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Homework Help: When are two equal and opposite forces about an axis not a couple

  1. Nov 24, 2012 #1
    1. The problem statement, all variables and given/known data

    A current flows in vertical square coil whose plane is at right angles to a horizontal uniform magnetic field B. The coil can rotate about a vertical axis.
    Which is correct
    A) The forces on the two vertical sides are equal and opposite.
    B) A couple acts on the coil
    C) No forces act on the horizontal sides of the coil
    D) If the coil is turned through a small angle about the vertical it will remain in position.

    2. Relevant equations
    The answer in the mark scheme is A

    3. The attempt at a solution
    What I cant seem to see is why B is not true also. Why are these two equal and opposite forces about an axis not causing a couple?

    Any help greatefully received
    Last edited: Nov 24, 2012
  2. jcsd
  3. Nov 24, 2012 #2
    A couple implies there's something to couple with.

    How would equal and opposite forces create a coupling?
  4. Nov 24, 2012 #3


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    hi mrcotton! :smile:
    in what direction are these opposite forces? :wink:
  5. Nov 24, 2012 #4

    Doc Al

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    Figure out the direction of the forces on the coil. What torque does each force exert about the given axis?
  6. Nov 25, 2012 #5
    Thanks for the responses.
    I think my sad old brain has got it.

    In this picture the left hand is not orientated correctly. If it were it would show the forces given by the red arrows and they just push towards each other. So when at 90 degrees to the field there is no couple.

    In my sketch I have tried to indicate the when at an angle between 90 degrees and 0 to the field the two wires experience forces that cause a torque and the wire will turn.

    Mr C
  7. Nov 25, 2012 #6


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    you nailed it! :biggrin:
  8. Nov 25, 2012 #7
    Sad thing is I remember that jarrred with my brain initially but I just couldn't hold my hand in that position long enough for the thought to sink in.
    Thanks again
    Last edited: Nov 25, 2012
  9. Nov 25, 2012 #8


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    Using the "left hand rule" would get me into trouble during exams, as I scrawled with pencil in right hand :wink:.
  10. Nov 25, 2012 #9
    alas in exams my digits only ever perfomed the function of counting.
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