a) Prove that for a particle in a potential V(r), the rate of change of the expectation value of the orbital angular momentum L is equal to the expectation value of the torque: where d (L) = (N) dt N=rx(−VV) This is the rotational analogue to Ehrenfest's theorem. (1) (2) = b) Show that d(L)/dt 0 for any spherically symmetric potential. This is one form of the quantum statement of conservation of angular momentum.