Analysis:
The second law of thermodynamics describes the direction of heat transfer:
The mechanical energy of regular molecular motion can be completely converted into the thermal energy of irregular molecular motion; Thermal energy cannot be completely converted into mechanical energy. A common expression of this law is that every spontaneous physical or chemical process always develops in the direction of entropy increase. Entropy is a kind of heat energy that cannot be converted into work. The change of entropy is equal to the change of heat divided by the absolute temperature. When the high and low temperatures are concentrated respectively, the entropy value is very low; When the temperature diffuses uniformly, the entropy value increases. When the objects are ordered, the entropy value is low; When the objects are out of order, the entropy value will increase. Now the whole universe is moving from order to disorder, from rule to irregularity, and the total entropy of the universe is increasing.
Clausius statement
It is impossible to transfer heat from a low-temperature object to a high-temperature object without causing other changes.
Kelvin expression
It is impossible to absorb heat from a single heat source and make it completely useful without other functions.
Kelvin expression can also be expressed as: the second perpetual motion machine cannot cause.
Establishment and expression of the second law of thermodynamics
The second law of thermodynamics was established shortly after the first law of thermodynamics (the law of conservation of energy), and its establishment was closely related to Carnot's research on heat engines in the 1920s. In the research work of improving the efficiency of heat engine, Carnot grasped the essence of heat engine, put aside various secondary factors, and abstracted an ideal heat engine (Carnot heat engine) that only works between high-temperature heat source and low-temperature heat source (cold source). He compared such a heat engine to a water turbine: "We can accurately compare thermal energy to a waterfall ... The strength of a waterfall depends on the height and quantity of liquid; The power of heat also depends on the amount of heat mass used and the' falling height' of the heat mass, that is, the temperature difference between two objects exchanging heat mass. " Kano's era is the era in which the theory of heat prevails, and this passage of Kano is also a reflection of the theory of heat. Now it seems wrong, of course, but the conclusion he got is correct: "It is not enough to provide heat to give power, but it must be cold. Without cold, heat is useless. " He has touched the edge of the second law of thermodynamics.
When British physicist Kelvin (formerly known as Thomson) studied the work of Carnot and Joule, he found some incongruities: according to the law of conservation of energy, heat and work should be equivalent, but according to Carnot's theory, heat and work are not exactly the same, because work can be completely turned into heat without any conditions, and the work generated by heat must be accompanied by the dissipation from heat to cold. In a paper from 65438 to 0849, he said: "The theory of heat needs serious reform and new experimental facts must be found." Clausius, a contemporary, also studied these problems seriously, and he keenly saw the disharmony in Carnot's theory. He pointed out that the conclusion in Carnot's theory that the work generated by heat must be accompanied by the transfer of heat to cold is correct, but the conclusion that heat (that is, thermal mass) remains unchanged is wrong. In the paper published in 1850, Clausius proposed that in the theory of heat, besides the law of conservation of energy, another basic law must be supplemented: "Without some kind of power consumption or other changes, it is impossible to transfer heat from low temperature to high temperature." This law was later called the second law of thermodynamics. Clausius's expression in modern textbooks is generally expressed as:
It is impossible to transfer heat from a low-temperature object to a high-temperature object without causing other changes.
In the second year (185 1), Kelvin put forward another expression of the second law of thermodynamics, which is generally expressed in modern textbooks as follows:
It is impossible to absorb heat from a single heat source and make it completely useful without other functions.
Simply put:
Thermal energy can not be completely converted into mechanical energy, but can only be transferred from high-temperature objects to low-temperature objects.