# Magnetic Fields question?`

1. Jul 18, 2011

### mayodt

Where did the formula r=mv/Bq come from in my answerbook for my grade 12 physics? Thanks.

2. Jul 18, 2011

### Staff: Mentor

Do you know the equation for uniform circular motion, given a force F that always points toward the center of the circular motion?

Do you know the equation for the force F on a charged particle q when it is travelling at velocity v perpendicular to a magnetic field B?

3. Jul 18, 2011

### mayodt

F=qvBsin(theta) is the equation right? and, I probably have seen the first one, but can't remember that specific one.

4. Jul 18, 2011

### Staff: Mentor

Correct, but what does it simplify to when the velocity and B-field are orthogonal (at right angles to each other)?

See Uniform Circular Motion at wikipedia, and remember that F=ma:

http://en.wikipedia.org/wiki/Uniform_circular_motion

.

5. Jul 18, 2011

### mayodt

Ohh okay I got it. Because the force of the magnetic field equals the net force (F=ma) so that means that qvb=ma and then since v=rw and a=vw it's qrwB=mvw, and then the w's cancel out and you rearrange. Thanks alot

6. Jul 18, 2011

Good job!