The internal energy of a system increases when its temperature is raised. This increase depends on the conditions under which the heating takes place. Suppose the system has a constant volume. If the internal energy is plotted against temperature, then a curve like that in Fig. 2A.9 may be obtained. The slope of the tangent to the curve at any temperature is called the heat capacity of the system at that temperature. The heat capacity at constant volume is denoted CVand isdefined formally as