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Homework Help: Enery dissipated

  1. May 10, 2005 #1
    Hi, I am struggling with this physics problem:

    A piece of copper wire is formed into a single circular loop of radius 11 cm. A magnetic field is oriented parallel to the normal to the loop, and it increases from 0 to 0.50 T in a time of 0.45 s. The wire has a resistance per unit length of 3.3 10-2 /m. What is the average electrical energy (in J) dissipated in the resistance of the wire.

    I really don't know where to start in this problem. I can find the circum of the loop and area of the loop. But I'm not sure how to relate resistance, magentic flux, and electrical enery- any help would be appricated!

    Thanks
     
  2. jcsd
  3. May 10, 2005 #2

    Doc Al

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    Staff: Mentor

    Faraday's law will allow you to calculate the induced EMF due to the changing magnetic flux through the loop of wire. Start there.
     
  4. May 10, 2005 #3
    I found the EMF to be -.0422
    (-NA delta B)/ delta t
    where N= the number of loops
    A = area
    B= magnetic field
    t= time
     
  5. May 10, 2005 #4

    Doc Al

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    Staff: Mentor

    OK. What are the units of EMF?

    Now treat the wire loop as a circuit. What's the average power dissipated in the resistance of the wire?
     
  6. May 10, 2005 #5
    the units are V, so .0422 V

    P= (V^2)/R (via P=IV and V=IR)

    so I took P= (.0422^2)/.0228

    The R came from the circum of the loop times the resistance per unit of length

    The answer I got was 1.85 J, but this isn't the right answer-- is there something I'm missing?
     
  7. May 10, 2005 #6

    Doc Al

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    Staff: Mentor

    Looks OK. That's the average power. (What units?)

    OK.

    How did you get this answer?
     
  8. May 10, 2005 #7
    I got it!!!! I made a calculation error earlier

    I got the answer by using this equation:
    E=Pt

    where P= equals the average power
    t= time

    THANKS so much for helping me through this problem, you are the best!
     
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