# Electron Radius in a Bending Magnet

Problem:

15 MeV electrons enter a bending magnet with a magnetic field of .7 tesla. What is the radius of the path of electrons?

Equation:

r = mV/Bq

B = .7 tesla
q = 1.6 x 10^-19 C

My problem here is that I don't know how to find the velocity of the electron for this equation. The supposed answer to the problem is 7.4 cm = r, but I'm just not getting that.

## Answers and Replies

vela
Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
With an energy of 15 MeV, the electrons are clearly relativistic. The formula you're using is only valid in the non-relativistic regime.

Do you know which equation I should be using then? This problem has been stumping me for days.

Thanks!

vela
Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
That's something you should be able to derive on your own or at least find on your own in your textbook or notes.

I was told to use the equation above by my professor. That's why I'm confused now.