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Homework Help: Electromagnetism, current in coil

  1. Jun 2, 2013 #1
    1. A coil of 25 turns of wire is suspended by a thread. When a current flows through the coil, the tension in the thread is reduced by 4.0 x 10-2 N. What are the magnitude and direction of the current? {magnitude of current; direction of current}

    a) 4.1 A; clockwise
    b) 0.16 A; clockwise
    c) 0.16 A; counter-clockwise
    d) 4.1 A; counter-clockwise

    2. Relevant equations

    B= [itex]\frac{\mu NI}{l}[/itex]

    3. The attempt at a solution

    F= IlB = Il([itex]\mu[/itex]NI/l) = I2[itex]\mu[/itex]N
    I = [itex]\sqrt{ \frac{F}{\mu N} }[/itex]

    so I = √4.0x10-2 / 4∏x10-7 x25 = 35.48A

    My answer doesn't match with any of theirs.

    Also, how would I find the direction of current? I don't know the direction of B (magnetic field).

    Any help would be greatly appreciated.
  2. jcsd
  3. Jun 3, 2013 #2

    Simon Bridge

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    Show your reasoning - why would you expect that a current through the wire would decrease the tension in the thread?
  4. Jun 3, 2013 #3

    rude man

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    Just posting to get on the thread. Seems like there's a lot of info missing. Seems like this problem gives a whole new meaning to "pulling yourself up by the bootstraps"! :smile:
  5. Jun 3, 2013 #4
    Is there uniform circular motion involved?
  6. Jun 4, 2013 #5

    Simon Bridge

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    You are the one with the whole problem statement in front of you, you tell me: is their uniform circular motion involved?
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