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Homework Help: Magnetic Force Between Wires Problem #2

  1. May 20, 2009 #1
    I'm sorry to post two questions in succession, this is another one on the same topic (I'm obviously having trouble with this whole concept for some reason...)

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

    Three long parallel wires are 3.8 cm from one another. (Looking along them, they are at three corners of an equilateral triangle.) The current in each wire is 8.00 A, but its direction in wire A (at the top of the triangle) is opposite to that in wires B and C (the bottom points of the triangle). A comes out of the page, while B and C go into the page. Determine the magnetic force per unit length on each wire due to the other two, and the angle of each force.

    2. Relevant equations

    F/l = [u (2I)]/[2pi (d)]

    3. The attempt at a solution

    I can plug in to the above equation to find 2.96e-5 N/m. Is this the force for all of the three points? For some reason, the solutions I have indicate that wire A has a different force (5.13e-5 N/m) and I'm not sure why. I am also very unsure how to determine the angles at which these forces point. Is this a vectors problem? I feel like I am missing some pieces of information and I am not sure how to put together what I know to solve this. I'm sorry that this attempt at a solution is so vague...
    Again, I would really appreciate some insight!!
  2. jcsd
  3. May 21, 2009 #2


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    Science Advisor
    Gold Member

    Yes. The force will always have an associated direction. You need to find the direction and magnitude of each force and add them together as vectors to find the total force and its direction. If I have two guys that can pull on a rope each with a force of 5 N, it matters what direction I set the two men to pull a crate. If I put them on the same side of the crate they can pull together with a net force of 10 N. If I put them on opposite sides of the crate they will pull against eachother for a net force of 0 N.
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