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Homework Help: A current-carrying circular loop placed next to a long straight wire

  1. Mar 30, 2012 #1
    1. The problem statement, all variables and given/known data

    A current-carrying circular loop of radius R is placed next to a long straight wire, as shown in the diagram below. The current in the straight wire points to the right and is of magnitude 3 A.

    see diagram: http://www.webassign.net/userimages/ikoskelo@sfsu/loop_straight-wires-B.jpg

    In order to produce a net magnetic field of zero in the center of the circular loop, the current in the loop itself must be:






    n/a current is zero

    2. Relevant equations

    Bwire=μI/2∏R , Bloop=μI/2R

    3. The attempt at a solution

    For the net B to be zero, Bwire must equal Bloop.
    I got this part correct,

    the answer to part a) is .63662A

    I said the direction is clockwise because the magnetic field created by the loop must go in the same direction as the wire (out of the page).

    My online homework says this is wrong. WHY is this wrong?!
  2. jcsd
  3. Mar 30, 2012 #2
    If the current in the straight wire is to the right, then the B field above it is out of the page. This means that the circle must have a B field into the page of equal magnitude to cancel out the B field of the straight wire. This means that the loop should have a clockwise current.
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