetic field. But what you can't do, as you've done above, is assume that the larger current makes the larger magnetic field. Thats only true on the line y=x, where the point is equidistant from each wire. Each B is proportional to I and inversely proportional to r.
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