Is forcing a closed system to expand (e.g. by pulling out a piston), causing it to cool, work done to the system or work done by the system? I assume it was work done to the system, but that means the first law of thermodynamics formula no longer balances if you assume an adiabatic change. With adaibatic, Q = 0, so: -w = + delta u. -w is equal to work done to the system, but delta u should be negative, not positive as the formula would make it. Any ideas what I'm doing wrong.