Isovolumetric piston

If the volume is constant, no work is performed.

One facet of the discussion of heat capacity provides a good example of a somewhat tricky point that has appeared occasionally in MCAT passages dealing with thermodynamics. In general, heat capacities, whether specific heat or molar heat capacity, describe the relationship for particular substances between heat flow and temperature change. In the First Law of Thermodynamics, we will be discussing the centrally important principle that heat flowing into a system will either change its internal energy or be used to perform work. A distinction can therefore arise in the discussion of heat capacities regarding whether or not the heat capacity describes a gas at constant volume versus one at constant pressure. At constant volume, no thermodynamic work can be performed, but, at constant pressure, work definitely must be performed, because, to maintain constant pressure, the system must be expanding as heat flows in. For this reason, the heat capacity of a gas at constant pressure is higher than at constant volume. More heat will flow in at constant pressure before a given temperature change occurs because the system will necessarily be using energy in the work of expanding its volume.