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Homework Help: Straight wires carrying current

  1. Oct 6, 2009 #1
    I've tried to find this repeatedly in my text book, but my teacher told us to ignore the book and then didn't do a great job explaining it, but here goes...

    A conventional current is flowing into the page (-Z direction) along a straight wire placed at the origin O perpendicular to the page. (see attachment)

    Question 1: The magnetic field due to the current at P will be in the direction of: +X, -X, +Y, -Y, or +Z?

    Using the right hand rule I got -Y direction. Is this correct?

    Question 2: If there was a second, straight wire, carrying a current, at P (perpendicular to the page), where the current is in the +Z direction (out of the page), what is the direction of magnetic force on this 2nd wire? The choices are: +X, -X, +Y, -Y, or -Z

    My guess is +X, but I'm not confident with that answer. I know there is another right hand rule to use here, but i don't get how it works. Can someone answer and explain?

    Thank you.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: Oct 6, 2009
  2. jcsd
  3. Oct 6, 2009 #2


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    Staff: Mentor

    There's no attachment, so I'd just be guessing to try to help you. Can you try posting the attachment?

    For the force thing in the 2nd part, use the equation F = qv X B. Are you familiar with that force equation for moving charges (with vector velocity v) in a magnetic field?
  4. Oct 6, 2009 #3
    I've attached the image here. Thanks!

    Attached Files:

  5. Oct 6, 2009 #4


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    Staff: Mentor

    Based on the diagram, your answers (not guesses!) are correct. Good job.
  6. Oct 6, 2009 #5


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    Staff: Mentor

    BTW, did you use F = qv X B to solve the 2nd part of the question, or some other method?
  7. Oct 7, 2009 #6
    In the problem, I wasn't given any values to plug into the formula F=qv x B. It just asks which direction the force would be going, not the magnitude. It's a big picture kind of question, which is my weak point. I'm better with values and solving equations.

    The question asks: If there were a second, straight wire carrying a current at P, perpendicular to the page, where the current is in the +Z (out of the page) direction, the magnetic force on this current carrying 2nd wire will be along either: +x, -x, +y, -y, or -z?

    Also, was I correct with my first answer of -y for the direction of the magnetic field?

    Thanks again.
  8. Oct 7, 2009 #7
    Oh sorry! I just saw your answer. Thanks for your help!!!
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