Question

Consider N harmonic oscillators with coordinates and momenta (qP₁), and subject to the Hamiltonian q H(q₁P₁) = -22-21 Σ 2m i=1 (a) Calculate the entropy S as function of the total energy E. (Hint. By appropriate change of scale the surface of constant energy can be deformed into a sphere. You may then ignore the difference between the surface area and volume for N >> 1. A more elegant method is to implement the deformation by a canonical transformation.) (b) Calculate the energy E, and heat capacity C, as functions of temperature 7, and N. (c) Find the joint probability density P(q.p) for a single oscillator. Hence calculate the mean kinetic energy, and mean potential energy, for each oscillator.

Answer 1

(a) Entropy S as function of total energy E:The Hamiltonian of the system can be written as,H= Σ[½p²/ m + ½ω²q²m]where ω = (k/m)1/2 is the angular frequency of the oscillator, and k is the spring constant.The entropy S can be calculated as:S = k_B ln Ωwhere k_B is the Boltzmann constant, and Ω is the number of states for the system at a given energy E. For N harmonic oscillators, Ω can be written as:Ω = [V/(2πh³)^N] ∫ d³q d³p exp(-H/k_BT)where V is the volume of the system, and h is the Planck constant. Now, we can write the Hamiltonian in terms of the new variables, Q and P, as:H = Σ{[P²/2m + mω²Q²/2]}.Since the Hamiltonian is separable in terms of the new variables, we can write the partition function as,Z = [∫ d³Q d³P exp(-H/k_BT)]^N= [∫ d³Q d³P exp(-P²/2mk_BT)]^N [∫ d³Q d³P exp(-mω²Q²/2k_BT)]^N= Z_P^N Z_Q^Nwhere,Z_P = [∫ d³P exp(-P²/2mk_BT)] = (2πmk_BT)^-3/2V_Pand,Z_Q = [∫ d³Q exp(-mω²Q²/2k_BT)] = (2πk_BT/mω²)^-3/2V_QThe total energy of the system can be written as,E = Σ[P²/2m + mω²Q²/2].From the above equations, the partition function can be written as,Z = Z_P^N Z_Q^N= [V_P/(2πmk_BT)]^(3N/2) [V_Q/(2πk_BT/mω²)]^(3N/2)= [V/(2πmk_BT/mω²)]^(3N/2)where,V = V_P V_Q = (2πmk_BT/mω²)^3/2.The entropy can now be calculated as:S = k_B ln Ω= k_B ln Z + k_B (3N/2) ln [V/(2πh³)]- k_B (3N/2)= k_B ln Z + (3N/2) ln (V/N) + (3N/2) ln (2πmk_BT/mω²h²)For large values of N, the surface area of the sphere can be approximated by its volume. Therefore, we can write the entropy as:S = k_B ln Ω= k_B ln Z + (3N/2) ln V + (3N/2) ln (2πmk_BT/mω²h²) - (3N/2) ln N(b) Energy E, and Heat capacity C as functions of temperature T, and N:The energy E can be written as,E = - ∂(ln Z)/∂(β)where β = 1/k_BT is the inverse temperature. Therefore,E = 3Nk_BT/2and,C_V = (∂E/∂T) = 3Nk_B/2(c) Joint probability density P(q.p) for a single oscillator:The joint probability density can be written as,P(q,p) = exp[-βH]/Zwhere Z is the partition function, which has already been calculated in part (a). The mean kinetic energy of the oscillator can be written as,K = ½= ½(ω²)= E/3N= k_BT/2where is the mean squared displacement of the oscillator, and the mean potential energy of the oscillator is given by,U = <½kx²> = ½ω² = E/3N= k_BT/2.