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Is there a formula for determining the velocity at which two magnets are attracted?

  1. Dec 14, 2009 #1
    Hi PF Pros... I wonder if you can help me...

    I am trying to see if there is a formula for how fast magnets attract. I know that there is the inverse square in terms of power loss and that magnetism is a field that travels at or near light speed.

    But, for example, if I have 2 cyllander NIB magnets (.5" wide x 1" long) with 5,000 Gauss each and they are 3 inches away from each other... at what speed will they meet? If I have the speed I can calculate the time/distance. (as for weight or other forces like friction, etc, please feel free to make something up for the equation).

    Thanks!

    ~Electrica
     
  2. jcsd
  3. Dec 16, 2009 #2

    vk6kro

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    Science Advisor

    Re: Is there a formula for determining the velocity at which two magnets are attracte

    No, there probably isn't a formula for this velocity.

    If anything, you might be able to find the force between two magnets, however it is very dependent on the spacing between the magnets.
    So, you would only have to move the magnets closer to each other by a small amount and they would be attracted to each other a lot more.

    There is a formula for this attraction here: (formula taken from Wikipedia article on magnets)

    ea0c076f1c59249aba590d07b31da41e.png

    where

    B0 is the magnetic flux density very close to each pole, in T,
    A is the area of each pole, in m2,
    L is the length of each magnet, in m,
    R is the radius of each magnet, in m, and
    x is the separation between the two magnets, in m

    8534cbb9499a30dfa335ad6b0f01400b.png relates the flux density at the pole to the magnetization of the magnet.

    Probably easier is to just measure the force with the following method:
    http://www.exo.net/~pauld/activities/magnetism/forcebetweenmagnets.html
     
  4. Dec 16, 2009 #3
    Re: Is there a formula for determining the velocity at which two magnets are attracte

    Well, there will be a potential energy due to that force that looks like the integral of Fdx, so you can set that integral equal to the kinetic energy when they collide (1/2 mv^2) and you will find the velocity right before impact (neglecting friction)... should be able to repeat this to find the velocity after travelling half the distance, etc. etc.

    But if you are holding them close, and you want to find the velocity right when you let them go, then... I'll think about it...
     
  5. Dec 16, 2009 #4
    Re: Is there a formula for determining the velocity at which two magnets are attracte

    Thanks a lot!!! I'll try that. Please do let me know if you come up with the other. -Electrica

     
  6. Dec 16, 2009 #5
    Re: Is there a formula for determining the velocity at which two magnets are attracte

    Thank you very much for this info!!! I have some reading to do! - Electrica

     
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