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Multiple choice question

  1. Oct 1, 2006 #1
    What is the answer to this question(see attachment) ?

    i thought the answer was (B) but it is (C)
     

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    Last edited: Oct 1, 2006
  2. jcsd
  3. Oct 4, 2006 #2
    anybody know why
     
  4. Oct 4, 2006 #3

    berkeman

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    Staff: Mentor

    Answer C is correct-amundo. What retards Q? Why does that same effect not retard R?


    EDIT -- Sorry, got my alphabet mixed up. ABC, PQR, etc. Fixed now.
     
    Last edited: Oct 4, 2006
  5. Oct 5, 2006 #4

    Pythagorean

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    Gold Member

    What happens when a magnet goes through a closed conductor?

    Can current flow through plastic?

    can current flow through an open circuit?

    As the closed loop conductor slides down the magnet, it generates a current around the loop, so gravity is doing work to produce a current, which takes up some of the work gravity uses to pull the ring down.
     
  6. Oct 5, 2006 #5

    berkeman

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    And just in case Pythagorean's hints aren't enough, please review what eddy currents are all about, and what helps them to happen, and what various things keep them from happening:

    http://en.wikipedia.org/wiki/Eddy_current
     
  7. Oct 7, 2006 #6
    i know that the open loop won't have as high a current as the closed loop of copper but should it still have some eddy currents. Afterall it is still a peice of metal experiencing a change in flux through it.
    Thats why i thought it should be
    p,r,q
     
  8. Oct 7, 2006 #7

    berkeman

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    Staff: Mentor

    Nope. The eddy current would need to flow around the enclosed surface with the changing flux. Break that outside path, and there is no total eddy current generating an opposing flux and force.

    My undergrad alma matter Physics department puts on a great demo during the annual Picnic Day (UC Davis's open house day each spring), where eddy currents and their practical effects are demonstrated. Slots in different metals, plastics, the whole deal. Very effective learning tools.
     
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