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A circular loop over a magnetic field directed outwards

  1. Jun 23, 2015 #1
    1. The problem statement, all variables and given/known data
    There is a counterclockwise current I in a circular loop of wire situated in an external magnetic field directed out of the page as shown. The effect of the forces that act on this current is to make the loop
    25e44451-5231-4a3c-a0c6-f7e4d005d9d8.png

    Select one:
    a. expand in size X
    b. contract in size
    c. rotate about an axis perpendicular to the page
    d. rotate about an axis in the plane of the page
    e. accelerate into the page

    2. Relevant equations
    Right hand rule is all that is needed

    3. The attempt at a solution
    So I got this wrong for reasons I am unsure of, however this is my reasoning for choosing a


    The right hand rule has many variations but this is the one i learned

    My thumb is in the direction of Current or Velocity
    My fingers point in the B Field direction
    My palm then is the direction of the force


    I can use this at any point on the circle and it pushes outwards, so should try and expand the loop right?


    The reasoning behind my answer being incorrect.
    "The external field has lines running out of the page, which we can think of as a North pole being just under the page. The current in the wire, obeying the RHR, effectively makes a magnetic field that also has lines running up out of the page. This means that the above page part is like a north pole and the below part is like a south pole. Since this south will be attracted down by the external field north, there is an accelerating force downwards into the page."

    I somewhat understand what this answer however I do not find any force directed down, is this not why things levitate when above high powered fields?
     
  2. jcsd
  3. Jun 23, 2015 #2
    Do you know the right answer ? Sorry I asked a stupid question.
     
  4. Jun 23, 2015 #3
    Definitively no, but I know that to me and everything else I read about this problem says it expands and I agree.

    I would like to know why my teachers answer is not correct. My only guess for this is the the movement is in the direction of the of the magnetic field and it should be parallel I believe.
     
  5. Jun 23, 2015 #4

    berkeman

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    I think your answer A is correct. Use the Lorentz Force: F = qv X B. The positive current points counter-clockwise, and the B field is up out of the page. When I use the right-hand rule, I point my right hand fingers in the direction of qv and then curl them in the direction of B. My thumb points in the direction of the force F, which pulls outward on the circular wire.
     
  6. Jun 23, 2015 #5

    SammyS

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    I agree with berkeman and you.

    By any chance, was your instructor referring to the direction of electron current, rather than conventional current ?
     
  7. Jun 25, 2015 #6

    rude man

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    Your answer (a) is correct. There can be no force on the coil in the into-out-of-page direction since that's the direction of the B field. F = i dl x B so F can never have a B-direction component.

    The offered argument is wrong also. Yes, there is a N pole behind the page and a S pole in front of the page, but the coil produces a S pole behind the page and a N pole in front.. Not that that's particularly relevant here.
     
  8. Jun 25, 2015 #7

    rude man

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    It wouldn't have made any difference.
     
  9. Jun 25, 2015 #8

    SammyS

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    It does make a difference.

    If the electrons are flowing in a counter-clockwise direction, then the loop will tend to shrink.
     
  10. Jun 25, 2015 #9

    rude man

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    Of course. But the given answer (in bold in post 1) is still wrong.
     
  11. Jun 27, 2015 #10
    I gave up trying to talk sense into her, its an online course to upgrade and I suppose the one mark is fine. She couldn't justify her answer she just said thats the answer given. thanks anyhow everybody
     
  12. Jun 27, 2015 #11

    rude man

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    Hope that is not representative of physics instructions in this country today. We've dumbed-down in too many ways already.
     
  13. Jun 27, 2015 #12

    SammyS

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    It's understandable for a teacher to give a wrong answer.

    However, when confronted with the correct solution, the teacher would hopefully be able to recognize it's correctness. Lacking that, the teacher should look further into the problem and consult a more enlightened source. A response like "that's the answer given" or "that's what the solution manual says" with no further investigation, is unforgivable.
     
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