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Does charge change with velocity?

  1. Aug 26, 2008 #1
    If object A travels close to light speed of light with respect to object B then object A will seem to have an increased mass as observed by B.

    If A is also a charged particle will the charge strength be increased also or will it remain at its rest velocity field strength as determined by B?
  2. jcsd
  3. Aug 26, 2008 #2


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    No. In fact one reason why length, time, etc. change with speed is to make sure that Maxwell's equations are "invariant" under change from one inertial frame of reference to another- and Maxwell's equation govern electromagnetic field strength.
  4. Aug 26, 2008 #3
    Shoot! every time I think I am starting to understand this stuff it throws me a curve.

    Does a field of a charged particle in motion have a Lorenz contraction?
  5. Aug 26, 2008 #4


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    Yes, and this is actually very useful in understanding why there must be a magnetic force if electric charge is the same in every frame. If you want to better understand the relation between electromagnetism and relativity I recommend looking at this page (especially all the useful diagrams), which is adapted from a similar discussion in Purcell's undergraduate textbook Electricity and Magnetism.
  6. Aug 26, 2008 #5
    I read the material and it all makes sense but it occurred to me you could explain the test charge response and the charges in the wire by replacing the Lorenz contraction with change in field charge magnitude due to a relativistic effects of a “charge in motion” principle.

    That is to say instead of the moving charges being closer together with the same field strength, the charges are the same distance apart and the strength of the charges are increased.

    The second thing I noted was that the charge field changes shape ( Lorenz contracts) when in motion.
    So if a motionless charged particle A field strength is measured
    and then the observer is accelerate to a velocity V, the field of A will contract in the direction of motion causing the field to weaken in the direction of motion and strengthen in a direction perpendicular to the direction of motion.

    It would seem that a charged particle is similar to rest mass with gravity,
    and that magnetic fields are similar to relativistic mass.

    I assume this line of reasoning has been followed before and that it breaks down somewhere.

    PS. I really enjoyed the reference material.
  7. Aug 27, 2008 #6


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    Actually, that "line of reasoning" does not break down, but was contradicted by experiment. This was in fact Lorentz' theory to explain the null result of the Michaelson-Morley experiment: that the force of moving electrons strengthened just enough to "contract" one arm of the equipment as given by Lorentz' equation. It was a variation of The Michaelson-Morley experiment, Kennedy's experiment, that show that was NOT the case, leading to Einstein's theory that it was not just the material in the arm that contracted but space itself.
  8. Aug 27, 2008 #7


    Staff: Mentor

    Also, this kind of directional dependence that you have noticed is another reason to drop the concept of relativistic mass. Don't think of mass as increasing relativistically either, just the energy and momentum.
  9. Aug 27, 2008 #8
    The reason I was interested in relativistic mass is simple.

    I was trying to determine if distant galaxies which are receding from us at a high velocity have a relativistic mass component causing an increase in the gravitational force they have on us.

    GR does not have force equations only tidal fields and I do not know how to change this to something I can relate to in the real world, like a force or acceleration.

    Newton’s equations do not work for near light speed velocities. .

    Has the solution of this question already been completed with GR?

    I am not capable to solve the GR equations.
    Some day I hope to be able to solve GR equations, but until then I keep my interest alive by asking questions and learning as I go.
  10. Aug 28, 2008 #9
    It is in fact the very fundamental Space-Time which we're living in that changes with Lorentz transformation, so everything relates to length, time, velocity...changes, but as one apple is still transformed into one apple, the charge of a particle will not change, although the charge density will. Mass doesn't change either, if not so, you'd be able to make a remote galaxy collapse just by moving yourself around!
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