6.3 Consider a system of N free particles in which the energy of each particle can assume two and only two distinct values, 0 and E (E> 0). Denote by no and n the occupation numbers of the energy level 0 and E, respectively. The total energy of the system is U (a) Find the entropy of such a system. (b) Find the most probable values of no and n' and find the mean square fluctuations of these quantities. (c) Find the temperature as a function of U, and show that it can be negative. (d) What happens when a system of negative temperature is allowed to exchange heat with a system of positive temperature