# An electron shot from the back of a T.V. help?

1. Apr 2, 2009

### acsis

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

If I shoot an electron from the back of a TV at a screen towards the front at .5c (e= speed of light) What magnetic field must be in the coils around the cathode ray tube (picture tube) if the electron would have made a circle radius 30m if it hadn't hit the screen and made it glow. (your TV show)

2. Relevant equations

r= (mv)/qB

3. The attempt at a solution

Using the equation rearranged to solve for B:

B=mv/qr

mass of an electron = 9.1 x 10^-31
charge of an electron = 1.602 x 10^-19
.5c=....well it's half of the spd of light so I assume : 1.5 x 10^8 since the spd of light is 3x10^8 if I remember correctly.

so:

B= (9.1x10^-31)(1.5x10^8)/(1.602x10^-19)(30)
B= .026?

Could someone check if I did this correctly, did I forget something?

2. Apr 2, 2009

### rl.bhat

Your calculation is wrong. Check it.

3. Apr 2, 2009

### acsis

Hm.

Now I'm getting

B = 3.06x10^-5

Is that correct?

I tried taking baby steps to see what I did incorrectly but I'm not sure if it's helping.

4. Apr 3, 2009

### rl.bhat

Yes. It is correct.

5. Apr 3, 2009

### acsis

Thanks, now I know those baby steps helped.

6. Apr 3, 2009

### flyway

Have checked those calculations and got 2.8402x10^-5... :uhh: