A particle of charge q and rest mass m is moving with velocity v where the magnetic field is B. Here B is perpendicular to v, and there is no electric field. Show that the path of the particle is a curve with radius of curvature R given by R = p/qB, where p is the momentum of the particle, p = mv. (Hint: Note that the force qv × B can only change the direction of the momentum, not the magnitude. By what angle Δθ is the direction of p changed in a short time?)