Electron Radius in a Bending Magnet

[mh1551]

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.

 

[vela]

With an energy of 15 MeV, the electrons are clearly relativistic. The formula you're using is only valid in the non-relativistic regime.

 

[mh1551]

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

Thanks!

 

[vela]

That's something you should be able to derive on your own or at least find on your own in your textbook or notes.

 

[mh1551]

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