I have a few questions about the time dependence of energy and probability etc. of systems. Firstly... Say I have a particle in an infinite 1-D well. I can work out the general wave function as a Fourier sum of the orthogonal sine functions. Hence, the average/expectation value of the energy is : < H > = SUM (C_n)^2 (E_n) where C_n are the Fourier coefficients and E_n is the energy of the particle in the nth state. To me, i cant see how the energy here is dependent on time. I have seen a question in a textbook asking for the average energy at say time t=t_0. How would one approach this? Am I missing something here?