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Homework Help: Magnetic field and wire current

  1. Apr 27, 2010 #1
    1. The problem statement, all variables and given/known data
    A long, straight wire with linear mass density of 45g/m is suspended by threads, as shown in the figure . There is a uniform magnetic field pointing vertically downward. A 7.0A current in the wire experiences a horizontal magnetic force that deflects it to an equilibrium angle of 10 degree. What is the strength of the magnetic field B?

    2. Relevant equations

    B=(U_o I) / (2 *3.14 * r)

    3. The attempt at a solution

    I have U_0 given, I given. but can't find r.

    How can find r to get B?

    Attached Files:

  2. jcsd
  3. Apr 27, 2010 #2


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    The equation that you have quoted gives the magnetic field produced by a very long wire. It is not relevant here. What equation do you know that gives the force on a current-carrying wire segment in a magnetic field?
  4. May 2, 2010 #3
    F=ILB maybe?
  5. May 2, 2010 #4
    That's the right equation.
    The next step is to draw a free body diagram and balance the forces.
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