The first law of thermodynamics is the law of energy conservation and transformation in the field of thermal phenomena, which reflects the conservation of different forms of energy in the process of transmission and transformation.

Expressed as: the increase of internal energy of an object is equal to the sum of the heat absorbed by the object and the work done on the object? . That is, heat can be transferred from one object to another, and can also be converted with mechanical energy or other energy, but the total value of energy remains unchanged during the conversion process. Its generalization and essence is the famous law of conservation of energy.

This law has been verified by many physicists such as J.R. Meyer and J.P. Joule, and was established in the form of scientific law in the middle of19th century on the basis of long-term production practice and a large number of scientific experiments.

When the system is in adiabatic state, the work only depends on the energy of the initial state and the final state of the system, and has nothing to do with the process. The energy of an isolated system is always conserved. After adiabatic cycle, the work done by the system is zero, so the first kind of perpetual motion machine is impossible (that is, a machine that does not consume energy to do work).