Editing Specific heat capacity (section)

===Variations===
The injection of heat energy into a substance, besides raising its temperature, usually causes an increase in its volume and/or its pressure, depending on how the sample is confined. The choice made about the latter affects the measured specific heat capacity, even for the same starting pressure <math>p</math> and starting temperature <math>T</math>. Two particular choices are widely used:
* If the pressure is kept constant (for instance, at the ambient atmospheric pressure), and the sample is allowed to expand, the expansion generates [[work (thermodynamics)|work]], as the force from the pressure displaces the enclosure or the surrounding fluid. That work must come from the heat energy provided. The specific heat capacity thus obtained is said to be measured '''at constant pressure''' (or '''isobaric''') and is often denoted {{nowrap|<math>c_p</math>.}}
* On the other hand, if the expansion is prevented{{snd}} for example, by a sufficiently rigid enclosure or by increasing the external pressure to counteract the internal one{{snd}} no work is generated, and the heat energy that would have gone into it must instead contribute to the internal energy of the sample, including raising its temperature by an extra amount. The specific heat capacity obtained this way is said to be measured '''at constant volume''' (or '''isochoric''') and denoted {{nowrap|<math>c_V</math>.}}

The value of <math>c_V</math> is always less than the value of <math>c_p</math> for all fluids. This difference is particularly notable in gases where values under constant pressure are typically 30% to 66.7% greater than those at constant volume. Hence the [[heat capacity ratio]] of gases is typically between 1.3 and 1.67.<ref name=Lange>Lange's Handbook of Chemistry, 10th ed., page 1524.</ref>