# Homework Help: Alpha particles

1. Jul 23, 2006

### whoknows12345

An alpha particle (the nucleus of a helium atom) consists of two protons and two neutrons. A horizontal beam of alpha particles is injected with a speed of 2.0 105 m/s into a region with a vertical magnetic field of magnitude 0.113 T.

(a) How long does it take for an alpha particle to move halfway through a complete circle?

I am trying to apply this equation:

T = 2pieM/qB

and I thought about it, do I use the speed as my q and for m, find the mass of (two protons and two neutrons) for my m?

2. Jul 23, 2006

### Andrew Mason

Rather than trying to apply an equation why not start by telling us what physics law applies here.

The T is the period of the circular motion, m is the mass, q is the charge, and B is the magnetic field. Look up the mass of a helium atom and use that (the electrons do not add much mass). What is the charge of a helium nucleus?

AM

3. Jul 23, 2006

### whoknows12345

i understand that, but when do I actually use my speed given to find the answer?

4. Jul 23, 2006

### Andrew Mason

What is the relationship between speed and magnetic force on a moving charge? That is what you have to use. You then have to relate that to circular motion to find the radius of the circular path. That will give you the distance the particle has to move through.

AM