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Homework Help: Resultant Magnetic Force

  1. Nov 11, 2009 #1
    Figure below shows a length of wire with a central arc, placed in a uniform magnetic field B that points out of the plane of the figure. If the wire carries a current i, what resultant magnetic force F acts on it? Express the answer in terms of B, L, R and i.

    I am stuck in this question for more than 2 hours and have no idea on how to approach the question.

    Please help. Thank you.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Nov 11, 2009 #2


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    Staff: Mentor

    You approach it by adding up the individual forces. The straight wire parts are easy, hopefully. The slightly harder part is integrating the force over the arc. What can you say about the horizontal components of force as you do the integration?

    Please show us the integral you have set up for the arc...
  4. Nov 11, 2009 #3
    i think the horizontal resultant force around the arc should cancel out, however, i m still stuck in setting up the equation for the vertical resultant force around the arc.
  5. Nov 11, 2009 #4


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    Staff: Mentor

    Correct about the horizontal cancellation. What integral equation do you use to calculate the force on a current in a magnetic field? Show us how you are starting to set up the integral.
  6. Nov 11, 2009 #5
    F = BiL
    I used the cosine Rule to express dL-> dL = (2Rsqu. - 2Rcos dθ)squ root
    therefore dF at particular point = Bi(dL)/sinθ

    But i think im in a totally wrong direction.

    I think i should set up a function so that the total area under the curve equals to the vertical force acting downwards. But i really have no idea where to start.
  7. Nov 12, 2009 #6
    can anyone please help?
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