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B Gaussian gun: Where does the extra kinetic energy come from?

  1. Mar 9, 2016 #1
    Probably a stupid question, but I do not get it...

    Here you can see an experiment demonstrating the Gaussian gun:

    Here I understand how momentum is conserved, i.e. the smaller mass of the ejected ball times its high speed is equal the bigger total mass of the recoiling masses times its smaller speed in the opposite direction. So fine so good.

    But what I do not understand is how does conservation of kinetic + magnetic potential energy hold here? Because fact is that before the collision with the neodymium magnet the ball has a much smaller kinetic energy than its counterpart with the same mass has after the collision. I assume that, like for gravity, something like a magnetic potential energy is converted into kinetic one. But while for gravity everything turns out fine with the final energy budget (in that case the ejected body would have been hold back) here I can't get rid of the impression of kinetic energy as produced out from nowhere..... I couldn't find much of a clear explanation on this. Probably I'm just not getting something very simple.... Can anyone help??
     
  2. jcsd
  3. Mar 9, 2016 #2

    BvU

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    You can see the incoming ball sticks to the magnet and stays there. You need energy to pull it away from the magnet. And he demonstrates that that is quite a bit of energy. So to reload your 'gaussian gun' you have to do work to approximately the equivalent of the kinetic energy that appeared to appear from nowhere....

    But it's a very interesting and indeed stunning demo ! Nice !
     
  4. Mar 9, 2016 #3

    Drakkith

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    Is there a reason that there are 3 balls on the end of the magnet prior to the collision? Is the magnetic force on the ball furthest from the magnet less than the nearest ball?
     
  5. Mar 9, 2016 #4
    Yes, at the distance of 3 ball diameters the magnetic force is less and the last ball can escape with less work to do against the field, that's why it leaves with higher speed.
     
  6. Mar 9, 2016 #5

    Drakkith

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    Okay, that's what I was thinking. Thanks!
     
  7. Mar 9, 2016 #6

    BvU

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    It comes by in the video...
    And you can even see the 'gun' recoil in the slow-motion sequence!
     
  8. Mar 9, 2016 #7
    Ok, but then, as I understand it, besides the momentum conservation law the classical textbook energy conservation law for elastic collisions, when magnets come into the play, it must be rewritten in the most general terms as:

    [itex] 0.5 m_{1} v_{1}^{2} + E_{magpot 1} + 0.5 m_{2} v_{2}^{2} + E_{magpot 2} = 0.5 m_{1} u_{1}^{2} + F_{magpot 1} + 0.5 m_{2} u_{2}^{2} + F_{magpot 2} [/itex]

    with [itex]E_{magpot 1}[/itex] and [itex]E_{magpot 2}[/itex] the magnetic potential of the two balls before the collision, [itex]F_{magpot 1}[/itex] and [itex]F_{magpot 2}[/itex] that after the collision and [itex]u_{1}[/itex] and [itex]u_{2}[/itex] the speeds after the collision (of course here [itex] v_{2} ; E_{magpot 2}; u_{1}; F_{magpot 1} [/itex] are zero in the above example of the Gaussian gun).

    If so, I might begin to feel to understand it. However, it is far from clear hot to express the magnetic potential of a permanent magnet.... as far as I can see there is no such easy formulation as for a gravitational or electrci potential.
     
  9. Mar 9, 2016 #8

    davenn

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    That's pretty cool, hadn't seen that demonstration before
     
  10. Mar 9, 2016 #9

    A.T.

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    It's even cooler when you make an array of the them:

     
  11. Mar 9, 2016 #10

    davenn

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    well I have the magnets and the ball bearings ... Think I will start with just the single setup as in the first video

    Dave
     
  12. May 20, 2017 #11
    Note: if you are using this for homework use absolutely nothing I am about to say.

    First off, most people seam to forget that a magnetic field is actually made from tiny particles(electrons)hurtling through space before curving around and joining back at the other end of the magnet, this creates a flow of energy called a field(this works kind of like the way a gyre works, but not exactly). now electrons do not have a habit of hurling themselves through space unless moved by another particle. Now this would seemingly work because the energy moves in circle therefor creating no loss. Yet, you should find that adding friction to that statement would make them act like any other object, seeing as they have mass. so where does the energy come from? well light most likely as light or electromagnetic waves carry all energy for electromagnetic forces. These waves should give the electrons in the magnet the power it needs to do its thing. None of this has been proven but is easily inferred do to previously proven laws.​
     
  13. May 20, 2017 #12

    Drakkith

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    This isn't correct. A magnetic field is part of the combined electromagnetic field of which photons are the force carrier particles. But even then there are no particles looping around and following the field lines. The field lines don't even exist. They are as imaginary as the topographic lines on a map that represent elevation. They exist in our diagrams to make it easier for us to visualize and model things, just like topographic lines or latitude and longitude lines.

    I'm sorry but none of that can be inferred. In fact, there's no need to infer anything at all. The details of what the laws say and how they're used are readily available in just about any college level physics textbook.
     
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