1, the first law of thermodynamics is the law of conservation of energy. Since Joule proved with irrefutable and accurate experimental results that the transformation between mechanical energy, electric energy and internal energy satisfies the conservation relationship, people think that the law of conservation of energy is a basic law that exists universally in nature.

The internal energy increment of a thermodynamic system is equal to the sum of the heat transferred to it by the outside world and the work done by the outside world, and the expression is: du = Δ q+Δ w.

2. There are several ways to express the second law of thermodynamics:

Clausius said that heat can be spontaneously transferred from a high-temperature object to a low-temperature object, but it is impossible to spontaneously transfer from a low-temperature object to a high-temperature object.

Kelvin Planck pointed out that it is impossible to absorb heat from a single heat source and completely convert it into work without any other influence.

Entropy expression: With the passage of time, the entropy in an isolated system will not decrease.

The expression is ds ≥Δ q/t.

3. When the third law of thermodynamics is usually expressed as absolute zero, the entropy of perfect crystals of all pure substances is zero. Or absolute zero (T=0K, that is, -273. 15℃) cannot be reached.

R.H. Feller and E.A. Guggenheim also put forward another expression of the third law of thermodynamics: no system can reduce its temperature to 0K through limited steps, which is called the principle that 0K cannot be achieved.