# Proton orbit direction

1. Oct 4, 2011

[Solved] Proton orbit direction

1. The problem statement, all variables and given/known data

A solenoid has a magnetic field in the +z direction. A proton in the center of the solenoid moves towards the sides sides of the solenoid. Will this proton move with a clockwise or counterclockwise motion?

2. Relevant equations

F = qv x B
B in a solenoid = (u0)(n)(I)

3. The attempt at a solution

I have no idea how to start this problem.

Last edited: Oct 4, 2011
2. Oct 4, 2011

### davo789

Are you aware of the right hand rule?

3. Oct 4, 2011

Which rule, the one for the cross product (F=qv*B) or the screw rule (magnetic field direction around a current-carrying wire)?

4. Oct 4, 2011

### davo789

For the cross product

5. Oct 4, 2011

Oh, as I move out from the center, that's where I get v from... B is known, so F tells me the direction around the center. Got it, thanks.

6. Oct 4, 2011

### davo789

Yep :-) What direction did you get, out of interest?

7. Oct 4, 2011

Looking from the origin down +z: if v moves left and B moves into the page, F goes down. This is counterclockwise. Alternatively, if v is moving right, the force on it pushes it up, so it's counterclockwise.

But if I look from somewhere down the +z axis towards the origin, it appears to spin in the opposite direction even though the forces at those points are the same.

8. Oct 4, 2011

### davo789

Usually questions like this are asking if you are looking down onto it (if the proton is at the origin, you are at +z). Also, I thought the question says that the B field is is in a +z direction?

9. Oct 4, 2011

If I'm at the origin and I look down the +z axis, B appears to go in to the page. I wrote both answers, one is if I'm at the origin and the proton is further along the z axis. The second is when I'm somewhere down the z axis looking in the opposite direction as the first case, and the proton is at the origin.

10. Oct 4, 2011

### davo789

Now I see! That seems fine.

11. Oct 4, 2011

Great, that's done.