conservation of energy

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Law of conservation of energy is one of the universal basic laws in nature. It is generally stated that energy will neither be generated nor disappear out of thin air. It will only be transferred from one form to another, or from one object to another energy The total amount of remains unchanged. It can also be expressed as: system The change of total energy can only be equal to the amount of energy transmitted into or out of the system. Total energy is systematic mechanical energy 、 internal energy （ heat energy ）And the sum of all forms of energy except mechanical energy and internal energy. If a system is in an isolated environment, it is impossible for energy or mass to pass into or out of the system. In this case, the law of conservation of energy is expressed as: "The total energy of an isolated system remains unchanged."

The law of conservation of energy was discovered in the 1840s. It was independently discovered by more than 10 scientists from different professions in five countries from different perspectives. among Mayer 、 joule 、 Helmholtz Is a major contributor. It is one of the most basic laws in natural science, which scientifically clarifies the view that motion is immortal.

Chinese name: conservation of energy
Foreign name: law of conservation of energy
expression: energy In the process of conversion or transfer, the total amount remains unchanged
Presenter: Mayer, Joule, Helmholtz

Proposed time: 1842
Applicable fields: All fields
Applied discipline: physics 、 Chemistry 、 biology
The earliest research time: 1475

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Energy definition

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Energy is a measure of the transformation of material movement, referred to as "energy". Everything in the world is in constant motion material All attributes of, motion It is the most basic attribute, and other attributes are specific expressions of motion. Energy is a measure of the ability of a physical system to do work.

Encyclopedia x ignorance: the first law of thermodynamics

energy (energy) Yes material Basic Physics attribute One, yes Material movement Unification of measurement 。

Energetic Company Same as the unit of work International System of Units The middle is joule (J). stay Atomic physics 、 Nuclear physics 、 Particle physics Commonly used in such fields as Electron volt (eV) as the unit, 1 electron volt=1.602,18 × 10 －19 Joule. Physical field, also used Erg (erg) as energy unit, 1 erg=10 －7 Joule.

Energy exists in many different forms; According to the different motion forms of matter, energy can be divided into mechanical energy 、 chemical energy , Internal energy（ heat energy ）、 electric energy 、 radiant energy Nuclear energy. These different forms of energy can pass through Physical effects or chemical reaction And transform each other^[1] 。 Various fields also have energy.

The English word "energy" originates from the Greek word: ἐνέέέέέέέέ, which first appeared in the 4th century BC Aristotle In their works. Galileo The idea of "energy" has appeared in the times, but there is no term of "energy". The concept of energy comes from the 17th century Leibniz The "vitality" idea of, definition On a object quality And the square of its velocity, which is equivalent to today's kinetic energy Twice as much. In order to explain the phenomenon that the speed is slowed down due to friction, Leibniz's theory believes that heat energy is composed of the random movement of the constituent matter in the object, that is, the internal energy of the object molecule. This idea is consistent with Newton's, although it took a century for this idea to be widely accepted.

The term "energy" was coined by T. Yang in 1807 King's College London speak natural philosophy It was introduced at that time. In view of the view of "vitality" or "rising force" at that time, it was proposed to use the word "energy" to express it, and it was related to the work done by objects, but it did not attract attention. People still believed that different movements contained different forces. 1831 France scholar Coriolis The concept of force doing work is also introduced, and a 1/2 coefficient is added before "vitality", which is called kinetic energy The relation between work and kinetic energy is given through integration. In 1853“ potential energy ”In 1856, the term "kinetic energy" appeared. It was not until the law of conservation of energy was confirmed that people realized the importance and practical value of the concept of energy.

space Property is Material movement Extensive embodiment of; time Attribute is the continuous embodiment of material movement; gravitation Attribute is the embodiment of the interaction caused by the uneven distribution of mass during the movement of matter; electromagnetism Property is charged particle External performance in the process of movement and change, etc. There are various forms of material movement, and each specific Form of material movement There are corresponding energy forms.

