(a) Write the expression for the magnetic force acting on a charged particle moving with velocity v in the presence of a magnetic field B. (b) A neutron, an electron and an alpha particle moving with equal velocities enter a uniform magnetic field going into the plane of the paper. Trace their paths in the field and justify your answer.
Solution
(a) F=q(v×B), magnitude qvBsinθ. (b) \textbf{Neutron:} q=0⇒F=0, so it continues in a straight line. \textbf{Electron} (negative) and \textbf{α-particle} (positive) feel forces in opposite directions, so they curve to opposite sides. The radius r=qBmv: the heavy α-particle (large m/q) describes a much larger circle than the light electron, which curves sharply.
