The second law of thermodynamics is that heat cannot be transferred from a low-temperature object to a high-temperature object without causing other changes.

legal interpretation

Heat can spontaneously pass from a hot object to a cold object, but it is impossible to spontaneously pass from a cold object to a hot object (Clausius); It can also be expressed as: the result of friction between two objects turns work into heat, but it is impossible to turn this friction heat into success without other effects.

For thermal processes such as diffusion, infiltration, mixing, combustion, electrothermal and hysteresis, although the inverse process still conforms to the first law of thermodynamics, it is impossible to occur spontaneously. The first law of thermodynamics does not solve the problem of direction, condition and limit in the process of energy conversion, while the second law of thermodynamics just stipulates this.

Passive expression

Kelvin said: It is impossible to absorb heat and do work from a single heat source without affecting the environment. Clausius pointed out that heat cannot be transferred from low temperature to high temperature without affecting the environment. Ostwald said: There is no second perpetual motion machine.

The equivalence of these three expressions can be proved by Carnot engine.

Two characteristics of the second law of thermodynamics;

1, isolation system

According to the theory of molecular motion, work is the regular motion of a large number of molecules, and thermal motion is the irregular motion of a large number of molecules. Obviously, the probability of irregular motion becoming regular motion is very small, while the probability of regular motion becoming irregular motion is very high.

An isolated system is not affected by the outside world, and its internal spontaneous process is always from a low probability state to a high probability state, so heat cannot succeed spontaneously.

2, suitable for macro programs

The second law of thermodynamics can only be applied to systems composed of a large number of molecules and macro processes in a limited range. It is not suitable for a few microscopic systems, nor can it be extended to the infinite universe.

It can also be said that the second law of thermodynamics cannot be strictly proved, otherwise the focus of the universe is the legendary "theory of heat death". You can look for it yourself if you are interested.