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Chapter 074 Principle of Conservation of Energy

He was originally from Switzerland and came to Russia when he was 3 years old. He worked as a doctor and taught in Petersburg. He is famous for his thermochemistry research.

In 1836 Hess reported to the Petersburg Academy of Sciences: "After continuous research, I am convinced that no matter what method is used to complete the combination, the heat emitted is always constant. This principle is so obvious that if I did not think that it has been Once it has been proven, it can also be accepted as an axiom without much thought.”

Convenience put forward a general statement in a lecture at the Academy of Sciences on March 1840, 3: "When any chemical compound is formed, heat is often released at the same time. This heat does not depend on whether the combination is carried out directly or indirectly through several reactions. conduct."

Later he applied this law extensively in his thermochemical research.

Hess's discovery reflected for the first time the basic principle of the first law of thermodynamics; the total amount of heat and work has nothing to do with the process path, and is only determined by the starting and ending states of the system.

It embodies the basic nature of the internal energy of the system - it has nothing to do with the process.

Hess's law not only reflects the idea of ​​conservation, but also includes the idea of ​​transformation of "force".

At this point, the laws of energy transformation and conservation have been initially formed.

In fact, the French engineer Sadi Carnot had established the idea that work and heat are equivalent as early as 1830. He wrote in his notes: "Heat is not something else, but a driving force, or it can be said that it changes Form of motion, it is a form of motion (of particles in an object).

When the power of the particles of an object disappears, heat must be generated at the same time, and the amount is exactly proportional to the power of the particles disappearing.

Conversely, if heat is lost, power must be generated. "

“Thus one can draw a general proposition: the dynamics existing in nature are quantitatively constant.

To be precise, it neither creates nor destroys; in fact, it only changes its form. "

Carnot gave the value of heat work equivalent basically correctly without any derivation: 370 kg m/kcal.

However, due to Carnot's premature death, his brother did not understand the significance of this principle although he had read his manuscript. It was not until 1878 that this manuscript was publicly published.

By this time, the first law of thermodynamics had already been established.

The law of conservation of energy is one of the universal basic laws of nature.

Generally stated as follows: Energy will neither be created out of thin air nor disappear out of thin air. It will only be converted from one form to another form, or transferred from one object to other objects, while the total amount of energy remains unchanged.

It can also be expressed as: the change of the total energy of a system can only be equal to the amount of energy transferred into or out of the system.

The total energy is the sum of the mechanical energy, internal energy (thermal energy) and any form of energy other than mechanical energy and internal energy of the system.

If a system is in isolation, no energy or mass can be transferred into or out of the system.

For this situation, the law of conservation of energy is stated as: "The total energy of an isolated system remains constant."

The law of conservation of energy was discovered in the 19s. It was independently discovered from different aspects by more than 40 scientists from various professions in 5 countries.

Among them, Meyer, Joule and Helmholtz are the main contributors.

Meyer is a doctor. During a voyage to Indonesia, Meyer, as the ship's doctor, received an important revelation while bleeding sick crew members. He discovered that venous blood was not as dim as people living in temperate countries, but rather like arterial blood. So fresh.

The local doctor told him that this phenomenon can be seen everywhere in the vast tropical areas.

He also heard sailors say that the water was warmer during storms. These phenomena caused Meier to ponder.

He thought that food contained chemical energy, which could be converted into heat just like mechanical energy.

In tropical high temperatures, the body only needs to absorb less heat from food, so the burning process of food in the body is weakened, so more oxygen is left in the venous blood.

He had realized that the input and output of energy in living organisms are balanced. In a paper entitled "Some Notes on the Mechanics of Heat" published in 1842, Meyer announced the equivalence and convertibility of heat and mechanical energy. His reasoning was as follows:

"Forces are causes: therefore, we may regard them comprehensively by applying the principle that 'cause equals effect'. If cause c has effect e, then c = e; conversely, if e is the cause of another effect f, then There are e=f and so on, c=e=f=...=c. In a series of cause and effect, a certain term or a part of a certain term will never disappear. This can be clearly seen from the properties of the equation. .This is the first quality of all causes, which we call immortality.”

Then Meyer used proof by contradiction to prove conservation (immortality):

“If a given cause c produces an effect e equal to itself, the act must cease; All the consequences of the remaining cause c will be >e, and will therefore contradict the premise c=e."

"Accordingly, as c changes to e, e to f, etc., we must regard these different values ​​as different forms in which the same object appears. This ability to assume different forms is the second of all causes. a fundamental quality. Putting these two properties together we can say that the cause is (quantitatively) immortal and (qualitatively) a transformable object.”

Meier then concluded, "Thus force (i.e. energy) is an immortal, transformable, immeasurable object."

Meyer's inference method is obviously too general and unconvincing, but his description of energy transformation and conservation is the earliest complete expression.

Meyer published his second paper in 1845: "Organic Movement and its Connection with Metabolism", which more systematically clarified the ideas of energy transformation and conservation.

He clearly pointed out: "Nothing can be created, and nothing can be changed into nothing." "In the dead and living nature, this force (note: energy) is always in the process of cyclic transformation. Nowhere is there a process that is not The form of force changes!”

Therefore he argued: "Heat is a force which can be converted into mechanical effects."

Meyer's paper also specifically discussed the relationship between heat and work, and derived the relationship between the gas specific heat at constant pressure and the specific heat at constant volume Cp-Cv equal to the expansion work R at constant pressure.

Therefore, we call Cp-Cv=nR the Meyer formula.

Meier then calculated the heat work based on the experimental data of Dillaroche, Berard and Dulon gas specific heat Cp=0.267 cal/g·degree and Cv=0.188 cal/g·degree.

The calculation process is as follows:

在定压下使1厘米3空气加热温升1度所需的热量为：Qp=mcpΔt=0.000347卡（取空气密度p=0.0013克/厘米3）。

......

To be continued

Chapter 076 Preview of the first half and second half