# Duration of electromagnetic disturbance

• I
Let us say I have a moving charge. At each point x,y,z in it's path from understanding there is a transverse electromagnetic wave being radiated (could also be viewed as a photon). The electric field at any point x1,y1,z1 in the path is disturbed. The moving charge does the same thing all along its path so many electromagnetic waves are created - an infinite amount, one at each point in the path. My question is about what happens to the wave/disturbance created at X1,Y1,Z1 after the charge has passed the point - let's say in some magical way the charge disappears infinitesimally right after X1,Y1, Z1 - there is no more moving charge. Let's also assume that the charge appeared magically an infinitesimal distance from X1,Y1, Z1. So the charge moved a very small distance and then disappeared. What is the fingerprint of the charge after it disappears in the vicinity of X1,Y1, Z1? I am assuming the disturbance spreads out over time from X1,Y1, Z1 - let's assume it's a vacuum that this is happening in. If I put a compass/charge close to X1,Y1, Z1 how long would it show the effect of the moving charge? How would it be calculated? What would happen to compasses/charges put further and further away from X1,Y1, Z1? Would the disturbance die down at all points - some points sooner than others, some points weaker effect than others? One zillion light year away would a super powerful detector of some kind detect the fact that the charge passed X1,Y1, Z1?

As always any feedback is appreciated in advance.

Staff Emeritus
I don't understand your question, but a) it is not true that motion is enough to produce radiation and b) any question that requires magic to pose is likely to be unsolvable without magic.

davenn
davenn
Gold Member
Let us say I have a moving charge. At each point x,y,z in it's path from understanding there is a transverse electromagnetic wave being radiated (could also be viewed as a photon).

As V50 hinted at ... a moving charge does not produce EM emission ...
see if you can google the reason why that is ?

Dave

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"Electromagnetic radiation, is a form of energy emitted bymoving charged particles. "

"When a charged particle—such as an electron, proton or ion—is in motion, magneticlines of force rotate around the particle. Since electrical current moving through a wire consists of electrons in motion, there is a magnetic field around the wire."

https://en.wikisource.org/wiki/Electromagnetic_effects_of_a_moving_charge

http://farside.ph.utexas.edu/teaching/em/lectures/node125.html
Fields due to a moving charge - Although the fields generated by a uniformly moving charge can be calculated from the expressions (1525) and (1526) for the potentials, it is simpler to calculate them from first principles. .....

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And from many other sources a changing magnetic field causes a changing electric field and changing electric field causes a changing magnetic field, ad infinitum.

All of the above seems to indicate that a moving charge produces some effect which I called electromagnetic waves. Maybe the effect is called something else. In any case I must be missing something as you seem to indicate that a moving charge is simply a moving charge - you make no reference to to waves, fields, etc. . Please elucidate what it is I am missing and once that is straightened out my question applies to whatever the effect is if it's not called electromagnetic wave What happens over time to whatever the charge does at a point in time?
[Mentors' note: This thread has been edited to remove an unnecessary digression]

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davenn
Gold Member
"Electromagnetic radiation, is a form of energy emitted bymoving charged particles. "

I have read through that link several times I don't read that specific quote in the link you gave and unfortunately, you missed the point
and you have written it, it is incorrect as it doesn't give the full story

A DC current through a wire will produce a electric and magnetic field that is static
EM radiation is generated by an accelerating charge ( one that is speeding up or slowing down, eg an AC current)

And from many other sources a changing magnetic field causes a changing electric field and changing electric field causes a changing magnetic field, ad infinitum.

that is only when the current flow ( charge is changing speed) is also changing

eg an AC current flow THEN you have a radiated EM wave

When the Electric and Magnetic fields are static, there is none of this ......

And from many other sources a changing magnetic field causes a changing electric field and changing electric field causes a changing magnetic field, ad infinitum

Dave

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Nugatory
Mentor
Google:And from many other sources a changing magnetic field causes a changing electric field

All of the above seems to indicate that a moving charge produces some effect which I called electromagnetic waves. Maybe the effect is called something else. In any case I must be missing something as you seem to indicate that a moving charge is simply a moving charge - you make no reference to to waves, fields, etc.
Electromagnetic waves are caused by accelerating charges, so a charge that is moving but not accelerating will not produce electromagnetic radiation. If not accelerating, the motion may or may not cause the electrical field to change and will produce some magnetic effects; but that's not electromagnetic waves.

Nugatory
Mentor
let's say in some magical way the charge disappears infinitesimally right after X1,Y1, Z1 - there is no more moving charge. Let's also assume that the charge appeared magically an infinitesimal distance from X1,Y1, Z1.
There is a problem here. You are asking what the laws of electromagnetism say will happen under these circumstances... but those laws don't allow charges to appear and disappear (which is why you had to invoke magic to pose the question). Thus, your question is pretty much equivalent to "I want to to apply the laws of physics when they don't apply" and that isn't going to lead to an illuminating discussion. (Don't be confused by the altogether respectable notion of "thought experiments". Thought experiments either start from physically possible conditions or they are stated in terms of "what would happen if I tried to [smething impossible]").

However, you should be able to find a way of reframing your question so that it has a sensible answer; thinking in terms of acceleration instead of velocity might be enough to get you there.

Nugatory/Davenn

Thanks for the clarification. Those are the kinds of answers I was expecting to get and most always have gotten on PF.

The articles I read prior to posting - many of them - did not make the critical distinction between accelerating and non-accelerating charges - you can see there is no reference to acceleration in any of the links I posted. So now I would very much appreciate it if 'moving charge' was replaced by 'accelerating charge' in my original post and the respective questions I asked about duration/extent of the effect near and far - time-wise and space-wise - be addressed for an accelerating charge.

Thanks in advance for any further guidance.

sophiecentaur
Gold Member
2020 Award
Those are the kinds of answers I was expecting to get and most always have gotten on PF.
I presume that you see that as a good reason for coming to PF?

I wonder if you are really asking about the actual shape of an electromagnetic pulse - as with the profile of the surface wave on water when a stone is dropped in? If the current were switched on (after ∞ delay) and then left on for ∞ then the time profile of the step waveform would be the same as the profile of the current waveform. If a real circuit were used then there would not be a simple step function at switch on and the edges of the pulse would be affected by the reactance of the circuit. (finite slope and possible 'ringing' at the edges)

Staff Emeritus