# The Electron - Size of Magnetic field vs Size of Electric field

1. Dec 13, 2005

### Buckeye

What is the radius or effective scattering cross-section of the magnetic field of a free electron and what is the radius or effective scattering cross-section of the electric field of a free electron? Admittedly the two fields are different aspects of the EM field of the electron.

2. Dec 13, 2005

### Norman

How do you seperate these two cross sections in an experiment? A scattering cross section needs a reaction, so what would your reacton be or in other words, what is being scattered and by what? I am not sure I understand what you are asking and if I do, then your question may be ill-posed.

Are you trying to get a feel for the relative strength between them?

3. Dec 13, 2005

### Buckeye

Good question. I'd love to know as well. Bohm (QT book) wrote that the magnetic field is roughly 100X smaller than the electric field, but I did not read if that was experimentally measured or estimated from a calculation.
Sorry, but that's the best way I know how to put it.
Actually, I'm interested in the effective volume and the radial density of both. Does that help?

4. Dec 13, 2005

### Norman

Ok, now we are getting somewhere. I think you are asking basically an classical electro/magnetostatics problem.

I will work it out and post what I get later tonight ok.
Cheers,
Ryan

5. Dec 13, 2005

### Norman

By the way, the statement about the cross sections in completely meaningless since you must have some sort of scattering event to define the cross section. You are just looking for the ratio of the energy densities. Which will tell you something about their relative strengths.

6. Dec 13, 2005

### Norman

at first glance this seems like it is very wrong. The magnetic field of the electron is due to the magnetic moment from the spin. Spin is a QM effect and therefore should be on the order of hbar which is very, very small. While the electric field is due to electrostatics and should be large when compared to hbar.

I would suggest openning your copy of Jackson and using the magnetic moment from spin to compute the magnetic energy density and then use standard electrostatics to compute the electric energy density. It should be very straight forward.

I will do it tonight or tomorrow morning.
Maybe someone else can weight in on this and see if my intuition is correct.
Cheers,
Ryan