heat capacity SI unit J.K^-1 If a quantity of heat D Q produces a change of temperature D T in a body, then heat capacity is defined as

heat capacity = D Q/ D T [J/K]

The quantity of heat D Q required to produce a change in temperature D T is proportional to the mass of the sample, m and difference in temperature D T.

D Q = cm D T (h4)

where c is a constant for a particular substance called the specific heat. The SI unit of specific heat is J.kg^-1.K^-1. Sometimes it is preferable to use molar specific heat which is the heat capacity per mole of a substance. In table HI the specific heat of some important materials are listed.

Table HI Specific heat at 20° C and 1 Atm.

Specific heat varies with temperature. When a substance transforms from one state to another, there is an abrupt change in the specific heat.

Temperature of a gas can be raised by two ways. First by keeping its volume constant, and second by keeping its pressure constant. Therefore for a gas we have to define two specific heats; specific heat at constant volume C_v and specific heat at constant pressure C_p. The two specific heats per mole of an ideal gas are related as,

C_p = C_v + R

where R is universal gas constant. For a solid or liquid the volume change is small, and in practice C_p is usually measured.