How is momentum transfered between magnets

Discussion in 'Physics' started by wes, Jul 12, 2012.

  1. wes

    Thread Starter Active Member

    Aug 24, 2007
    I know according to newtons 3rd law (every action causes a reaction) that two magnets facing each other N-N will cause a opposing force on each one. Conservation of momentum holds obviously because one magnets momentum is cancelled by the other one. However in Newtonian mechanics this action causes a reaction is usually considered instant.

    So how is momentum transferred between magnets on small timescales or large distances where the momentum transfer or force take's nanoseconds, say 1 second to traverse the space between the magnets (earth to the moon almost)? I know supposedly the magnetic force is carried by virtual photons, so is the momentum first transferred to the virtual photons then when the virtual photon interacts with the other magnets field, the momentum is somehow transferred to it's virtual photons which make up its field and then the to the magnet itself causing it to move?

    Really confused on this one, lol, I have a bunch of ideas of how it might happen, some involve bubbles analogy's, lol, but have no idea which is right or even close to right.
  2. Papabravo


    Feb 24, 2006
    In general the answer to your question is that momentum is not transferred between magnets unless there are some other facts or assumptions present.

    For example if I have two bar magnets on a table 1 meter apart, I can calculate or measure the force of attraction or repulsion between them. If neither one of them is moving then there is no momentum transfer.

    You need to describe the situation in much greater detail for me to give you a better, more satisfying answer.
  3. takao21203

    Distinguished Member

    Apr 28, 2012
    Remember these iron dust experiments? Ordinary magnets don't really extend their force over long distance. The field lines somehow correlate to the actual dimension of the magnet.

    The virtual photons cause momentum, however, it is just a model, the iron dust aligns as if they were field lines, which don't really exist.

    Imagine rather virtual field lines carrying virtual photons somehow extend into the matrix of energy, matter and space as we know it. They do this all the time. If they are confronted with suitable matter, a force will be excerted.

    I don't know myself, if supposedly a metallic object appears from one nanosecond to the other, 100000 km apart from a large magnet-

    a) will it take 1/3 second for the force to have effect?
    b) will the virtual field lines already have stretched out, and the effect will be immediately?

    My guess is b)
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  4. wes

    Thread Starter Active Member

    Aug 24, 2007
    I was assuming in space, so any force generated on either would always cause them to move and thus have momentum.

    Also I actually thought that might be one way it happens, that the field lines have already stretched out, so when the other magnet encounters the field, it feels a force and thus moves away, then after 1/3 of a second like in your example, the other magnet moves. So the momentum in the time delay is in the magnetic field moving back toward the other magnet. I kinda viewed it as a bubble with this one, lol. Each magnet generats it's own magnetic field( bubble), when another magnets magnetic field ( bubble ) interacts with it's bubble, this causes a force (pressure) on both bubble's at about halfway between each magnet. If one magnet is much stronger than the other, the interaction happens closer to the weaker magnet. The force ultimately causes both to move away. Not sure how that would work in attraction though, lol.
    Last edited: Jul 12, 2012
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  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
    Assume that you have 2 magnets, each a small object free to move in space, and they interact with each other. They will interact thru their fields ( all forces are associated with a field, and the fields all have quanta ) So momentum is conserved, as always ( how ). First you must count all components that have momentum, this includes both magnets and the fields ( or its photons, same thing more or less. ).

    You seem to be worried about the time for the field to propagate, this is only an issue when the magnets are moving near the speed of light since this is the rate of propigation of the e and m field.

    One weird thing that comes up sometimes ( and I am not really qualified to tell you much about it ) is the so called advanced field that seems to anticipate its cause. You may want to google this.
  6. WBahn


    Mar 31, 2012
    The magnets establish a field around them that can exert forces on other objects. This is the same as a body establishing a gravitational field around it that can exert forces on other objects. It is my understanding that all of these fields propogate changes at the speed of light (call it one foot per nanosecond in free space).

    Thus, if you had two magnets that were separated by 186,000 miles (one light-second) but had strong enough fields to interact, moving one would result in no change in the force acting on the other for a full second. But that delay is true of any interaction since, if it weren't, you would have a means of conveying information at super-light speeds.
  7. wes

    Thread Starter Active Member

    Aug 24, 2007