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Contracting Loop in a Magnetic Field

  1. May 16, 2015 #1
    1. The problem statement, all variables and given/known data

    An elastic circular loop in the plane of the paper lies in a 0.81 T magnetic field pointing into the paper. If the loop's diameter changes from 19 cm to 6.8 cm in 0.45 s,

    a. what is the magnitude of the average induced emf?

    b. If the loop's resistance is 2.3 Ω, what is the average induced current I during the 0.45 s?

    2. Relevant equations

    emf = change in flux * B / change in time

    magnetic flux = B*A cos Theta


    3. The attempt at a solution

    Using the equation from above,

    the distance that I am going to use is .19m - .068m = .122 m (change in distance)

    magnetic flux = (0.81T)(pi)(0.122m)^2 cos 0

    = 3.7875*10^-2 T/m^2

    emf = (3.7875*10^-2 T/m^2) / 0.45s = 8.4167*10^-2 Volts

    Which is wrong but I am not sure where I went wrong.
    Maybe something with the distance? Because I feel like that's the one place where I could have messed up...

    Thanks in advance!
     
  2. jcsd
  3. May 16, 2015 #2

    BiGyElLoWhAt

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    You made a mistake here:
    You need to calculate the change in area, not the change in diameter.
    You want this: ## \Delta A = A_{final}-A_{initial} = \pi r_{final}^2-\pi r]_{initial}^2 ##
    Try that and see if it works.
     
    Last edited: May 16, 2015
  4. May 16, 2015 #3

    TSny

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    This is where the mistake is. Try finding the initial total flux and then the final total flux (after 0.45 s). Use these to get the change in flux.

    [You can ignore my post. BiGyElLoWhAt already posted]
     
  5. May 16, 2015 #4

    BiGyElLoWhAt

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    Beat ya ;)
     
  6. May 16, 2015 #5

    TSny

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    :H
     
  7. May 16, 2015 #6
    For initial flux : .81T (pi*r (.19^2) = 9.18633e-2
    For final flux: .81T (pi*r (.068^2) = 1.17666e-2

    so the change in flux is 1.17666e-2 - 9.18633e-2 = - .0800967

    so I plug that in to find Emf:

    magnitude of emf (- .0800967/0.45s) and it still isn't the right answer.
     
  8. May 16, 2015 #7

    TSny

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    Looks good except you need to interpret the word "magnitude".
     
  9. May 16, 2015 #8
    I got the right answer for both parts. Thanks to you both for the help!
     
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