Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Current Carrying Wires-No force b/t them?

  1. Apr 10, 2004 #1
    Current Carrying Wires--No force b/t them?

    Faraday's law predicts that two current-carrying wires, when placed parallel to each other, will exert an attractive force on each other. Conversely, two wires that are placed anti-parallel to each other will exert a repulsive force on each other.

    While I was vacuuming my apartment today, I decided to test this phenomenon. I folded the wire into two parts such that one part of the wire was touching the other, with its current flowing in the opposite direction. The result? Nothing. According to Faraday's law, however, the wires should have exerted an attractive force on each other.

    I then folded the wires such that one part was touching the other, with its current flowing in the same direction. The result? Again, nothing. But according to Faraday's law, the wires should have exerted a repulsive force on each other.

    How can I make sense of this result?

    Of course, the electrical input is AC 60hz, not DC. But this shouldn't matter. If the wires are parallel to each other, then on the positive half cycle, both currents will be in the same direction, and the wires will exert a repulsive force on each other. Likewise when the input is on its negative half cycle. The frequency of the input would not seem to make any difference.

    Maybe the rubber coating around the wire stifles the magnetic field created by the current flow? I've never read anything that would suggest this.

    Any comments would be greatly appreciated.
  2. jcsd
  3. Apr 10, 2004 #2

    Hint: Suppose your vacuum's drawing 5 amps, what's the net current through the wire? Actually, it's a cord (that's another hint!).
    Last edited: Apr 10, 2004
  4. Apr 10, 2004 #3
    Doh!!! One cord, two wires inside, one with current going into the load, the other with current coming out. So they cancel. It never occurred to me that the answer was so simple--and that the question was so stupid!

    Anyway, thanks for clearing that up :biggrin:
    Last edited: Apr 10, 2004
  5. Apr 12, 2004 #4


    User Avatar
    Homework Helper

    Whoa, now, wait a second. Can you explain further. I didn't understand your experiment.

    What does your experiment have to do with vacuuming the apartment?

    What wires are you talking about (the ones in the power cord)? Did you cut up the power cord to access the wires (not all that safe :rolleyes: )? Did you just consider the power cord as a wire?

    How did you measure the force? (This is my most curious question. If you've ever done this experiment in a controlled lab environment, or even just calculate it, you'll know that the force is absolutely miniscule at a seperation on the order of millimeters at a current on the order of amps. I.e. separation of 1 mm between two lengths of wire a foot long carrying 10 A gives you a magnetic force about equal to the weight of a single pea).
    Last edited: Apr 12, 2004
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook