heat capacity

Put a pot of water and a pot of the same weight of cooking oil on identical burners and the oil heats up much faster. Heat capacity is the answer to the question: how much heat energy must I pour in to raise this object's temperature by one degree? A large heat capacity means the object behaves like a heat sponge, soaking up a lot of energy for only a small rise in temperature.

Heat capacity C is defined as C = dQ/dT, the heat Q (in joules) needed per degree of temperature change (in kelvin K or degrees Celsius; a change of 1 K equals a change of 1 degree C). It is an extensive property, meaning it depends on how much material you have: two liters of water have twice the heat capacity of one liter. The unit is J/K. Because atoms store the incoming energy as more vigorous vibration about their lattice sites, heat capacity really measures how many vibrational modes the material can load energy into. It is measured two ways: Cp at constant pressure (the usual lab case, where the sample is free to expand) and Cv at constant volume. Cp is slightly larger because some heat also does the work of expansion.

Heat capacity decides how quickly a part warms or cools: a heavy cast-iron pan holds heat and cooks evenly, while a thin foil warms instantly. Engineers use it whenever they compute how much energy a furnace charge, a brake disc, or a building's thermal mass will absorb. Do not confuse the capacity of the whole object (heat capacity, which grows with size) with the material's per-kilogram value (specific heat, a size-independent material property).