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Homework Help: Magnetic forces on two current carrying wires

  1. Jul 14, 2006 #1
    "Find the force per unit length on each of two long, straight, parallel wires that are 24 cm apart when one carries a current of 2.0 A and the other a current of 4.0 A in the same direction."

    Tell me if I'm doing this right:
    I use d = 0.24m, and the equations B = (mag. perm. * I)/(2*pi*d), and F = ILB.

    The force per unit length on each wire is then: F/L = IB.

    Plugging in I and B for each of the wires, I get:
    F/L (for the 2.0A wire) = 3.3 * 10^-6 N/m (toward the other wire)
    F/L (for the 4.0A wire) = 1.3 * 10^-5 N/m (toward the other wire)

    I think it's wrong because the answer is supposedly: "6.7 * 10^-6 N/m; attractive". But I would think that the forces are different on each wire, because the wires are carrying different currents...
  2. jcsd
  3. Jul 14, 2006 #2
    Your formulae are correct, however I can't see how you obtained your two different answers.
    Remember, the expression you wrote explicitly is
    [tex]\frac{\mu_0I_1I_2}{2\pi d}[/tex]

    Can you possibly have two different answers ? :)
  4. Jul 14, 2006 #3
    oh ooops.
    I did (F/L)1 = I1B1, not I1B2!
  5. Jul 14, 2006 #4
    Thought so :biggrin:
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