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Semicircular Permanent Magnet Rail Gun - continuous motion?

  1. Sep 18, 2014 #1
    Doubtless you are all familiar with a rail gun made by using permanent magnets.

    An example is given here:

    http://sci-toys.com/scitoys/scitoys/magnets/gauss.html

    Assuming the railgun could me made curved, won't it be possible to build a curved track to transport the projectile - assume a steel ball on a track - to the starting point so continuous motion of the steel ball is achieved?

    This is not perpetual motion since the magnet would eventually lose their strength.
     
  2. jcsd
  3. Sep 18, 2014 #2

    jambaugh

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    Nope, look carefully at the configuration before and after. You have to reset it which require energy input.
     
  4. Sep 19, 2014 #3
    Well I am open to proof that it cannot work, however I am not sure about the reason for saying it cannot work.

    To simplify matters, just imagine a permanent magnet rail gun:


    //////////
    0======
    \\\\\\\\\\

    The steel ball is place at the start of the gun with a velocity zero or greater at the start of the rail gun. Due to the forces of the magnets the steel ball moves down the rail in the diagram to the right.

    //////////
    ======0=
    \\\\\\\\\\


    //////////
    ======= 0
    \\\\\\\\\\

    Something similar has been shown on Discovery TV, and there is no question that it works.

    There are two questions that can be asked here:

    1. Can the ball, possessing a velocity, have sufficient momentum to be directed back to the starting point using a circular track?

    In the demonstrations I have seen on You Tube the projectile does pick up substantial speed using the rail gun, so this should be possible.

    2. If the steel ball is re-routed to the start of the rail gun, will it be accelerated down the rails once again?

    I cannot at present see a reason why the answers to both these questions should be in the negative.
     
  5. Sep 19, 2014 #4

    A.T.

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    The attraction between the balls and magnets is like a conservative field. You can convert potential into kinetic energy and vice versa, but their sum is at best constant, and dissipates through losses in real life.

    You should ask yourself: If all you need is a couple of magnets and steel balls, why hasn't anybody build a working circular one yet?
     
  6. Sep 19, 2014 #5

    davenn

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    I guess you didn't read to the bottom of the page on the link you gave ??


    cheers
    Dave
     
  7. Sep 19, 2014 #6

    davenn

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    but that wouldn't happen for zillions of years, so in reality ( well your reality anyway)
    you are claiming perpetual motion

    and as such, any further discussion is against forum rules
    so expect this thread to be closed


    cheers
    Dave
     
  8. Sep 19, 2014 #7
    Sure I read that, but that is not a suitable example regrettably. The system I am describing does not use several balls and collisions to work - it is a simple magnetic rail gun as shown here:

    www.youtube.com/watch?v=Vo2-Qb3fUYs
     
  9. Sep 19, 2014 #8
    OK OK, before you close the forum - imagine an electromagnetically driven rail gun - energy is input from a battery into two electromagnets that accelerate a steel ball on a track. Now will this motion continue as long as the
    * electro magnets * are switched on?

    http://en.wikipedia.org/wiki/File:Rail_Gun.png
     
  10. Sep 19, 2014 #9

    A.T.

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    I think that misses the point. I'm not sure that "magnet losing it's strength" constitutes any source of energy, no matter how negligible. Isn't the magnetized state of lower potential energy than the demagnetized? It would seem to me that you have to put (thermal) energy into the magnet to demagnetize it.
     
    Last edited: Sep 19, 2014
  11. Sep 19, 2014 #10

    A.T.

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    This argument in post #4 still applies here.
     
  12. Sep 19, 2014 #11

    davenn

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    yeah true :wink:
     
  13. Sep 19, 2014 #12

    CWatters

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    For the purposes of this thread a magnetic field can be considered conservative...


    http://en.m.wikipedia.org/wiki/Conservative_force

    Eg if you move in a circle no work is done. Basically you can't ever "close the loop" and get the energy out that is needed to overcome losses.

    Hasn't stopped lots of misguided people trying and posting their efforts and fakes on YouTube.
     
  14. Sep 19, 2014 #13

    Doc Al

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    OK, time to close this thread. Discussions of perpetual motion schemes are not permitted.
     
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