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Homework Help: Coils mutual force exercise: help

  1. Mar 23, 2008 #1
    1. The problem statement, all variables and given/known data

    In a compact cicrcualr coil "A" (radius R= 25cm),formed by N=150 loops of wire, flows I=10A current.
    There's another compact circular coil "B" over "A"(radius r= 1.5 cm, loops n= 30) coaxial with "A" distant X ,in which i= 4A flows .
    in the same direction of "I" (look at the image).

    Calculate the mutual force between the two coils, and is it repulsive or attractive ?
    Calculate X value when the force is Maximum.

    http://img179.imageshack.us/img179/7660/bobinacn6.jpg [Broken]

    HELP ME I DON'T KNOW HOW TO SOLVE IT.....sorry for the translation English isn't my mother tongue.

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Mar 26, 2008 #2


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    Science Advisor

    The problem is very complicated if done exactly, because you would need B off the axis of one of the coils. Assuming R=1.5 cm is small enough, you can approximate the smaller coil by a magnetic dipole. Then the force on the larger coil is not hard to calculate.
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