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B How come accelerating charges produce EMR, but others don't?

  1. Aug 1, 2017 #1
    I am a high school teacher preparing to teach grade 12 physics for the first time in the fall. I can't quite understand this and it is really bothering me...

    In the textbook my school uses, it says that accelerating charges produce electromagnetic radiation, while charges moving at a constant speed do not produce EMR.

    I am having trouble understanding why this is the case. I feel like I might be having trouble because I'm not exactly sure what the term "changing electric field" means.

    Does an electron moving at a constant speed not have a changing electric field? If not, what qualifies an electric field as a CHANGING field as opposed to a static one?
  2. jcsd
  3. Aug 1, 2017 #2


    Staff: Mentor

    It does have a changing electric field, but radiation is more than merely a changing field. It is a field which transports energy away from the radiating body. As a charge moves at constant velocity the energy in the field remains localized near the charge.
  4. Aug 2, 2017 #3
    Things like to be in their lowest energy state. An accelerated charge, or actually an accelerated charged particle, is gaining energy and as such sheds the excess energy by emitting photons, which is what electromagnetic radiation is.

    A changing electric field can be seen as a current flow. In an electric wire, the current flow is the flow of electrons along the surface of the wire, the greater the current the greater the number of electrons. As each electron contributes to the overall electric field, the more electrons that are flowing, the greater the electric field. If the current changes, the number of electrons flowing changes thus causing the electric field to change.

    A static electric field comes from electric charges (particles) that are not moving.

    Also, I might add, the electromagnetic field is not the same thing as the electric or magnetic fields.
  5. Aug 2, 2017 #4
    So if the current is constant, does it produce a static electric field or a changing electric field?

    If it is a constant electric field then I understand why it would not produce EMR.

    But if it is a changing electric field, then I still don't understand. I thought the idea behind EMR is that a changing magnetic field produces a changing electric field, and vice versa, allowing the EMR to propagate.
  6. Aug 2, 2017 #5
    My understanding is that the charge needs to be accelerating in order to radiate. The idea was originated by J J Thomson and J J Larmor in about 1897.
    If you imagine an electron at constant speed, its electric lines of force stick out radially like spines of a hedgehog. But if it accelerates, the information telling the World that it has done something new can only travel out radially at the speed of light. So the spines now tilt backwards, as if they experienced wind resistance. This means that they have a field component acting along the path of acceleration, rather than radially as for a non accelerating charge, and this will travel outwards at the speed of light as radiation. Of course, a distant stationary observer to one side of the electron path will also observe a magnetic field as the pulse of electric field passes him (Maxwell).
    The backward tilt of the field lines acts against the acceleration, and means that additional work has to be done, and this work is partly lost as radiation. Another part of the work is stored in the magnetic field created as the electron goes faster, effectively as momentum, or in electrical terms, as inductive energy.
    The following Wiki might be of interest: https://en.wikipedia.org/wiki/Larmor_formula
    I hope the real physicists reading this will be kind to me!
  7. Aug 3, 2017 #6
    You are correct that a constant electric current produces a static electric field. This can be easily seen in the operation of electrical circuits, transformer in particular. The difference between DC and AC electricity. In DC, or direct current the current flow is rather constant and when flowing though a transformer only flows through the primary side. If an AC, or alternating current is used, the current is constantly changing, so that as the current flows through the primary coil, with the constantly changing electrical current, causes the electric field, in the primary coil, to be constantly changing which becomes a magnetic field, which while changing produces an electrical field in the secondary winding of the transformer. A similar action happens in capacitors, except that it only operates uses the electric field.

    However, you may be confusing terms here. The electric field, and related magnetic field are, essentially, extensions of charged particles, like electrons, protons, and ions, and have a + or - charge associated with them. EMR or electromagnetic radiation is photons, which is what light and radio waves are, and has no charge.

    Part of the problem might be that Maxwell's equations, while useful, give a view of nature that is at odds with current understanding. Maxwell saw electromagnetic radiation as a wave. Einstein got his Nobel prize for demonstrating, in part, that electromagnetic radiation is particles, photons. The common picture one sees of Maxwell's electromagnetic radiation, with the electric field at right angles to the magnetic field, may not be the best model to use. I prefer the stream of vibrating balls as a more instructive model.

    Electromagnetic radiation (photons) originates as excess energy that electrons emit. It consists of discrete packets of energy.

    Just to add confusion to the subject, an electric light powered by DC electricity, as in a flashlight, produces electromagnetic radiation (light) without a changing electric field. It all comes down to shedding excess energy.
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