Movement of charged particles in a magnetic field-physics problem

Question

I found this interesting physics problem on the internet:

An electron-positron pair is created from a photon with an energy of 2 MeV passing by a nucleus with a high atomic number. In the Wilson cloud chamber placed in a magnetic field, both particles move in the same plane, in a circular arc with a radius of 7 cm.

What is the magnitude of the magnetic induction vector?

(The heavy atomic nucleus takes up only momentum, almost no energy.)

Thank you for any help!

Answer 1

Hi Ian B.,

So electron rest mass-energy =0.511 MeV, the pair has 1.022 MeV as rest mass-energy. Looks like they want you to convert that to kinetic energy of the formed pair -- don't forget the relativistic mass is operative. Once you have velocities, the rest is just current / magnetic flux, etc. calculation. Neat that they add! And I guess you can calculate the magnetic field that's directing them as well.

1. Cheers, --Mr. d.