macroscopic object Of mechanical movement The corresponding energy form is kinetic energy ； Molecular motion The corresponding energy form is internal energy (thermal energy); The energy form corresponding to atomic motion is chemical energy ； charged particle Of Directional movement The corresponding energy form is electric energy ； photon The energy form corresponding to (electromagnetic field) movement is Light energy (electromagnetic wave energy), etc. In addition to these, there are Wind energy 、 Tidal energy Etc. When the motion forms are the same, the motion characteristics of objects can adopt some physical quantity Or chemical quantity. Corporeal mechanical movement Can be used speed 、 acceleration 、 momentum And other physical quantities; electric current Can be used Current intensity 、 Voltage 、 power And other physical quantities. However, if the motion forms are different, the only physical quantity that can describe and compare the motion characteristics of matter is energy, which is the common characteristic of all moving matter.

Different forms of energy can pass through Physical effects or chemical reaction And transform each other. Corresponding to various forms of motion of matter, energy has various forms. The mechanical energy expressed as the whole of an object or system in mechanical motion, such as kinetic energy 、 potential energy Etc. In thermal phenomena internal energy , which refers to each molecule Kinetic energy of random motion, potential energy of intermolecular interaction atom and Nucleus The sum of the energy within, but excluding the mechanical energy of the overall movement of the system. The internal energy of thermal motion (formerly called heat energy) is known by the mutual conversion between it and mechanical energy (see First law of thermodynamics ）。 Various fields also have energy^[2] 。

mechanical energy 、 chemical energy , internal energy (thermal energy), electrical (magnetic) energy radiant energy 、 nuclear energy There are various ways for different types of energy to transform each other. For example, the most common electrical energy（ alternating current and Battery ）It can be transformed from a variety of other forms of energy, such as mechanical energy – electrical energy（ Hydropower ）Nuclear energy – internal energy (thermal energy) – mechanical energy – electrical energy conversion（ Nuclear power generation ）Chemical energy – electric energy conversion（ Battery ）Etc^[1] 。

Essence of expression

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Common expression: energy will neither be generated nor disappear out of thin air. It will only be transferred from one object to another, or from one form to another. In the process of transformation or transfer, the total amount of energy remains unchanged.^[3]

First law of thermodynamics General energy conservation and transformation laws are embodied in all processes involving macro thermal phenomena. The first law of thermodynamics confirms that the heat absorbed by the system from the surrounding medium in any process medium The amount of work done is conserved with the increment of internal energy of the system.

A kind of envisaged perpetual motion machine

The first law of thermodynamics is the law of conservation of energy, which is a summary of human experience, and can also be derived by analogy with the law of conservation of matter. The energy of thermodynamic system is expressed as internal energy 、 quantity of heat and Work The first law of thermodynamics is an expression of the conservation of energy. The conclusions derived from it have not yet been found to contradict the facts. According to the first law of thermodynamics, it can be envisaged that to build a machine, it does not rely on the external supply of energy, nor reduce its own energy, but constantly does work without consuming energy. People call this hypothetical machine the first type of perpetual motion machine. The first law can also be expressed as“ Permanent motion machine of the first kind Is impossible to cause ”。 Conversely, the first type Perpetual motion machine It can never be caused, which proves that the first law is correct.

After the thermodynamic system goes through a process from state 1 to state 2, the internal energy of the system will generally change. According to the law of conservation of energy:

Δ U = Q － W （1）

Where Δ U = U two － U one Is the internal energy increment of the system; Q Is the heat absorbed by the system from the environment during this process; W It refers to the work done by the system to the environment in this process. Equation (1) is the mathematical expression of the first law of thermodynamics.

