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Homework Help: Electro magnetic Induction problem. Simple one, but check wehre I am wrong.

  1. Feb 26, 2007 #1
    Electro magnetic Induction problem. Simple one, but plz check wehre I am wrong.

    1. The problem statement, all variables and given/known data

    A magnet moving down, goes through a coil and exits it below. Plot the graph showing variation of induced emf in coil with time.

    2. Relevant equations

    emf = - Flux/Time

    3. The attempt at a solution
    As the magnetic field B increases, emf becomes more and more negative.
    Then while leaving, B decreases, emf becomes more and more positive.


    The emf is first negative, forms a V shape in the negative emf region and then an inverted V in the positive emf region when it leaves.

    4.Answer in the textbook
    The emf is first positive ... in other words just the opposite of what I said.

    Where did I go wrong ?
  2. jcsd
  3. Feb 26, 2007 #2
    From what two points are you measuring the emf?
  4. Feb 26, 2007 #3
    From either ends of the coil
  5. Feb 27, 2007 #4
    Isn't the "B" in this system a vector?

    (Those tricky vectors...)


  6. Feb 27, 2007 #5
    Ya. But how will that matter?
  7. Feb 27, 2007 #6
    I think I get it now.
    Initially B and Area vectors are antiparallel, so flux is negative. Hence, the induced emf is positive.
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