Are photons present in a constant electromagnetic field?

[alvaros]

Homework Statement

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 ?

Homework Equations

E = h . f ??

The Attempt at a Solution

None

 

[Dick]

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

 

[alvaros]

This comes from another thread:

Consider two identical charges moving along the x and y axes with same speed away from the origin. The electrical forces between them is repulsive, but now just find out how the magnetic forces between them behave. You don't have to calculate -- just roughly think of the charges as currents and find the direction of the associated magnetic fields like we do for currents.

The total force of one on the other is equal to the force of the other on the former, but they are not opposite.

Ultimately, momentum will be conserved, because the fields themselves carry momentum.

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 doesn't 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 ?

 

[alvaros]

I don't think this is a silly question, do you ?

 

[Dick]

I don't think this is a silly question, do you ?

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'.

 

[alvaros]

Dick:

Not particularly, but I'm having trouble seeing what the exact question is

a) lot of questions b) I don't know either what the exact question is

Ultimately, momentum will be conserved, because the fields themselves carry momentum.

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,

Consider two identical charges moving along the x and y axes with same speed away from the origin.

there are no photons.

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

If any question have any sense...

 

[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.

 

[alvaros]

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