Solenoid and Proton Motion: Clockwise or Counterclockwise?

[Cade]

[Solved] Proton orbit direction

Homework Statement

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?

Homework Equations

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

The Attempt at a Solution

I have no idea how to start this problem.

 

[davo789]

Are you aware of the right hand rule?

 

[Cade]

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

 

[davo789]

For the cross product

 

[Cade]

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.

 

[davo789]

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

 

[Cade]

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.

 

[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?

 

[Cade]

If I'm at the origin and I look down the +z axis, B appears to go into 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.

 

[davo789]

Now I see! That seems fine.

 

[Cade]

Great, that's done.