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Homework Help: Ampere's Law

  1. Nov 23, 2004 #1
    I'm kinda stuck on this question:

    1. Calculate the magnitude ofthe magnetic f ield at a point midway between two
    long, parallel wires that are 1 .0 m apar t and have currents of1 0.0 A and 20.0 A,
    respectively, ifthe currents are (a) in opposite directions and (b) in the same

    The answer is
    (a) 1.2 x 10^-5 T
    (b) 4.0 x 10^-6 T

    2. Also, this question (i thought it was easy, but i kept on getting 4.0 x 10^-5, but the answer is 1.0 x 10^-5).

    A long, solid, copper rod has a circular cross-section ofdiameter 10.0 cm. The
    rod has a current of5.0 A, uniformly distributed across its cross-section.
    Calculate the magnetic field strength 2.5cm from the centre of the rod.

    P.S. At this point, the only equation i am using is
    B = (mew)oI
    Where (mew)o is the is a proportionality constant,called the permeability of free space.
    Last edited: Nov 23, 2004
  2. jcsd
  3. Nov 24, 2004 #2
    Magnetic field due to an infinte wire is given by
    [tex]\frac{\mu_0i}{2 \pi r}[/tex].
    And the direction is the same as the direction of cross product of a vector along the current and another in the direction of r.

    So, in one case the magnetic fields reinforce each other and in the other, are in the oppsite direction.

    You need to replace the current by [tex]\frac{Ir^2}{R^2}[/tex] since the contribution to the magnetic field is only from the portion of current outside the 2.5 cm cylinder.

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