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Homework Help: Work done by rectangular wire loop with current in magnetic field

  1. Oct 25, 2011 #1
    1. The problem statement, all variables and given/known data

    A rectangular wire loop of height h and width w, centered on the origin, carries current I in the direction shown in the figure. The angle between the positive x-axis and the plane of the loop is θ, defined as shown in the figure below. (When θ = 0 the loop lies in the x-z plane.) This entire region of space is filled with a uniform external magnetic field B pointing in the +x direction.



    Part A
    The magnitude of the work, |W|, involved in the rotation of the loop from θ = 0° to θ = 60° is:
    (a) |W| = 9.09 × 10-5 J Correct Answer
    (b) |W| = 2.37 × 10-4 J
    (c) |W| = 4.25 × 10-4 J
    (d) |W| = 5.87 × 10-4 J
    (e) |W| = 8.13 × 10-4 J

    Part B
    When the loop rotates from θ = 0° to θ = 60°, the external magnetic field
    (a) does no work on the loop.
    (b) does positive work on the loop. Correct Answer
    (c) does negative work on the loop.

    2. Relevant equations

    U= - dipole moment B cos(theta)

    3. The attempt at a solution

    For part A,
    I tried

    -( I * Area * B * cos(60)) - (-( I * Area * B * cos(0))) , however, it is wrong answer.

    For part B,
    I am confused about the work done by field (W) and the energy (U), what is difference?
    How do I know a work is positive or negative in this case?

  2. jcsd
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