Problem 4.23 (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: d (L) = (N), dt where N=rx(−VV). (Thisis the rotational analog to Ehrenfest's theorem.) (b) Show that d (L)/dt: O for any spherically symmetric potential. (This is = one form of the quantum statement of conservation of angular momentum.)