Starting from the initial state, the system reaches the final state through any process, and the internal energy increment δU is equal to the difference between the heat Q transferred to the system by the outside world and the work A done by the system. Mathematical expressions can be written as:
δ U = U2-U 1 = Q-A or q = δ u+a.
Provisions: system heat absorption Q>0, system heat release Q.
δQ=dU+δA
DU in the formula is the total differential of internal energy; Δ q and Δ a represent the micro heat transferred in the infinitesimal process and the micro work done externally, respectively, and they are not fully differential.
Extended data:
Any system has a single-valued state function-internal energy, and the internal energy of an isolated system is constant. The internal energy of an object is the sum of the kinetic energy of irregular thermal motion of microscopic particles and the interaction potential energy between them when the object is at rest.
The experimental basis of the macroscopic definition of internal energy is that the adiabatic work values of the system in the same initial and final states are all equal, regardless of the path. It can be seen that the work done by the outside world to the system in the adiabatic process is only related to the change of a function between the initial state and the final state of the system, and has nothing to do with the path.
This state function is internal energy. It can define the adiabatic work as the work done by the outside world through the system: U2-U 1 =-As, where the negative sign indicates that the external work is positive work. The unit of work is joule. In the process of pure heat transfer, the internal energy change of the system can be used to define the heat and its value, that is, q = U2-U 1, in which the heat absorption of the system is defined as positive (q is greater than 0). The unit of heat is also joule.