In formula (1) U Yes Status function , i.e. Δ U The value of only depends on the initial state and final state of the system, and has nothing to do with the specific process of the system from the initial state to the final state, where Q and W It is related to the process. Attention shall be paid to the application of formula (1) Q and W The positive and negative sign of is: system heat absorption Q >0, system heat release Q <0； The system does work on the environment W >0. Environment does work on the system W <0。

If there is a slight change in the system state, the first law of thermodynamics is written as:

d U =δ Q －δ W 　（2）

Where δ Q And δ W They are respectively small heat and small work of the process. They are not full differential, so they are represented by "δ" instead of "d", which is different from full differential^[3] 。

The first law of thermodynamics can also be expressed as Permanent motion machine of the first kind (A machine that can do work automatically without consuming any fuel and energy) is impossible.

When system It is open, and there is not only thermal and mechanical interaction between it and the medium, but also material exchange, so the expression of the first law of thermodynamics should also add an increase or decrease of energy caused by material exchange.

Mechanical energy is the energy form of objects in mechanical phenomena, including kinetic energy and potential energy (potential energy), that is, mechanical energy=kinetic energy+potential energy.

In a closed mechanical system (conservative mechanical system), only conservative forces do work, and there is no mutual conversion between mechanical energy and other forms of energy, then Conservation of mechanical energy , system energy is expressed as mechanical energy. Energy conservation is specifically shown as Law of conservation of mechanical energy 。 The law of conservation of mechanical energy is a special case of the law of conservation of energy.

The law of conservation of energy shows that energy can only change from one form to another and cannot be generated or eliminated out of thin air. The conservation of energy is a mathematical conclusion derived from the translational symmetry (translational invariance) of time (see Nott's theorem ）。

According to the law of conservation of energy, the incoming energy is equal to the outgoing energy plus internal energy Change.

This law is physics Fairly basic criterion 。 According to the translational symmetry of time (translational invariance), Laws of physics (theorem) holds at any time.

In special relativity, the law of conservation of energy is Law of conservation of mass and energy 。 The law of conservation of mass and energy is a special form of the law of conservation of energy. Mass energy formula E = m c two The corresponding relationship between mass and energy is described. In classical mechanics, mass and energy are mutually independent, but in relativistic mechanics, energy and mass are the same representation of two aspects of mechanical properties of objects. In relativity, mass is expanded to mass energy. Originally independent in classical mechanics mass conservation and Conservation of energy Combine to become unified Law of conservation of mass and energy , fully reflecting the unity of matter and motion.

The relativistic energy of a single mass particle includes its Static mass And its kinetic energy. If the kinetic energy of a mass particle is zero (or Relative static Or a system with kinetic energy in the momentum center system, its total energy (including the kinetic energy inside the system) and its static mass or Constant mass The relation is known as E = m c two 。

So as long as the observer's Reference system No change, the conservation of energy to time in special relativity is still valid, the energy of the whole system is still unchanged, and the energy measured by observers in different reference systems will be different, but the energy values measured by each observer will not change with time. The constant mass, defined by the energy momentum relationship, is the minimum value of the system mass and energy that can be observed by all observers. The constant mass is also conserved, and the values measured by all observers are the same.

According to a large number of experiment The law of conservation of energy is confirmed, that is, when different forms of energy are converted to each other, their magnitude is conserved. Joule thermal work equivalent experiment It is a famous experiment that early confirmed the law of energy conservation, and then established energy conversion and conservation in the macro field First law of thermodynamics 。 Compton effect Confirm that the law of energy conservation is still correct in the micro world, and then gradually realize that the law of energy conservation is determined by the invariance of time shift, thus making it a universal law in physics (see Symmetry and conservation law ）。

It should be noted that the concept of energy has its scope of application. According to general relativity, under certain conditions, the measure of energy can no longer be used.

Relevant interpretation

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Energy transformation and conservation classification

The idea of the first law of thermodynamics was first proposed by German physicist J. Mayer in 1842 on the basis of experiments. After that, the British physicist J. Joule did a lot of experiments and used various methods to find Thermal equivalent The results are consistent. In other words, there is a certain conversion relationship between heat and work. After accurate experimental measurement, it was known that 1 calorie=4.184 joules. In 1847, the German scientist H. Helmholtz gave a strict mathematical description of the first law of thermodynamics and clearly pointed out that "the law of conservation of energy is one of the basic laws that are universally applicable to all natural phenomena." By 1850, it had been recognized in the scientific community.

