EM Waves: Oscillation vs. Translational Motion

[Gear300]

I've already figured that oscillating charges produce electromagnetic waves...but if the charge was simply in translational motion through space, would that produce EM waves?

 

[granpa]

no..

 

[Gear300]

no??

 

[atyy]

If a charge is stationary and not radiating in reference frame R, then in any other reference frame S moving at constant velocity relative to R, the charge will be moving at a constant velocity, but not radiating. This is because radiation is light, and light has the same speed in R and S. So if it radiates S, it must radiate in R, contradicting our initial assumption.

A uniformly accelerating charge will radiate, since acceleration is absolute.

 

[Gear300]

I see...somewhat...thanks for the clarification

An additional question...lets say that the reference frame S was accelerating...how would this change?

 

[atyy]

An additional question...lets say that the reference frame S was accelerating...how would this change?

I don't know off the top of my head, Maxwell's equations look weird in accelerating frames. It's usually easier to do classical physics in inertial frames. In inertial frames, only accelerating charges radiate.

In quantum field theory, there is the "Unruh effect" in which an accelerating observer sees radiation due to his acceleration.

 

[Gear300]

I see...thanks

 

[olgranpappy]

I've already figured that oscillating charges produce electromagnetic waves...but if the charge was simply in translational motion through space, would that produce EM waves?

A charge in uniform motion through a medium (i.e., not through vacuum) actually can radiate. This is not in contradiction with the theory of relativity simply because in the presence of a medium there *is* a preferred frame (the frame in which the medium is at rest). Thus, if we try and "boost" such that the particle is at rest then the entire medium will be in motion.

An example of the above type of radiation is "Cherenkov radiation" (see PF library for more info). Also, a type of radiation called "transition radiation" can occur for a particle in uniform motion (but again this depends on the presence of a medium).

 

[Naty1]

A stationary charge has a stationary electromagnetic field surrounding it...You can tell because if you place another similar charge nearby, both stationary, they repel each other. The field is present regardless of motion, but what you observe changes with relative motion:

When relative velocity occurs, electromagnetic waves induce emf...If you move in the right way relative to a moving charge you may detect the magnetic portion of the field...or not...depending on whether there is relative motion..

For a stationary charge with a stationary observer: you see only an electric field. Start moving and you see only a magnetic field...motion tranforms between the two:

from http://en.wikipedia.org/wiki/Electromagnetic_field:

The field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field.

Reciprocal behavior of electric and magnetic fields
The two Maxwell equations, Faraday's Law and the Ampère-Maxwell Law, illustrate a very practical feature of the electromagnetic field. Faraday's Law may be stated roughly as 'a changing magnetic field creates an electric field'. This is the principle behind the electric generator.

The Ampère-Maxwell Law roughly states that 'a changing electric field creates a magnetic field'. Thus, this law can be applied to generate a magnetic field and run an electric motor.

 

[Dale]

A charge in uniform motion through a medium ... An example of the above type of radiation is "Cherenkov radiation" (see PF library for more info).

Umm, a particle emitting Cherenkov radiation is undergoing enormous acceleration as it slows down to below the speed of light relative to the medium.