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Homework Help: Force within coil: compression or expansion?

  1. Jul 20, 2004 #1
    A coil of wire is formed of many loops. These loops, though tracing a circular path, may be though of as being parallel to each other. We know that whenever parallel wires carry and electric current, there will be a mechanical force generated between those two wires.

    When electric current is passed through a coil of wire, does the inter-loop force tend to compress the coil, or extend it? Explain your answer.
  2. jcsd
  3. Jul 20, 2004 #2
    I am a bit confused. In the first paragraph, you are talking about parallel wires, while in the 2nd paragraph, you are talking about forces within the wire. Please clarify.

    - harsh
  4. Jul 20, 2004 #3

    Doc Al

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    Staff: Mentor

    Start by figuring out what happens between parallel wires carrying currents. If the currents are in the same direction, do the wires repel or attract?
  5. Jul 20, 2004 #4

    Tom Mattson

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    I think the intent behind the problem statement is for you to use the expression for the force exerted on a long, straight wire by a magnetic field, despite the fact that the wires are bent into loops. Locally, it's a good approximation.

    What you need to do is figure out which way the local magnetic field points at the location of a particular segment of wire. You do this by considering the field due to the current in nearby segments of wire. Then use the right hand rule to get the direction of the force. That will tell you whether the force is compressive or tensile.
  6. Jul 20, 2004 #5
    Start by doing this:
    1. Draw two parallel wires on a piece of paper.
    2. draw an arrow in the middle of both wires going in the same direction
      The arrow represents the driection of current flow
    3. apply the right hand rule to determine the orientation of the B-field produced by both wires.
    4. What do you know about magnets in proximity and their pole orientation?

    Think the above through.

    Good luck.
  7. Aug 7, 2004 #6
    take a col (or a solenoid)..... the force between these loops near the axis is maximum...this force compresses the solenoid...the force direction is given by right hand rule of vectors...by rotating the fingers in the direction of current...and thumb gives the direction of field..
  8. Aug 7, 2004 #7
    Related Problem

    A related but slightly different problem was posed in the first level of an engineering entrance examination in India last year:

    A circular coil of radius R carrying current I in the clockwise direction (as seen by you--facing the coil) is placed in a uniform magnetic field of magnitude B, pointing

    (a) into the plane of the paper (screen)
    (b) out of the plane of the paper (screen)

    In each case the magnetic field vector is perpendicular to the plane of the loop. For each case, determine whether the coil will

    (1) Translate
    (2) Rotate
    (3) Expand
    (4) Contract

    (I have modified the question slightly to make it more germane to the issue being discussed here.)

    The idea here is to find the direction of the force on the current carrying conductor as suggested by Tom Mattson and faust9. Note that you're given everything to compute this force F mathematically, but you don't need to do that to chose one correct option from the four given for either direction of B.
  9. Aug 8, 2004 #8
    maverick....was it IIT screening?
  10. Aug 9, 2004 #9
    Yup sure was! :biggrin:
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