# Homework Help: Photons in a constant field

1. Dec 13, 2007

### alvaros

1. The problem statement, all variables and given/known data
If you move a charge right-left left-right .. you produce an electromagnetic field. This electromagnetic field is made up of photons
But if the charge is at rest or the charge is moving at constant velocity
are there any photons anywhere ?

2. Relevant equations
E = h . f ??

3. The attempt at a solution
None

2. Dec 13, 2007

### Dick

There might be. But they wouldn't be related to the presence of the charge. It's not radiating.

3. Dec 14, 2007

### alvaros

There is a delay between the position of the Xcharge and the force exerted in the Ycharge because they are moving. I mean when one charge moves the other charge doesnt feel the decrease of the B field until a time t = d / c.

The ( change of ) field moves at c and "carry momentum" but its not made up of photons.
True ?

4. Dec 16, 2007

### alvaros

I dont think this is a silly question, do you ?

5. Dec 16, 2007

### Dick

Not particularly, but I'm having trouble seeing what the exact question is. Sure a disturbance in the field propagates at speed c, and you can think of that disturbance as 'photons'. But a uniformly moving charge doesn't generate any 'disturbances'.

6. Dec 17, 2007

### alvaros

Dick:
a) lot of questions b) I dont know either what the exact question is

If the fields move at c, then momentum moves at c, so momentum is conserved just once the field has moved, after a delay,
like one photon hitting a charge, the photon gets momentum when its emitted and gives momentum when its collides with the charge
but in this case,
there are no photons.

Dont you need QM in order to calculate momentum ? Its momentum continuous ?

If any question have any sense...

7. Dec 17, 2007

### Dick

You don't need QM to understand the problem. It's classical EM. The charges create a propagating field which can carry momentum (radiation) only when they accelerate.

8. Dec 18, 2007

### alvaros

Thanks. I must know first what the question is. Im very confused.