Sources of electromagnetic waves / photons

[hobz]

I have questions. Questions that I have been trying to figure out for myself, but now I have realized that I need professional help.

 

An electron has a charge, and is a source of an electric field.

A moving charge has the property that it is a source of a magnetic field.

What should the charge be moving in relation to, in order to consider it a "moving" charge? Are not all charges moving in some sense, and thus be affected by all other charges however slightly?

 

I set up a measuring device, at some point in space, that can measure electric force as well as the magnetic force, so that I can measure moving charges (as well as non-moving).

If I shoot one charge in some direction, so that it is moving at a constant velocity, then would my measuring device measure an impulse in the electromagnetic field?

 

Accelerating a charge gives rise to a constant oscillating electromagnetic wave. Can this be interpreted as a series of continuous impulses (as described above)?

 

Do electromagnetic waves, created by an accelerating charge, propagate spherically (e.g. from the origin of an radio antenna)?

If I accelerate the charges (e.g. in an antenna) frequently enough, say to make the electromagnetic waves have a wavelength of visible light, would people all around the antenna be able to see this light?

I have read that the human eye can detect photons, which suggest that if the answer to the question above is: yes, then the electromagnetic waves have the property of interaction with the cells in eyes via photons.

But since the charges in my antenna are accelerated using limited energy (as opposed to unlimited energy), then if I crowd the space around the antenna with eyes (or solar cells), then would I not be able to detect photons in every eye/solar cell, since the electromagnetic wave is (assumed by me) omnipresent? And if so, there would be an unlimited number of photons caused by limited amount of energy, which really tells me something is wrong.

 

I cannot seem to figure out how photons can have a direction, if they have a wave interpretation. Which brings me to my next question.

Do photons have a wave interpretation? I imagine the electromagnetic field to be a large carpet, and a photon to be a beetle crawling (in a precise direction) below that carpet.

 

Can photons only be created by-- and related to-- quantum leaps in excited atoms?

[thedarkshade]

What should the charge be moving in relation to, in order to consider it a "moving" charge? Are not all charges moving in some sense, and thus be affected by all other charges however slightly?

I'm not sure if I really understand here, but there is a difference between electrostatic and electricity flow

[swansont]

Yes, all free charges are moving, and they can be accelerated and radiate — that's what happens with an impulse to the electric & magnetic field. Bound electrons do not radiate unless they are making a transition between states.

 

An oscillating dipole radiates in a non-uniform pattern (called, not surprisingly, a dipole pattern) whose strength goes to zero along the dipole axis. In general, the radiation pattern depends on the acceleration.

 

In the situation where you had a large number of detectors, you would detect only as many photons as were present; i.e. E = nhf, where the photon energy is hf, and you have n photons. If you had more than n detectors, then some would not detect a photon.

 

You can get photons from the quantum jumps in atoms and nuclei, as well as accelerating a charge (bremsstrahlung) and in matter/antimatter annihilation.

[hobz]

thedarkshade: Thanks for your reply. swansont answered this. What I meant was: do non-moving charges exist?

 

swansont: Thank you very much. Your answer suggests that the wave in the electromagnetic field is pure abstraction, and what is "real", is how the wave interacts with the detectors; through the photon.

Is this correct?

 

If not these detectors were photon detectors, but rather Coulomb force detectors, then as the field stars oscillating, so would the charges inside the detectors (thus allowing me to measure the force of their "movement").

Now all these charges are oscillating around the antenna, and creating their own oscillations in the electromagnetic field. Will these oscillations oppose that of the antenna, and if so, why? Would it be like oscillating a cork in a pool of water, and when the waves from the edge of the pool are reflected, and superpositions with the cork-wave, then the it would become harder to oscillate the cork?

[swansont]

I wouldn't say that the field is pure abstraction, because you can have interactions that depend on the field. An antenna is an example — the classical view works just fine, with the electrons responding to an oscillating field.

[hobz]

I see.

Does the response of the electrons depend on the number of electron that respond (are present)?

That is, if I have 10 electrons at the same radius from the antenna, do they experience a force 10 times weaker that if there were only one electron? And is this due to a sort of electromagnetic inductance caused by the 10 electrons?