Thermodynamics

These considerations may be summarized in the first law of thermodynamics: the internal energy of an object is increased by the amount of work done on it, and by the amount of heat added to it. Mathematically, if U_f is the internal energy of an object at the end of some process, and U_i is the internal energy at the beginning of the process, then U_f - U_i = W + Q, where W is the amount of work done on the object, and Q is the amount of heat added to the object (negative values are used if work is done by the object, or heat is transferred from the object). As is usual for an equation, all quantities must be expressed in the same units; the usual mechanical unit for energy (in the International System of Unitsformerly the MKS system) is the joule, where 1 joule equals 1 kg-m²/s².