Magnitude of the Electric Field?

Question

An electron accelerated from rest through a voltage of 410 V enters a region of constant magnetic field. If the electron follows a circular path with a radius of 17 cm, what is the magnitude of the magnetic field?

I'm using r = mv/qB and I am trying to find B. I know r, m, and q, but not sure what v would be here.

Thanks Doctor Q!

I got B = 4.02 E-4 T and the back of the book says B = 0.40 mT so looks like I got it!

Answer 1

You need to work out the speed (or velocity) of your electron by using-;

qE = 1/2 mv^2

(1.6 x 10^-19) x 410 = 1/2 x (9.11 x 10^-31) x v^2

V^2 = (410 x (1.6 x 10^-19)) / (1/2 x (9.11 x 10^-31))

Then just square root the v^2 to give the velocity - just a calculator job...

Then you can use your original equation to give you the path radius..