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Homework Help: Two parallel lines with current

  1. Feb 22, 2009 #1
    1. The problem statement, all variables and given/known data
    http://img294.imageshack.us/img294/3157/lines.gif [Broken]

    Two long parallel wires are separated by 0.2 meters and carry currents of 3 and 5 amps as shown in the figure. What can we say about the force on wire #2?

    There is no force on wire #2

    The force on wire #2 is towards wire #1

    The force on wire #2 is away from wire #1

    The force on wire #2 is into the page

    The force on #2 is out of the page

    There is no force on #2 but there is a torque.

    3. The attempt at a solution

    I believe that wire #2 has a stronger force on it, but I don't quite know what direction it would be in. Trying to use the right hand rule, i believe it is in the -k direction, but am unsure. I just for some reason don't feel entirely confident.

    Any help is gratefully appreciated!!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 22, 2009 #2


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    You must look at the diagram sideways, with the two currents coming right toward your eye. IF it is conventional positive current, use the right hand rule around wire 1 to see which way the magnetic field circles around the wire (counterclockwise). Draw or imagine a series of circular B lines around wire 1, including one that touches wire 2. B is tangent to the circle so it is to the right at I2, which is located below I1. Use the other right hand rule to determine the direction of the force when I is out, B to the right.

    The force on wire 2 has to be equal and opposite to the force on wire 1 by Newton's 3rd Law.
  4. Feb 22, 2009 #3


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    Leaving aside that Newtons Laws of action reaction might have something to say about that. that's not a choice.

    How would you apply the Lorentz force equation?


  5. Feb 22, 2009 #4
    Both of you thank you for your replies. Delphi, I hadn't though of looking at it that way, but is that just to make it seem easier? I don't really get lost with directions in my head.

    Lowly, using a combination of those two links (good info btw), I feel like r = +i, and B = +k, which would cross into -j. Is that right? Am I grasping this concept ok? From the first link, I understand that the field is going into wire 2 from the direction of my wrapped fingers.

    Edit: I just figured that since the field from wire 1 would actually come down on top of wire two, causing a resultant in the negative k direction. Would this be right? Does the field spiral away from wire 1?
    Last edited: Feb 22, 2009
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