# Period of a Proton's Orbit arount magnetic field

1. Oct 2, 2014

### Chase11

1. The problem statement, all variables and given/known data
A proton moves perpendicular to a magnetic field of .005 T. What is the period of the proton's circular orbit?

2. Relevant equations
1. T=2pi/v
2. FB=qv x B

3. The attempt at a solution
I know I need to solve for velocity first so I can use the period equation, but I don't understand how I can do that without being given the force. If I knew the force, I could solve for v using equation 2 and then plug that value in to equation 1 to find the period.

2. Oct 2, 2014

### Simon Bridge

What sort of force is associated with circular motion?
(note: eq. 1 is incorrect.)

Last edited: Oct 2, 2014
3. Oct 2, 2014

### Chase11

I always thought it was gravitational force, but I have never done a problem in which I need to find the period of a proton moving around a magnetic field. I can't find any other equations that I can use for the period, as I don't have enough information. Is gravitational force what you are referring to?

4. Oct 2, 2014

5. Oct 2, 2014

### Chase11

Okay so are you saying that I need to solve for the centripetal force, and that will be the force that I use in my equation 2? I'm still confused though, because the centripetal force formula requires me to know the velocity and the radius. I see why it would be a centripetal force, I just don't understand how I would be able to solve for that with the information I am given.

6. Oct 2, 2014

### Simon Bridge

How many forces are on the proton?

7. Oct 2, 2014

### Chase11

I honestly thought it was just the magnetic force, we've never done anything where I needed to know anything other than the magnetic field, velocity, and charge. Does gravity act as a force on a proton? And then you said it is like a centripetal force.

8. Oct 2, 2014

### Simon Bridge

The centripetal force applies to circular motion - lots of things go in circles: not just planets.
In this case, the magnetic force is to the proton what gravity is to the planet.

There is one force on the proton: that is the magnetic force.
Because this force makes the proton go in a circle, that force is also the centripetal force.
So how od you write that out mathematically?

Last edited: Oct 3, 2014
9. Oct 3, 2014

### Chase11

FB=qv x B= mv2/r

Is that right? I would still need the radius to be able to solve this.

10. Oct 3, 2014

### BvU

Keep going, maybe it cancels at some moment...

Remember

11. Oct 3, 2014

### Chase11

I got it thanks.

12. Oct 4, 2014

### Simon Bridge

Yes.
No you don't.
You don't know v or r, but you do know v/r (see the correct equation 1).