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I Violation of conservation of momentum?

  1. Mar 28, 2016 #1
    According to amperes force law two parallel wires with current flowing in the same direction will experience a magnetic force attracting them together. consider two 1cm long wires parallel to each other, separated by a distance of 1cm. Pass a current through both of the wires for the time it takes for the magnetic field from each to reach half way between them, then turn the current off on the left wire. The magnetic field from the left wire from when it was still on will continue to propagate outwards towards the right wire and will pass the right wire, this will cause the right wire to attract to the left. At the same time, the magnetic field from the right wire will pass the left wire, however since the wire has no current there will be no force attracting it to the right wire. Then turn the current off on the right wire and wait till the magnetic fields dissipate. then start the process all over again very quickly. This should produce a small thrust in one direction. Is there anything wrong with this picture??
     
  2. jcsd
  3. Mar 28, 2016 #2

    berkeman

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    Thread closed temporarily for Moderation...
     
  4. Mar 28, 2016 #3

    berkeman

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    Thread re-opened. There has to be an issue, but I'm not seeing it. In the simplest case, you are asking about why a caterpillar would not work with timing of the currents in the two wires. Hopefully somebody can point out the flaw for us. :smile:
     
  5. Mar 28, 2016 #4
    there are infact problems with the timings that I gave in the example, but I'm quite sure there exists timing that will work
     
  6. Mar 28, 2016 #5

    berkeman

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    Well, I'm pretty sure it won't work, since it would violate fundamental physics. The simplified problem statement is to time the two currents so that the B field from the left wire reaches the right wire as the current is turned on there, generating a force which does not happen in the left wire.

    Sorry that I'm not seeing the obvious problem right off the bat. Others will find it for us...
     
  7. Mar 28, 2016 #6
    The proper timings are the following. separate the wires by 1.5cm. Turn on both the wires until the magnetic field reaches 0.5cm away from the wires, then shut power off on the left wire, while keeping the right wire powered until the magnetic field from the left wire reaches it.
     
  8. Mar 28, 2016 #7

    berkeman

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    I know, your question is clear. Please just wait for some responses from our physicists. Thank you.
     
  9. Mar 28, 2016 #8

    Dale

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    This is the same issue as in your other thread. This arrangement will produce EM radiation (waves). The waves will carry momentum and you will get an equal and opposite momentum on the wires. This is another example of a radiation reaction drive, not a reaction less drive.

    It is not a violation of conservation of momentum, the momentum is in the EM waves. Also, again you will be better off just shining a laser backwards. The EM momentum will be more collimated.
     
  10. Mar 28, 2016 #9
    Okay, thanks. I've been wondering about this for a while. would there be any near field effects in this example?
     
  11. Mar 28, 2016 #10

    Dale

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    That depends on the frequency of your current switching. Near fields still conserve momentum, so that doesn't change the above comments.
     
  12. Mar 28, 2016 #11
    That's a shame. Oh well
     
  13. Mar 28, 2016 #12
    Which wire do you say is emitting radiation? Would it still be the case if the device was only used once?
     
  14. Mar 28, 2016 #13

    berkeman

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    Okay, I think I finally figured out the flaw. Interesting thought experiment accually.

    The simplified situation is a pulse in current from the left wire that generates a B field at the right wire just as there is a current pulsing in the right wire, generating a force. The part that I missed at first is that to generate a pulse, you need to have a positive and then a negative transition. Same goes for the current on the right wire. If you look at the overall set of transients, you get a net zero force from the transient B fields and the transient currents in the right wire. I was only thinking about the positive onset currents and B-fields, and not taking into account the decreasing B-field and currents. Whew.
     
  15. Mar 28, 2016 #14

    Dale

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    Both. EM radiation is produced any time the current changes.

    The conservation of momentum is built into Maxwell's equations. You cannot get around it.
     
  16. Mar 28, 2016 #15

    David Lewis

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    Some energy will be consumed overcoming radiation resistance. When you add that to the mix, momentum increase and decrease balances.
     
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