The aurora is caused when electrons and protons, moving in the earth's magnetic field of 5x10^-5 T , collide with molecules of the atmosphere and cause them to glow. What is the radius of the circular orbit for (a) an electron with speed 1.0×10^6 and (b) a proton with speed 3.0×10^4 .
What kind of equation or background information is useful for a problem like this?