1. The problem statement, all variables and given/known data hello everyone. I have to derive the classical Dulong Petit law by using kinetic theory of gases and the equipartition of energy. The heat capacity at a constant V for simple crystalline solid. So, it should be explained without the quantum theory. 2. Relevant equations Cv (heat capacity at constant v) = 3R = 3Nak (avogadro Na and k of boltmann's constant) this is what I want to derive. 3. The attempt at a solution the formula Cv = Cp - R is what I have derived from simple formula's like dU = dQ + dW. If Cp = 4R then Cv = 3R, but why should this be the case? I know it has something to do with the degrees of freedom. I think the solid at high temperature has 3 translational degrees of freedom and 3 vibrational degrees of freedom which would mean it has 6 degrees of freedom. How should I use this? according to the equipartition theorem the energy of the degree of freedom is kT/2 thus with 6 degrees of freedom: E=Na6(kt/2)=3RT (because R=Nak) so now I have an energy which is equal to 3RT didn't the classical physics (around 1819) predict that the heat capacity was independent from the temperature (by only allowing the body to absorb very small amounts of heat)? clearly I'm missing some vital next steps. can anybody help me? Thank you!