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Physical Movement Caused by Current

  1. Feb 23, 2009 #1
    1. The problem statement, all variables and given/known data
    http://img5.imageshack.us/img5/348/snake.gif [Broken]
    The figure above shows a piece of flexible wire lying on a frictionless table top. The wire can slide around and change its shape freely, but it cannot move perpendicular to the table.

    Part I
    If a current were to begin to flow counterclockwise in the wire which of the following would happen?

    1. The wire would tend to stretch out into a circle
    2. The wire would contract down to a tangled mess
    3. Nothing
    4. There is not enough information to tell.

    Part II
    If the current went clockwise, would that affect your answer to part a?

    3. The attempt at a solution
    For this problem, I feel like I should be looking at Newton's third law, which states, for any action, there is an equal or opposite reaction. So I look at the wire and I get the impression that the wire has a uniformly distributed charge, meaning that no part of the wire feels attracted to each other. If I had to have my choice at the solutions above, I would either pick 1 or 3 because I don't know the nature of two similarly charge wires. Would they push each other away for just ignore each other?

    For part II, I don't feel like this would make a difference, because I feel like the direction in this problem is negligible.

    Any help appreciated!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 23, 2009 #2


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    Homework Helper

    Charge is not exactly the way to look at things. You have current in the wire. It's not static.

    But what do you have when you have current flowing?


    Is the Lorentz force law part of your curriculum?

    Your sense of Part II is correct. Whatever it is for I it is for II.
  4. Feb 23, 2009 #3
    He never has called it the Lorentz force law, but we use it.

    I guess you are right about the charge vs current. I still am unsure which answer it would be though. Would two coinciding magnetic fields cancel each other out, and is that what I am getting in this situation? Or would they only cancel each other out when the force coming from all angles is uniform, resulting in a circle being made?
  5. Feb 23, 2009 #4


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    Look at the way the line starts laid out in a serpentine fashion. With this topology if there is non-uniformity then won't there be lines in more extreme example laying next to each other, but in doing so won't the current on each segment necessarily be going in opposite directions?

    Perhaps if you determine the relative direction of the force on wires carrying current in opposite directions, i.e do they attract or repel, then maybe you can get a handle on what happens? If they attract then figure a jumble. If they repel then maybe it blossoms into a circle?
  6. Feb 25, 2009 #5
    Thanks for the help. The part of information I didn't know what that if current flows parallel with the same magnitude but in opposite directions, the wires would attract, and if they went in the same direction, they would repel.

    So in this case, the wire would bow out into a circle.

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