Chapter 074 The Principle of Energy Conservation

Originally a Swiss, he went to Russia when he was 3 years old, worked as a doctor, and taught in Petersburg. He is famous for his thermochemical 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 by it is always constant. This principle is so obvious that if I do not think it has been It can be proved, and it can be accepted as an axiom without thinking about it."

On March 27, 1840, he put forward a general expression in a speech of the Academy of Sciences: "When any chemical compound is formed, heat is often released at the same time, and this heat does not depend on whether the compound is carried out directly or indirectly through several reactions. conduct."

Later he applied this law extensively in his thermochemical research.

This discovery by Hess for the first time reflects the basic principle of the first law of thermodynamics; the total amount of heat and work has nothing to do with the process path, but only depends on the initial and final 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 ​​"force" transformation.

So far, the law of energy conversion and conservation has been initially formed.

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

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

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

"Thus one can draw a general proposition: The motive forces existing in nature are quantitatively invariant.

Precisely, it neither creates nor perishes; in fact, it only changes its form. "

Carnot did not make a derivation but basically correctly gave the value of thermal work equivalent: 370 kgm/kcal.

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

At this time, the first law of thermodynamics has already been established.

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

The general expression is: energy is neither created nor destroyed, it is only transformed from one form to another, or transferred from one object to another, while the total amount of energy remains the same.

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, there is no possibility of energy or mass being transferred into or out of the system.

For this case, the law of conservation of energy states: "The total energy of an isolated system remains constant."

The law of conservation of energy was discovered in the 1840s by more than 10 scientists of various professions in 5 countries and independently from different aspects.

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

Meyer is a doctor. During a voyage to Indonesia, Meier, as the ship's doctor, got an important revelation when he bled the sick sailors. He found that the venous blood was not as dull as people living in temperate countries, but 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 sea is hotter during storms. These phenomena caused Meyer to contemplate.

It occurred to him that food contains chemical energy, which, like mechanical energy, can be converted into heat.

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

He has realized that the input and output of energy in living organisms are in balance. In his 1842 publication titled "Some Notes on the Mechanics of Thermodynamics", Mayer announced the equivalence and convertibility of thermal and mechanical energies, and his reasoning was as follows:

"Forces are causes: we may therefore treat them with full application of the principle, 'cause equals effect'. Let a cause c have an effect e, then c = e; conversely, let e be the cause of another effect f, then There are e=f, ​​etc., c=e=f=...=c In a chain of causality, a certain item or a certain part of a certain item will never disappear, which can be clearly seen from the nature of the equation .This is the first property of all causes, which we call indestructibility."

Then Mayer used the method of proof by contradiction to prove the conservation (indestructibility):

"If a given cause c produces an effect e equal to itself, the action must cease; c becomes e; if after producing e, c remains in whole or in part, there must be a further effect, equivalent to All consequences of the remaining cause c will be > e, and thus will contradict the premise c = e."

"Correspondingly, since c becomes e, e becomes 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 cause of all Putting these two properties together we can say that the cause is (quantitatively) indestructible and (quantitatively) a transformable object.”

Mayer concluded, "Therefore force (i.e. energy) is an indestructible, transformable, and immeasurable object."

Meier's deduction method is obviously too general and unconvincing, but his description of energy conversion and conservation is the earliest complete expression.

Mayer published his second paper in 1845: "Organic Motion and Its Connection with Metabolism", which more systematically clarified the ideas of energy conversion and conservation.

He clearly pointed out: "Nothing cannot be born, and nothing can become nothing", "In the dead and living nature, this force (press: that is, energy) is always in the process of circular transformation. There is no process anywhere that is not The form of power changes!"

So he argued: "Heat is a force~lightnovelpub.net~it can be transformed into a mechanical effect."

In Mayer's thesis, the relationship between heat and work was also discussed in detail, and the relationship between the difference between the specific heat of gas at constant pressure and the specific heat at constant volume, Cp-Cv, equal to the expansion work R at constant pressure was introduced.

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

Then Mayer calculated the heat work based on the experimental data of Dillaroche, Berard and Dulong's gas specific heat Cp=0.267 cal/g·degree, Cv=0.188 cal/g·degree.

The calculation process is as follows:

The heat required to heat 1 cm3 of air and raise its temperature by 1 degree under constant pressure is: Qp=mcpΔt=0.000347 calories (take the air density ρ=0.0013 g/cm3).

 …

to be continued

Chapter 076