Consider an electron in a hydrogen atom.
(a) Show that if the electron is in the ground state, then necessarily ⟨pe⟩=0. No calculation allowed.
(b) Show that if the electron is in an n=2 state, then ⟨pe⟩ need not vanish. Give an example of a wave function for the energy level n=2 that has a non-vanishing ⟨pe⟩ and compute ⟨pe⟩ for this state.