The aurora is caused when electrons and protons, moving in the earth's magnetic field of $5\mathrm{.}010^5$ T

$,$ collide with molecules of the atmosphere and cause them to glow. What is the radius of the circular orbit for an electron with speed $4\mathrm{.}010^6$ m$/$s

$?$ Assume that the electron moves in a plane perpendicular to the magnetic field. What is the radius of the circular orbit for a proton with speed $6\mathrm{.}010^4$ m$/$s

$?$