Isothermal Expansion, Details Because it is an ideal gas and the process is quasi-static, the ideal gas law is valid for each point on the path. Numerically, the work equals the negative of the area under the PV diagram. Because the gas expands, Vf > Vi and the value of the work done on the gas is negative. If the gas is compressed, Vf < Vi and the value of the work done on the gas is positive.\

Adiabatic ▪ No heat exchanged ▪ Q = 0 and DEint = W

Isobaric ▪ Constant pressure ▪ W = P (Vf – Vi ) and DEint = Q + W

Isothermal ▪ Constant temperature ▪ DEint = 0 and Q = -W