Confirm as Conserved quantity The existence of energy began at the end of the 17th century, when G. Leibniz observed the Earth field of gravity in particle Energy（ mv two /2+ mgh ）Conservation. Since 1840s, Joule has confirmed that heat is only a form of energy, laying the foundation for the first law of thermodynamics. 1905 Einstein Put energy and material Static mass The famous Mass energy relation Formula. To explain Beta decay The part of energy "disappeared" in the process, W Pauli It is proposed that there must be an unrecognized particle 。 Later E Fermi Name this particle neutrino The "disappeared" energy is recovered.

The first law of thermodynamics confirms that there is a single value State function —— internal energy ， Isolated system The internal energy of is constant. The internal energy of an object is the Microparticle Irregular Thermal movement The sum of kinetic energy and potential energy of interaction between them. The experimental basis for macro definition of internal energy is that the adiabatic work done by the system between the same initial and final states is equal to route irrelevant. thus it can be seen, Adiabatic process The work done by the outer bound to the system is only related to the change of a function of the system between the initial and final states, and has nothing to do with the path. This state function is the internal energy. It can be accessed through system What we do to the outside world Adiabatic work A s Defined: U two － U one =－ A s , where the minus sign indicates that the external work is Positive work 。 The unit of work is the joule. In a pure heat transfer process, the heat and its value can be defined by the change of internal energy of the system, namely Q = U two － U one , here the system heat absorption is defined as positive（ Q Greater than 0). quantity of heat It is also in joules.

Both heat and work are Process quantity They only appear when the system state changes, and their values are not only related to the initial and final states of the process, but also related to the path the process experiences. Both work and heat are measures of changes in internal energy, indicating that there should be some equivalence between them. Historically, the numerical expression of this equivalence is called thermal work equivalent.

The first law of thermodynamics is an extension of the law of conservation of energy to non isolated systems. At this time, the energy can work W Or heat Q To or from the system in the form of.

Description method:

1. The increase of the internal energy of the object is equal to the heat absorbed by the object and the amount of Work The sum of.

2. When the system is in an adiabatic state, the work only depends on the energy of the initial state and the end state of the system, and has nothing to do with the process.

three Isolated system The energy of is always conserved.

4. After the adiabatic cycle, the work done by the system is zero, so the first type of perpetual motion machine is impossible (that is, a machine that does not consume energy to do work).

5. When two systems interact, the work has a unique value, which can be positive, negative or zero.

Theoretical interpretation

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stay Einstein In the special theory of relativity, energy is Four-dimensional momentum One of weight 。 At any Closed system , at any Inertial frame When observing, this vector Each component of (one is energy, the other three are momentum ）Will be conserved and will not change with time length Will also be conserved（ Minkowski The vector length is a single particle Static mass Is also the constant mass (i.e. constant energy) of the system composed of multi mass particles.

stay quantum mechanics The energy of a quantum system consists of Hamiltonian operator Described by the self adjoint operator of the operator Acting on the system Hilbert space (or wave function Space). If the Hamiltonian operator is a time-varying operator, it will appear as the system changes probability The measurement of energy does not change with time, so expected value It will not change with time. Energy conservation of localization in quantum field theory can be achieved by energy momentum tensor Operator coordination Nott's theorem Find. Because on Quantum Theory There is no global time operator in, and the uncertain relationship between time and energy only holds under some specific conditions position and momentum Between Uncertain relation As quantum mechanics The nature of the foundation is different (see Uncertainty principle ）。 The energy at each fixed time can be measured accurately and will not be affected by the uncertain relationship between time and energy. Therefore, even in quantum mechanics, energy conservation is a clearly defined concept.

The laws of conservation of energy are many Laws of physics Characteristics of. with mathematics The conservation of energy is the result of Nott's theorem. If physical system If continuous symmetry is satisfied during time translation, its energy（ time Of Conjugate physical quantity ）Conservation. Conversely, if physical system None during time translation Symmetry , its energy is not conserved, but if this system exchanges energy with another system and the resultant larger system does not change with time, the energy of this larger system will be conserved. Since any time-varying system can be placed in a larger non time-varying system, the conservation of energy can be achieved by properly redefining the energy. For the physical theory in flat space-time, because quantum mechanics allows non conservation in a short time (such as positive anti particle pairs), it is not observed in quantum mechanics Conservation of energy 。

The law of conservation of energy expresses continuous symmetry and conservation law Correspondence of. The conservation law is the most basic law that must be obeyed in the process of material movement. It has become physics One of the most common and profound ideas in. For example, the laws of physics do not change with time, which means that they have some symmetry about time. Nott's theorem is deeply related to quantum mechanics, because it only uses the principle Can distinguish between Heisenberg Uncertainty Principle dependent physical quantity (such as time and energy). The famous law of conservation of energy is given for the invariance of time shift.

Space and time are shown as uniform and Isotropy Of, Coordinate system Of origin translation and Axis The rotation of Symmetric transformation , they form a nonhomogeneous Lorentz group, also called Poincare group 。 In the Poincare group, the physical quantity corresponding to the translation generator is energy momentum vector 。 The conservation of energy, momentum and angular momentum is directly related to the uniformity and isotropy of space-time, and it does not depend on the specific content of matter. Whether microscopic or macroscopic, particle or field, all materials moving in uniform and isotropic space-time comply with energy momentum and angular momentum Conservation law of^[4] 。

experimental verification

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Joule thermal work equivalent experiment Is famous for early confirmation of energy conservation experiment 。 On the premise of keeping the total energy unchanged, the inherent energy, kinetic energy and potential energy can be transformed into each other. The most typical example is in positron Annihilation with negative electron photon In the process of Dynamic mass ）。 As in Nuclear fission In the process, part of the inherent energy is converted into kinetic energy. For a composite system composed of multiple components, the overall inherent energy (or static mass) is the sum of the inherent energy (or static mass) of each component and the interaction potential energy. For example, stable Nucleus The static mass of nucleon （ proton and neutron ）The sum of the static masses of is small, and the difference between the two is called Quality loss The corresponding energy is Nucleus Of binding energy (from nucleon Interaction potential energy); Nuclear energy is the nuclear binding energy released in the process of nuclear reaction. It is Mass energy relation Direct evidence.

The law of conservation of energy and momentum （ angular momentum ）The most typical example of the successful application of conservation law is Elementary particle In the experiment neutrino Discovery of. Neutrino is a kind of basic particle with small static mass, uncharged and extremely weak interaction with matter. In the late 1920s and early 1930s, the research on the energy spectrum of nuclear beta decay found that the energy taken away by electrons (i.e. beta rays) emitted after decay was smaller than the energy it should take away according to the law of conservation of energy (it seems that part of the energy was lost), and Nucleus Of spin And Electronics The spin of is not consistent with the angular momentum synthesis rule in quantum mechanics. In order to explain this phenomenon, we should either give up energy and Conservation law of angular momentum , or assume that there is a kind of basic particle that cannot be observed, namely neutrino, in order to maintain these conservation laws. Physicists finally chose the latter and used other elementary particle experiments to prove that neutrino (and Antineutrino ）The law of conservation of energy and the law of conservation of momentum (angular momentum) are still valid in these processes.

above Special relativity The concepts and definitions of energy, mass and momentum, as well as the law of conservation of energy and momentum（ angular momentum ）Conservation law, or more general energy Law of conservation of momentum (The conservation of angular momentum is included in it), not only applies to mechanical phenomena, but also applies to the whole Flat spacetime Physics in^[5] 。

Discovery History

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Brief overview

The proposition of "the law of energy conversion and conservation" must be based on three foundations: ① a correct understanding of the nature of heat; ② Discovery of transformation between various forms of material movement; ③ Corresponding scientific thoughts. By the 19th century, all three conditions had been met.

