Heat is less easily defined than work. In genera! terms, the temperature of a substance increases if heat is added to it. Therefore, heat was, for a long time, thought to be an invisible fluid named 'caloric' which flows along a temperature difference from one system to the other. We now know that this is false and that heat is not contained in a system but is manifested only as an interaction of the system with its surroundings as the system changes from one state to another. Like work, it can be considered as energy in transit. Heat is best defined through the *First Law of Thermodynamics*:

where U is internal energy, M mass, T temperature, c_{v} and c_{p} specific heat capacity at constant volume and pressure, respectively, and Q is heat, W work, p pressure, V volume, and H enthalpy.

Therefore,

If phase change occurs as a result of heat transfer, the latent heat of evaporation and/or the latent heat of solidification have to be included in Equation (3).

The sign convention is that heat transfer is positive into the system and negative out of the system.