The first law of thermodynamics: In general, there is no adiabatic isolation between the system and the outside world, and the interaction between the system and the outside world can be completed by doing work and heat transfer. Let the system go through a certain process from the equilibrium state 1→2, in which the external work done to the system is: a, and the heat absorbed by the system from the outside is: q, then according to the law of energy conversion and conservation, the energy provided by heat transfer and work should be converted into the increment of the state function v of the internal energy of the system. V 1-V2=A+Q (5.8) Note: ① Provisions on the symbol of q: Q>0 indicates that the system absorbs heat from the outside; Q<0 indicates that the system releases heat to the outside world. ② About A: A > Provisions on signs; 0 means that the outside world is actively working on the system.