# Homework Help: AP mag. problem

1. Nov 16, 2004

### Physicsisfun2005

A 2.0 keV positron is projected into a uniform magnetic field B of 0.10 T with its velocity vector making an angle of 89 degrees with B. Find
a. the period
b. the pitch p
c. the radius of the helix

We have never done a problem similar to this in class so i'm am not sure where to start........

2. Nov 16, 2004

### dduardo

Staff Emeritus
Here's a hint:

$$\v{F} = Q ( \v{E} + \v{u} x \v{B} ) = \frac{m v^2}{r}$$

Last edited: Nov 16, 2004
3. Nov 16, 2004

### Chi Meson

The 2.0 keV is the kinetic energy of the particle, from which you can get the speed (but remember to convert to joules).

4. Nov 16, 2004

### Physicsisfun2005

ahhh.........mag force = centripital force

5. Nov 17, 2004

### Chi Meson

Oui!
Ja!
Yes!

6. Nov 17, 2004

"pitch"?

7. Nov 17, 2004

### Chi Meson

If you look at a spring that is hanging vertically, you can see that it winds "up" to the right or to the left with a certain angle to the horizontal. This angle is the pitch. THe path of a charged object travelling through a magnetic filed will also be a helix (like a spring).

8. Nov 17, 2004

### Physicsisfun2005

we went over it in class and i got the correct answers

if u wanna know here they are:

a. period is 3.6e-10 sec.
b.pitch is 1.63e-4 m
c.radius is 1.48e10-3 m

Was easy once I figured that i needed to resolve the velocity vector in horizontal and vertical components :rofl: