1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Parallel Wires and Magnetic Field

  1. Mar 27, 2012 #1
    1. The problem statement, all variables and given/known data
    Consider 2 parallel wires with currents running down them in the same direction. The first is fixed in place unable to move. The second is allowed to move. They exert an attractive magnetic force on one another, and the second wire moves towards the first. The second moves and speeds up; work is done. Magnetic fields do no work on moving charged particles. Is this statement violated in this situation?

    2. Relevant equations

    3. The attempt at a solution
    Not really sure. It seems like the magnetic force would be enough to allow the 2nd wire to move, although the field itself does no work, the magnetic force can?
  2. jcsd
  3. Mar 27, 2012 #2
    To solve this you have to really look hard at the forces. No work is done on the charges. But that doesn't mean no work is done on the wire.

    Work is F.dl. The charges are moving perpendicular to the force, so that's zero. Is there anything that is moving along F?
  4. Mar 27, 2012 #3
    I'm so confused! So F and v are always perpendicular so that is why there cannot be any work done. F=ILB and there is current through the wire so does that have something to do with it?
  5. Apr 6, 2012 #4
    Think of it like this; the charge is moving along the wire but the force is perpendicular to the wire. So the force pulling on the wire (sideways) can't be doing work on the charges along their direction of motion.

    That doesn't mean that work isn't being done to accelerate the wire sideways- it is. But the sideways acceleration doesn't do work against the current, it does work against the mass of the wire to accelerate it sideways.

    The work goes into moving the wire sideways, not pushing or pulling on the charges along the direction of the wire.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook