# Statistical mechanics: Particles with spin

by SoggyBottoms
Tags: mechanics, particles, spin, statistical
 P: 61 1. The problem statement, all variables and given/known data We have N particles, each of which can either be spin-up ($s_i = 1$) or spin-down ($s_i = -1$) with $i = 1, 2, 3....N$. The particles are in fixed position, don't interact and because they are in a magnetic field with strength B, the energy of the system is given by: $$E(s_1, ...., s_n) = -mB \sum_{i=1}^{N} s_i$$ with m > 0 the magnetic moment of the particles. The temperature is T. a) Calculate the canonic partition function for N = 1 and the chance that this particle is in spin-up state $P_+$. b) For any N, calculate the number of microstates $\Omega(N)$, the Helmholtz free energy F(N,T) and the average energy per particle U(N, T)/N 3. The attempt at a solution a) $$Z_1 = e^{-\beta m B} + e^{\beta m B} = 2 \cosh{\beta m B}$$ $$P_+ = \frac{e^{-\beta m B}}{2 \cosh{\beta m B}}$$ b) The number of possible microstates is $\Omega(N) = 2^N$, correct? I know that $U = -\frac{\partial \ln Z}{\partial \beta}$, but I'm not sure how to calculate Z here.