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Homework Help: Faraday's Law

  1. Oct 13, 2014 #1
    1. The problem statement, all variables and given/known data

    A rectangular loop of wire, of sides 5 and 3 cm moves away from a a long straight wire at 4ms-1 carrying a current of 2 amps. What is the EMF induced in the moving loop, when the near side of the loop is 3cm away from the wire?

    2. Relevant equations

    B = μ0I/2πr
    EMF = -dΦ/dt

    3. The attempt at a solution

    EMF = -dΦ/dt = -dAB/dt where A is the area, and B is the field.

    I am not sure what to do now as the answers I have seen treat A (the area) as changing?

    As I see it the area of the loop (A) is constant, and B changes as the loop moves out along the radius (the field gets weaker). Can anyone clarify which is right?
  2. jcsd
  3. Oct 14, 2014 #2


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    A stays the same.
    What's unclear to me is which of the two (3 cm or 5 cm) sides is parallel to the current wire ...
  4. Oct 14, 2014 #3

    rude man

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    You can also use the Blv law. That is actually preferred since for moving media emf = -dΦ/dt doesn't always hold.

    The Blv law is emf = (B x l).v
    or more generally
    d(emf) = (B x dl).v
    l = length of wire
    v = velocity of wire
    Apply the formula to both loop lengths (parallel to the wire) and subtract.
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