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Homework Help: Ampere's Law of wire

  1. Apr 19, 2007 #1
    1. The problem statement, all variables and given/known data
    Calculate the magnitude of the magnetic field at a point midway between two long, parallel wires thar are 1.0 m apart and have currents of 10.0 A and 20.0 A, respectively, if the currents are:
    a) In opposite directions and
    b) in the same direction

    2. Relevant equations
    B=kI/(2 pi X r)
    B is the magnetic field
    k is the proportionality constant... 4pi X 10^-7
    and r is the radius

    3. The attempt at a solution
    When it said that the currents move in opposite directions, i thought that the magnetic field at a point is B(net)= B(2) - B(1), and i got 4.0 X 10^-6 T. When it said that the currents move in the same direction, i did B(net)= B(2) + B(1), and i ended up getting 1.2 X 10^-5 T. But the answer at the back of the book is the opposite, the answer that i got for a turns out to be the answer for b and vice versa... are my equations wrong?
  2. jcsd
  3. Apr 19, 2007 #2
    in fact , you are wrong in directions. try use you right hand to sure when to add or minus
  4. Apr 19, 2007 #3

    so you calculation must be all right , conside more about it:

  5. Apr 19, 2007 #4
    you must distinguish between the diretions of currents and magnetic field .

    remember the magnetic field is vector field,its directions has complicated relation with the currents.

    does that help, my pretty baby?
  6. Apr 19, 2007 #5
    thank you very much enricfemi! :D
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