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Don't know where to begin mag field question

  1. Mar 22, 2009 #1
    Don't know where to begin....mag field question

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

    http://s685.photobucket.com/albums/vv212/dewdr0p714/?action=view&current=physicsexam2q2.jpg

    Each support string connects to a corner of the rectangular ceiling. The current carrying wires are .5m apart. The current carrying wires have a mass per unit length of .150 kg/m. The currents are identical and in opposite directions.
    Point A is 1m away from the wire on the right
    Point B is exactly between the 2 wires
    Point C is 1.5 m away from the wire on the left

    (all the points are in the same plane as the 2 wires)

    b) Determine the magnetic field B at Point A, Point B, and Point C

    2. Relevant equations

    i think this formula is supposed to be used but I'm not sure...B = F / |q0|(vsinθ)



    3. The attempt at a solution

    I just dont know where to begin with this problem...can anyone give me any pointers?
     
  2. jcsd
  3. Mar 23, 2009 #2

    lanedance

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    Re: Don't know where to begin....mag field question

    hi dewdrop, do you know amperes law?

    the equation you gave is for force on a moving charge in a magnetic field
     
  4. Mar 23, 2009 #3
    Re: Don't know where to begin....mag field question

    i tried to solve the problem....can someone please check my work? i think i may have made a mistake somewhere along the way
    I = 300 is given for the question

    B for Point A = [μ0I / 2pi]*[(1/x) - (1/r+x)] = (4pi *10^-7)(300) / (2)(pi) * (1/1m) - (1/1.5m) = 1.97*10^-4 T

    B for Point B = 2μ0I / (pi)(r) = 2*(4pi *10^-7)(300) / (.25)(pi) = 9.6 *10^-4 T

    ***i guess my biggest question is what am i supposed to use for the radius value?
     
  5. Mar 23, 2009 #4

    lanedance

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    Re: Don't know where to begin....mag field question

    r will be the distance from the point to the current carrying wire

    points
    - be careful with direction, at each point is the magnetic field from each wire up, or down?
    - define a positive direction before attempting the problem
    - eg. B is +ve in the up direction
    - also writing B = 2μ0I / (pi.r) makes more sense as we know what is divided by

    that said, your first calc looks ok (field opposite directions)
    2nd 2 contributions at radius of 0.25 - also looks good
     
    Last edited: Mar 24, 2009
  6. Mar 24, 2009 #5
    Re: Don't know where to begin....mag field question

    thanks for your help! =]
     
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