I am studying for a thermo exam, and one of the problems I am doing deals with adiabatic expansion of a piston in a cylinder. When solving for work, the solution guide uses m*Cv*(T2-T1). I don't understand why they know how to use Cv instead of Cp. The pressure changes, so obviously you wouldn't use Cp, but isn't there also a change of volume since it is adiabatic expansion? It would seem that nether Cv nor Cv would be valid. What am I misinterpreting?