The first law of thermodynamics is the law of conservation of energy.

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The internal energy increment of a thermodynamic system is equal to the sum of the heat transferred to it and the work done by the outside world. If a system is isolated from the environment, its internal performance will not change. ) expression: △U=W+Q

Symbolic law

The mathematical expression of the first law of thermodynamics is also applicable to the case that an object does external work, radiates heat to the outside and reduces internal energy. Therefore, when using △U=W+Q, there are usually the following provisions: ① External work is done on the system, W>0, that is, W is positive. ② The system does work to the outside world, that is, the outside world does negative work to the system, and W0, that is, Q is positive; ④ The system releases heat to the outside, and Q0, that is, △U is positive; ⑤ The internal energy in the system is reduced, △ U

Understand; Understanding

Understand 1 from three aspects. If the internal energy of an object is changed simply by doing work, then the change of internal energy can be measured by the amount of work done. At this time, the increase (or decrease) of the internal energy of an object is equal to the value of the work done by the outside world to the object (or the object to the outside world), that is, △ u = w 2. If the internal energy of an object changes only through heat transfer, then the change of internal energy can be measured by the heat transferred. At this time, the increase (or decrease) of the internal energy of the object △U is equal to the value of the heat Q absorbed (or released) by the outside world, that is △U=Q 3. In the process of doing work and heat transfer at the same time, the change of internal energy of an object is determined by doing work and transferring heat. In this case, the increment △U of the internal energy of an object is equal to the sum of the heat Q absorbed from the outside and the work W done to the outside. That is △U=W+Q

conservation of energy

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Energy can neither be produced nor disappeared out of thin air, it can only be transformed from one form to another, or transferred from one object to another.

Diversity of energy sources

The motion of an object has mechanical energy, the motion of a molecule has internal energy, the charge has electric energy, and the motion inside the nucleus has atomic energy and so on. It can be seen that different forms of energy correspond to different forms of movement in nature.

Different forms of energy conversion

"Friction generates heat" is to convert mechanical energy into internal energy to do work by overcoming friction; When the water in the kettle boils, steam acts on the lid and lifts the lid, indicating that internal energy can be converted into mechanical energy. Electric current can convert electric energy into internal energy by doing work with electric heating wires. . . These examples show that different forms of energy can be transformed into each other, and this transformation process is completed by doing work.

Significance of conservation of energy

1. The transformation and conservation of energy is an extremely important method to analyze and solve problems, which is more common than the law of conservation of mechanical energy. For example, when an object falls in the air with resistance, the mechanical energy of the object is not conserved, but the total energy including internal energy is conserved. 2. The law of conservation of energy is one of the three great discoveries of natural science in19th century, and it also solemnly declares the complete disillusionment of the first perpetual motion machine. 3. The law of conservation of energy is a powerful weapon to understand and transform nature, which relates to a wide range of natural science and technology fields.

The first perpetual motion machine (impossible to manufacture)

A machine that can continuously do external work without consuming any energy. It can't exist because it violates the law of conservation of energy.

Edit the second law of thermodynamics in this paragraph.

There are several ways to express the second law of thermodynamics: Clausius stated that heat can be spontaneously transferred from a hot object to a cold object, but it is impossible to spontaneously transfer from a cold object to a hot object; Kelvin Planck pointed out that it is impossible to absorb heat from a single heat source and convert it into work without other influences. The expression of entropy is time-dependent, and the entropy in an isolated system will never decrease.

relationship

The two expressions of the second law of thermodynamics (the first two) seem to be unrelated, but they are actually equivalent, that is, the other can be deduced from one.

meaning

Every expression of the second law of thermodynamics reveals the directionality of macro-processes involving a large number of molecules, which makes people realize that macro-processes involving thermal phenomena in nature are directional.

Microscopic significance

All natural processes always follow the direction of increasing disorder of molecular thermal motion. The second perpetual motion machine (impossible to manufacture) only absorbs the heat from a single heat source and converts it into useful work without causing other changes. The efficiency of the second kind of perpetual motion machine is 100%. Although it does not violate the law of conservation of energy, a large number of facts have proved that a heat engine cannot have only one heat source under any circumstances. If it wants to continuously turn the absorbed heat into useful work, it will inevitably transfer some heat to a low-temperature object, so the efficiency will not reach 100%. The second perpetual motion machine violates the second law of thermodynamics.

Edit the third law of thermodynamics in this paragraph.

The third law of thermodynamics is usually expressed as absolute zero, and the entropy of perfect crystals of all pure substances is zero. Or absolute zero (T=0K) cannot be achieved. 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.

Edit this paragraph separately.

zeroth law of thermodynamics

The zeroth law of thermodynamics: If each of the two thermodynamic systems is in thermal equilibrium with the third thermodynamic system, then they must also be in thermal equilibrium. The zeroth law of thermodynamics is the basis of the three laws of thermodynamics.