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I might have asked this to many times but there just does not seem a good understandable answer about this.

From the following Link

A charge moving at constant velocity also cannot produce electromagnetic waves as the constant motion means that there is no change in the electric and magnetic field of the moving charge.

if charge moves in a constant speed, there is definately electric and magnetic fields changing at each point due to distance between the charge and each point increasing or decreasing so we got time varying electric and magnetic field at each point in space.

Why then does the quote say that there is no change ? I am saying it is. Maybe observer should not be considered as a point in space but then who should he considered observer ?

Giorgi Lagidze
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3 Answers3

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You are getting caught up in the difficulty of using words to express the concept.

Under no circumstance are the people who are telling you the correct information are saying that the E and B fields are invariant in time, because as you have correctly noted, the E and B fields everywhere will be changing with time as the charge passes by in its constant velocity.

The statement, however, is that such a changing E and B field do not make electromagnetic waves.

One needs accelerating charges to get electromagnetic waves. Constant velocity is zero acceleration, and so there will not be electromagnetic waves, no kinks.

The obvious way to see that constant velocity will not get you electromagnetic waves is to do a Lorentz transformation to make the charge stationary. Then it will be a pure E field that is time-invariant, and so obviously no electromagnetic waves.

You can work through the horrendous mathematics to get the Lorentz transform of the fields to get you the correct answer, or you can think of things in any form of relativity (even Galilean, even though that cannot get the maths down correct). Or any other argument that will convince you.

naturallyInconsistent
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The question in the link asks about electromagnetic waves, not the one time change of an electric field at some place, wich will occur, if a charge particle moves with constant speed moves along one line relative to an observer.

trula
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One key difference between water and EM waves is there is a medium for water waves. Your finger has a velocity with respect to the water. For EM, there is no medium.

Suppose you see waves from a charge moving at constant velocity. If you run alongside a moving charge at constant velocity, you see the same waves. That would mean you get waves from a charge at rest.

This shows that you don't get waves. But it isn't very satisfying. Perhaps a better answer might be an elaboration on "because special relativity."

mmesser314
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