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Homework Help: Current in a wire and compass needle

  1. Mar 16, 2010 #1
    1. The problem statement, all variables and given/known data
    A compass needle points 19.1 degrees E of N outdoors. However, when it is placed 9.80 cm to the east of a vertical wire inside a building, it points 55.7 degrees E of N. What is the magnitude of the current in the wire? The Earth's field there is 4.80×10-5 T and is horizontal.

    2. Relevant equations
    B = uI/(2*pi*r)

    3. The attempt at a solution
    Can someone point me in the right direction for this problem because I have no idea how to start. I'm pretty sure I have to use the Biot-Savart law, but I just don't know how to incorporate the angles given into the equation.
  2. jcsd
  3. Mar 16, 2010 #2
    Try drawing a vector diagram of known fields (be sure to account for both the wire and the earth in your diagram with correct angles in between).
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