The law of conservation of energy discovered in the middle of the 19th century is a very important law in natural science natural science The inevitable result of the gradual accumulation of law knowledge to a certain extent, the law of conservation of energy is the law connecting mechanical energy and internal energy (heat energy).

During the period from the end of the 18th century to the middle of the 19th century experience And a large number of production practices scientific experiment The first law of thermodynamics is established. In this process, German doctor J. Mayer and British physical scientist J. joule They made important contributions and reached the same conclusion through independent research. In 1842, in his article "On the Force of the Inorganic World", Mayer proposed the principle of mutual conversion of mechanical energy and heat Specific heat capacity at constant pressure with Specific heat capacity at constant volume The difference is used to calculate the value of thermal work equivalent. On Organism Movement and the new supersedes the old 》This book describes 25 cases of transformation of sports forms. Joule has done a lot of work since 1840 Current heating effect And thermal work equivalent (see Joule thermal work equivalent experiment ）。 From 1840 to 1845, he successively published "On Voltaic battery The resulting heat《 electrolysis The heat released in the metal conductor and battery pack when heat effect And the mechanical effect of heat, as well as temperature Change. He directly obtained the value of thermal work equivalent through various accurate experiments, and the consistency of the results laid a solid experimental foundation for the law of energy conservation and conversion. In addition to Mayer and Joule, many scientists have also contributed to the establishment of the first law of thermodynamics. For example, M. Segan made a statement in 1839 Thermochemistry in reaction heat Articles on laws irrelevant to intermediate processes; In 1843, L. Cordin published the experimental results of determining the thermal work equivalent; In 1847, H Helmholtz stay Centripetal force Based on the assumption of Thermal movement as well as Electromagnetic motion The law of mutual transformation and conservation. During this historical period, scientists from all countries were able to independently discover the laws of energy conservation and transformation, which was determined by the production conditions at that time. From the beginning of the 18th century to the second half of the 18th century, steam engines were manufactured, improved, widely used in Britain's iron smelting and textile industries, and Heat engine efficiency The research on friction heat generation in machines has greatly promoted people's understanding of the law of energy conversion^[6] 。

Discovery process

In 1798, C. Renford submitted to the Royal Society the theory of thermal motion derived from the barrel experiment Experimental report 。 1800, D· David use vacuum The experiment of melting ice with medium friction supported Renford's report. In 1801, in his Theory of Light and Color, T. Yang said light It has the same nature as heat, emphasizing that heat is a kind of motion. Since then, the theory of hot movement has gradually replaced Heat mass theory 。

At the turn of the 18th and 19th centuries natural phenomena The mutual transformation between them was discovered successively: after the transformation from heat to work and the discovery of the chemical effect of light, the infrared ray was discovered in 1800 heat effect （ Resonance effect ）。 Battery Just invented, I found Thermal effect of current and electrolysis Phenomenon. In 1820, it was found that Magnetic effect of current Discovered in 1831 Electromagnetic induction phenomenon 。 Discovered in 1821 Thermoelectric phenomenon The reverse phenomenon was found in 1834, and so on.

At the turn of the century, the idea of regarding nature as "vitality" was that Germany“ natural philosophy ”The main point of view. such Philosophy Put the whole universe It is regarded as the product of historical development caused by a certain root force. At that time, this philosophy was dominant in Germany and some countries in Western Europe.

Kano was the first person who proposed the conversion of thermal work. He believed that "heat is nothing more than a kind of power, or simply a kind of motion in the form of conversion. Heat is a kind of motion. For a small part of an object, if the power is eliminated, then at the same time, there must be heat that is strictly proportional to the amount of power eliminated. On the contrary, where the heat is eliminated, there must be power. Therefore, we can establish the proposition that the quantity of power is nature China is unchangeable. To be more precise, the amount of power can neither be generated nor eliminated. " At the same time, the rough value of thermal work equivalent is given.

