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Forces Between Two Magnets

  1. Oct 20, 2014 #1
    I've been searching for a simple answer online and on this site, but I haven't been able to find anything that explains this for me.

    Simply, is there an equation that can be used to calculate the force of attraction between two magnets?
    I am interested in when two magnets are touching, how much force will be required to separate them?
    And when they are x distance apart, how much pull strength will they exhibit?

    Thank you
    Sean
     
  2. jcsd
  3. Oct 20, 2014 #2

    sophiecentaur

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    Hi and welcome to PF.
    You haven't been able to find anything much about this because the 'strength' of real permanent magnets is hard to specify. The problem is that you are dealing with (even ideally) four magnetic poles, an N and a S on each one. At a large distance, they can be characterised by their Dipole Moment (whatever shape they are) and the force is easier to calculate but, when you are interested in the useful strength (for actually making use of them) it is much harder.
    It is much easier to discus the Magnetic Force between two electromagnets - even easier to calculate the force between two parallel wires, carrying current.
    Have you looked at the Wikipedia article? Does it have enough information for you?
    Is your interest a general one or do you have a specific application and are you wondering what to buy?
     
  4. Oct 20, 2014 #3
    I did take a look at that wikipedia article, but it is pretty advanced for me...
    I do have a specific application that I am looking to use this for and, really, I need to figure out what I first mentioned, I figure there must be some way to calculate it...?
     
  5. Oct 22, 2014 #4

    CWatters

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    There is no simple way. It's possible to calculate the force that a relay coil applies to the contacts (eg to work out the return spring force required) but I bet relay manufacturers only use that as a starting point and do measurements to confirm.

    http://info.ee.surrey.ac.uk/Workshop/advice/coils/force.html#nfringe

    Perhaps see "Pull Force Case 3 Magnet to Magnet" on this calculator page...

    https://www.kjmagnetics.com/calculator.asp

    Note:

     
  6. Oct 22, 2014 #5
    If the magnets are of same area A then:
    gif.latex?F%3D\frac{B^{2}}{2\mu%20_{0}}\cdot%20A.gif
     
  7. Oct 23, 2014 #6
    Thanks guys for all the responses!
    zoki85- that looks like the formula that I'm looking for, what do the symbols represent?
     
  8. Oct 24, 2014 #7
    A- contact surface area between two nearly touching magnets
    B- magnetic induction in a very short gap between attracting magnets
    μ0- permeability of free space

    The formula assumes B is constant in the gap
     
  9. Nov 7, 2014 #8
    thank you
     
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