Editing Heat capacity (section)

===Heterogeneous objects===
The heat capacity may be well-defined even for heterogeneous objects, with separate parts made of different materials; such as an [[electric motor]], a [[crucible]] with some metal, or a whole building.  In many cases, the (isobaric) heat capacity of such objects can be computed by simply adding together the (isobaric) heat capacities of the individual parts.

However, this computation is valid only when all parts of the object are at the same external pressure before and after the measurement. That may not be possible in some cases.  For example, when heating an amount of gas in an elastic container, its volume ''and pressure'' will both increase, even if the atmospheric pressure outside the container is kept constant. Therefore, the effective heat capacity of the gas, in that situation, will have a value intermediate between its isobaric and isochoric capacities <math>C_p</math> and <math>C_V</math>.

For complex [[thermodynamic system]]s with several interacting parts and [[state variables]], or for measurement conditions that are neither constant pressure nor constant volume, or for situations where the temperature is significantly non-uniform, the simple definitions of heat capacity above are not useful or even meaningful. The heat energy that is supplied may end up as [[kinetic energy]] (energy of motion) and [[potential energy]] (energy stored in force fields), both at macroscopic and atomic scales.  Then the change in temperature will depend on the particular path that the system followed through its [[phase space]] between the initial and final states.  Namely, one must somehow specify how the positions, velocities, pressures, volumes, etc. changed between the initial and final states; and use the general tools of [[thermodynamics]] to predict the system's reaction to a small energy input. The "constant volume" and "constant pressure" heating modes are just two among infinitely many paths that a simple homogeneous system can follow.