# Photon striking charged particle

1. Sep 10, 2004

### Rothiemurchus

How is the force that a photon exerts on a charged particle calculated?

2. Sep 10, 2004

### marlon

What exactly do you mean, here?

The foton is the force carrier of the elektromagnetic-interaction. I mean if two charged particles interact then this interaction is mediated via the exchange of photons. how many photons that are active in the interaction is determined by conservation laws like spin balance or energy conservation.

These interactions are described by the Feynmanndiagrams of QED.
The sterngth of the interaction between matter-particles is expressed by the corresponding potential...

regards
marlon

3. Sep 10, 2004

### Rothiemurchus

If I had a large sheet of paper and shone a torch on it then what would be the force on that paper due to scattering?

4. Sep 10, 2004

### humanino

Yes a force due to radiation pressure to be precise.

5. Sep 10, 2004

### Rothiemurchus

A force/unit area (pressure) due to radiation over a large area is given by
energy density / speed of light

This could not be applied to an electron for example because the electron does not have an area and so energy density on its surface would be meaningless.
Is that a correct interpretation of the situation for a single electron?

6. Sep 10, 2004

### humanino

Yes that seems perfectly correct to me.

7. Sep 11, 2004

### marlon

Looks ok to me too.

8. Sep 12, 2004

### Gonzolo

For a single electron, you can think in terms of a EM wave instead of a photon and use the Lorentz force :

$$F = qE + qvB$$

It can be shown with this that with complete absorption, the momentum transfered to the electron is :

$$p = W/c$$

With this, we can take the particle view and say the force on the electron :

$$F = p/t = W/ct$$

where t is the time it took for the electron to go from 0 to whatever speed it was given using :

$$W = K = \frac{1}{2}mv^2$$

Last edited by a moderator: Sep 12, 2004