Magnetic field for electron in cathode ray tube at 0.5c

[acsis]

Homework Statement

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)

Homework Equations

r= (mv)/qB

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.
and radius = 30m

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?

 

[rl.bhat]

Your calculation is wrong. Check it.

 

[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.

 

[rl.bhat]

Yes. It is correct.

 

[acsis]

Thanks, now I know those baby steps helped.

 

[flyway]

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.

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