Accelerating a charge produces radio-waves which is a form of energy. This energy has to come from somewhere. It would seem to suggest it would require greater force to move a charged mass than an equal neutral mass. If not, what accounts for this energy emission?
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$\begingroup$ I was sure this had been asked before but I cannot find the duplicate. The nearest is How is the energy loss by an accelerating charge expressed in the equations of motion? but this is rather more involved than your question so I'm reluctant to flag it as a duplicate, though you may find it interesting reading. The tl;dr version of this answer is yes, it does take a greater force to accelerate the charged mass. $\endgroup$– John RennieCommented Apr 16, 2022 at 6:09
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$\begingroup$ Yes. See "Abraham lorentz force" which is the recoil force due to EM propagation. Charges falling under gravity however do not radiate. According to -en.m.wikipedia.org/wiki/… $\endgroup$– jensen paullCommented Apr 17, 2022 at 9:12
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2 Answers
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In a simple experiment in a large vertical vacuum tube, to ignore the complications of air, if you drop two neutral masses, they end up with the same velocity at the bottom. Let one be charged. It would radiate some electromagnetic energy according to Maxwell's electrodynamics, and it will lose velocity with respect to the neutral one, as the electromagnetic radiation takes away energy and momentum.
Where does the energy come from? Both masses have the same potential energy at the top which is converted to kinetic energy while falling, the charged one losing kinetic energy by radiating some light.
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$\begingroup$ Could you be specific as to how to calculate this difference in end velocities? $\endgroup$ Commented Apr 16, 2022 at 6:24
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$\begingroup$ @no , it is complicated journals.aps.org/ppf/pdf/10.1103/PhysicsPhysiqueFizika.1.3 $\endgroup$– anna vCommented Apr 16, 2022 at 6:31
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$\begingroup$ Wouldn't this cause a contradiction as a free-falling observer would see the charged mass repelled from the surface where no such force exists? $\endgroup$ Commented Apr 16, 2022 at 7:06
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$\begingroup$ @my2cts have a look at the link I gave journals.aps.org/ppf/pdf/10.1103/PhysicsPhysiqueFizika.1.3 $\endgroup$– anna vCommented Apr 16, 2022 at 8:12
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1$\begingroup$ This link says it does radiate, but I remember a discussion concluding that it does not. Have to look it up. en.m.wikipedia.org/wiki/…. $\endgroup$– my2ctsCommented Apr 16, 2022 at 11:43
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What was observed
A moving electron is deflected in a magnetic field according to the vector product of magnetic field direction and velocity direction. EM radiation is emitted in the process. The electron, which has an electric and a magnetic field, loses kinetic energy through the emission - photons have a moment - and comes to a standstill in the centre of a spiral path. A magnetic field interacts with the electron and its kinetic energy becomes photons.
In a conductor, the (periodic) acceleration of surface electrons produces polarised EM radiation. An electric field interacts with the electron and its kinetic energy becomes photons.
All subatomic particles can be accelerated with light - for example with a laser. When braked by light, kinetic energy is converted into photons. When accelerating, part of the light is transferred to the particle (the gain in kinetic energy does not come out of nowhere), another part is re-emitted. The particle radiates. This also works with neutrons!
Even with undirected EM radiation, a subatomic particle constantly gains and loses kinetic energy and constantly absorbs and emits photons. For a macroscopic body, this accounts for its temperature.
What is interpreted
Some think that free-falling particles radiate. This is wrong. A free-falling particle follows its geodesic path and is not subject to any acceleration. We exclude deceleration by the gravitational field itself, as it has not been observed.
When accelerated by light, a macroscopic body has the advantage that the re-emission of light does not only occur back from its surface, but also into the body. Ultimately, the body is heated and re-emits IR radiation. However, a larger portion of the momentum that the directed light exerts on the body remains in the body and the macroscopic body accelerates faster than its subatomic particles do individually.
answered Apr 17, 2022 at 4:15