[Encyclios]

Symbols and sign conventions for work and heat

Heat and work are indicated, respectively, with the symbols Q and W and their metrological dimensions are obviously those of energy: in the International System, they are measured in Joules (symbol: J), while in the Technical System they are measured in kilocalories (symbol: kcal). Furthermore, although heat and work are not state properties, it makes sense to consider the amount of heat exchanged per unit of mass (or weight) of the system and the work done per unit of mass (or weight) of the system. Again, for the energy balances of a system it is always necessary to give a sign to the numerical value of heat and work, depending on the direction of their flow; there are then two different conventions for work and heat:

• heat is positive if energy is administered to the system, while it is negative otherwise;
• work is positive if energy is supplied to the environment, while it is negative otherwise.