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B Electromagnetic waves

  1. Mar 3, 2017 #1
    We took electromagnetic waves this week, and They specified that only accelerated charges make electromagnetic waves.

    So from my previous reading on the internet about speed of causality, I came up with this. That if a charge is moving at a constant velocity, It's field follow is it instantaneously. As there is no need for information to travel because you can predict its trajectory...but if it accelerates, then you cant and if you are observing it from outside you cant instantaneously know that it had started to accelerate. So at each moment it produces a field "Piece of information" That moves at the speed of light which takes time to reach you. then when it stops you will go back to the normal constant velocity so there is a gap between these two which represents the acceleration and it travels away. With this true, You can make any shape you want like a sin wave through an antenna.

    But I have a tiny problem with this, How is momentum conserved when a for example electron is placed in the outside field where the charges appear to not have moved yet? Somehow fields conserve momentum? They have their independent energy and momentum?

    (Probably all of this is non-sense), Please if it is wrong, can you explain it in simple highschool terms? because all websites takes about complicated stuff.
  2. jcsd
  3. Mar 3, 2017 #2


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    You can't instantly make a particle appear in a field. It has to be moved there from another location and will thus have the appropriate amount of momentum and energy.
  4. Mar 3, 2017 #3


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    Yes, EM fields carry momentum. This has been discussed many times before on PF, so please do a search. For instance: https://www.physicsforums.com/threads/doubts-regarding-electromagnetic-fields.878266

    Note also that electromagnetic waves do not simply represent delayed (because of the finite speed of light) induction. The effect of the motion of a charged particle always travels at c, but only acceleration will produce an EM wave.
  5. Mar 3, 2017 #4
    No I was asking about if you have to electrons for example, a bit far of each other. If one accelerates, the other will not notice the first one accelerating and at that time the 2nd electron will get a force from where the accelerated electron was. so I was asking how can momentum be conserved

    I was asking how this affects momentum conservation in newton mechanics and were the result we used to calculate in previous years were simply wrong?

    I did search for topics about it, and the similar threads thingy showed me couple but this one didn't pop up. I will check it out

    I don't get your last statement, Can you clarify please?

    I don't want to get in depth with these stuff, I only want to know if the concept I made above is true and that 12 years of my life was not wasted on newton mechanics
    Last edited: Mar 3, 2017
  6. Mar 6, 2017 #5


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    In the OP, you seem to be inferring that you get an EM wave because information needs time to propagate. This is incorrect. Changes in the electric field always propagate at c. In the case of a particle moving at constant speed, the other particle doesn't know that it is going at constant speed, and it will feel the field created the the moving particle with a time delay do the finiteness of the speed of light.
  7. Mar 6, 2017 #6


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    The momentum is conserved because it is transferred to the EM field in the form of an EM wave. This wave then accelerates the 2nd charge, transferring momentum to it.

    I'm not quite sure what you're asking. Are you asking about Newtonian mechanics as it was known prior to the discovery of EM waves? Once classical electrodynamics was formulated there was no longer an issue (as far as I know), as the momentum was then considered to be transferred from the 1st particle to the EM field and then from the field to the 2nd particle.

    Newtonian mechanics is used in about 99% of all of engineering and physics. It's also required to know before you advance to quantum physics or relativity. You certainly haven't wasted your time.
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