A system has two energy levels: 0 and epsilon = 2.0 x 10^-21 J. At T = 300 K: (a) compute Z; (b) find the probability of the upper level; (c) find the average energy. Solution: kT = (1.38x10^-23)(300) = 4.14 x 10^-21 J. epsilon/kT = 0.483. (a) Z = 1 + e^(-0.483) = 1 + 0.617 = 1.617. (b) P_upper = e^(-0.483)/Z = 0.617/1.617 = 0.382. (c) <E> = (0 x 1 + epsilon x 0.617)/Z = (2.0x10^-21)(0.617)/1.617 = 7.63 x 10^-22 J.