This thought of Kano was not taken seriously until 1878, 46 years after his death. In 1842 before that, Michael of Germany first started from "natural philosophy" and interpreted 25 transformation forms of force from the causal chain of "cause equals result" in a speculative way. In 1845, he also used Specific heat capacity at constant pressure And Specific heat capacity at constant volume Difference: C p － C v = R The thermal work equivalent value is calculated as 1 calorie=365g · m.

1843, England experimental physicist Joule did more work to determine more accurate equivalent values. In 1850, the result published was: "to produce a pound of water (in the vacuum Medium weight, whose temperature is between 55 ° and 60 °) quantity of heat , 772 pounds needed to fall one foot Mechanical work 。” Joule's work is "conservation of force" principle It has laid a solid experimental foundation.

German scientists Helmholtz In 1847, he published the book On the Conservation of Force. Proposed that all natural phenomena should interact with the central force particle To explain. This proves that vitality and tension Conclusion of the conservation of central force by the sum of. And then discussed Thermal phenomenon The relationship between electrical phenomenon, chemical phenomenon and mechanical force, and the possibility of applying the principle of "conservation of force" to life organism. As Helmholtz's way of discussion is very physical, its influence is greater than that of Mayer and Joule.

The discoverers of the law still call energy "force"; Moreover, the expression of the law is not accurate enough, but the law of energy conversion and conservation has been found in essence. Comparing the two expressions, we can see that "the conservation of force" is much more profound than "perpetual motion machine cannot cause". "Conservation of force" refers to all forms of motion of matter that have been recognized; At the same time Philosophical thought Under the guidance of (Mayer), on the basis of experiments (Joule), the theory was established with the axiomatic structure (Helmholtz).

Although the principle of "conservation of force" has the relationship between joule's thermal work equivalent and electric heat equivalent, as well as various relationships introduced by Helmholtz, they are all independent and have not yet been unified Analytic expression To express.

Analytic expression

The analytic expression of the law can only be carried out if the concepts of "heat", "work", "energy" and "internal energy" are accurately defined. In the 18th century, the concept of "heat" was the quantity of heat and mass. In 1829, J· Poncelet In the process of studying the steam engine, it is clearly defined that work is the product of force and distance. The concept of "energy" was in 1717, and J. Bernoulli was discussing Virtual displacement When used. In 1805, T. Young called force energy, and thus defined Young's modulus. But its definition has not been accepted by people. A group of people of insight realized the significance of the law and carried out fruitful work to improve it. One of the most famous is the British W· Thomson And R. Clausius of Germany. It is they who put forward the first and second laws of thermodynamics on the basis of predecessors and established thermodynamics The building of theoretical system.

In 1850, Clausius He published a paper on the power of heat and the laws of heat itself that can be deduced from it. point out Carnot theorem Yes, use Thermal movement Explain and add proof. It is believed that the single principle is that "in all cases where work is generated by heat, there is a heat that is proportional to the generated work to be consumed, on the contrary, such amount of heat can also be generated by consuming the same amount of work." Add a principle that "under the condition of no consumption of any force or other changes, any amount of heat can be moved from a cold body to Thermosome , which contradicts the behavior of pyrogen. " To demonstrate. Think of heat as a State quantity 。

W. Thomson

Clausius finally obtained the analytical formula of the first law of thermodynamics:

d Q =d U -d W

Then the law of energy conversion and conservation The second law of thermodynamics Of entropy The statements of "" together form the basis of the theoretical system of thermodynamics.

In 1853, Thomson put forward the definition of energy again: "We express the energy of the material system in a given state as the sum of various actions measured in mechanical work units outside the system when it transits from this given state to any fixed zero state in any way." U It is called internal energy. People began to Newton "Force" and characterization of Material movement "Energy" is distinguished and widely used. On this basis, the Scottish physicist W. Rankin renamed the principle of "conservation of force" as“ Conservation of energy ”Principle.

Since 1854, Clausius has done a lot of work to find a kind of Method of proof To explain this principle. In 1860, Principle of conservation of energy It is widely recognized.

