First law of thermodynamics: First law of thermodynamics states that, Heat and mechanical work are mutually convertible. According to this law, a definite amount of mechanical work is needed to produce a definite amount of heat and vice versa. According to First law of thermodynamics Total energy of a system remains constant. First law of thermodynamics deals with conservation of energy.

First law of thermodynamics also states that energy can neither be created nor destroyed, through it can be transformed from one form to another. According to this law, the energy due to heat supplied must be balanced by external workdone plus the gain in internal energy due to rise in temperature.