# Electromagnetic Forces on Charged Particles

Q:
Can you use electro-magnetism to slow down and speed up the atoms around the magnet? How slow will they go? Is there a tempeture change in the atoms when they slow down?
- Charlie (age 42)
A:
A magnetic field has the curious property of changing the direction of a moving charged particle but not  its overall speed.   In a region where there is a constant magnetic field the path of a charged particle will be a circle!   An electric field on the other hand can change the speed of a charged particle.   Depending on the relative algebraic signs of the charge and the electric field you can either accelerate or decelerate the charged particle.  If the particle, or atom, has no net charge then both magnetic and electric forces will be zero.

LeeH

(published on 10/22/2007)

## Follow-Up #1: Electromagnetic forces on charged particles

Q:
So if you can change the speed of the charged particle is it possible to stop an atom electro-magneticaly? Is there an anti-photon? Is there a recommended book to read on this?
- charlie (age 42)
A:
Welcome back Charlie,
If the atom has an electric charge, for example if it is ionized, then according to the laws of electricity and magnetism one can in principle bring it to a stop. Otherwise, no.  But, you have to consider that there are other forces involved, for example thermal excitation, that can prevent a complete halt to all motion.

According to the standard lore the photon is its own anti-particle.  That means, for example, the following reaction could take place...
photon + photon  -->  electron + positron.
In fact a similar reaction has been observed at the SLAC linear accelerator at Stanford.

For recommended reading...  any first year college course textbook on Electricity and Magnetism would be fine. Unfortunately they are terribly expensive.   There is a raft of web sites that contain good elementary explanations of E and M processes.   I Googled "electricity and magnetism"  and came up with a million hits.
One of the first was
This is a nice little site with lots of elementary explanations.   Try investigating yourself.   Regards,

LeeH

(published on 10/22/2007)

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