The First Law of Thermodynamics
The First Law of Thermodynamics is sometimes thought to be merely an extension to the Law of Conservation of Energy, which was discovered in the late seventeenth century for mechanical systems. The First Law introduces the important thermodynamic state variable internal energy, and the law posits that energy may be converted from one of its forms to another form. The relationship between heat (thermal energy) and work was suggested by Count Rumford. In the caloric theory of heat, the temperature of a substance was considered to be determined by the quantity of caloric gas which it contained. The concept of heat capacity is only used when the addition of thermal energy to or withdrawal of thermal energy from the system produces a temperature change; the concept is not used when a phase change is involved. The heat capacity is dependent on the size of the system and is therefore an extensive property.