Formation process

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Energy can neither be created nor destroyed. The conservation of energy is one of the universal laws of material motion. Material movement There are various forms, and they can transform each other. Before and after transformation, the total amount of energy, as a measure of material movement, remains unchanged. The concept of conservation of energy has long been proved by physicists in the field of mechanics. However, this concept of conservation has been extended to internal energy (heat energy) for two or three hundred years. There have been various wrong understandings of thermal energy in history. From the 18th century to the mid-19th century, the natural science community was dominated by the theory of heat and mass for a long time. This one-sided theory believes that there is a kind of fluid in matter, called heat mass. The heat transfer caused by temperature difference is regarded as the flow of heat from high-temperature objects to low-temperature objects; The friction heat generation is considered to be the result of heat and mass release. This theory contradicts many experimental facts. 1798 Longford When developing the gun barrel, it is observed that the amount of heat generated is not proportional to the amount of metal chips drilled and ground, and if a blunt drill bit is used to continue drilling and grinding quantity of heat It is almost infinite, which means that heat cannot be a substance. Later, after the work of H. David, J. Mayer, H. Helmholtz, etc., especially the thermal work equivalent experiment conducted by J. Joule between 1840 and 1848, people gradually realized that thermal mass does not exist. Heat transmission or conversion, like the transmission or conversion of mechanical work and electrical work, is also a kind of energy transmission or conversion, and the total energy is constant during transmission or conversion. So, Conservation of energy Is recognized on a universal basis^[7] 。

In 1860, the law of conservation of energy "soon became the cornerstone of all natural sciences physics Every new theory should first be tested whether it is consistent with Principle of conservation of energy " But the discoverers of the principle only focus on the conservation of quantity to generalize the law, without emphasizing the transformation of motion.

Until the beginning of the twentieth century, an important basic concept in thermodynamics - the 18th century definition of heat was still used. This definition is based on the theory of heat and mass. There is still one weak cornerstone in the building of thermodynamics. Therefore, in 1909, C. Kara redefined internal energy: "Any object or system of objects Equilibrium state There is a state function U Is called its internal energy when the object passes through a Adiabatic process After reaching the second state, the increase of its internal energy is equal to the work done to it by the outside in the process W 。”

U two － U one = W

So defined internal energy It has nothing to do with heat, only mechanical energy and Electromagnetic energy of At this time, the first law of thermodynamics, the second law of thermodynamics and the whole thermodynamic theory abandoned the theory of heat and mass.

Historical influence

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Negative perpetual motion machine

allegedly Perpetual motion machine The concept of India , introduced in the 12th century Europe 。 It is recorded that the earliest and most famous design scheme of perpetual motion machine in Europe was proposed by V. Hennecko of France in the 13th century. Subsequently, people who study and invent perpetual motion machines continue to emerge, although many scholars point out that perpetual motion machines are impossible.

Renaissance Italian scholar Da Vinci I have spent a lot of energy on research Perpetual motion machine Finally, we get the conclusion that perpetual motion machine can not be manufactured. The contemporary J. Cardin (who was the first to give the solution Cubic equation It is also believed that perpetual motion machine is impossible. The first type of perpetual motion machine violates the law of conservation of energy, and Type II perpetual motion machine It violates the second law of thermodynamics.

With the recognition of the impossibility of perpetual motion machine, the patent offices of some countries decided not to accept the invention of perpetual motion machine patent application 。

Empirical statement

In the 13th century, people began to have the desire to make perpetual motion machines. In the 15th century, Great artist 、 scientist and engineer Da Vinci Invested in the research work of perpetual motion machine. In 1475, Leonardo da Vinci carefully summed up the lessons of failure in history and reached an important conclusion: "Permanent motion machines cannot be created." He also realized that the reason why machines cannot move forever is related to friction. So I was right friction Conduct in-depth and effective research. But Leonardo da Vinci was never right friction Obstructing machine movement to make scientific explanation, but not recognizing friction（ mechanical movement ）And Thermal phenomenon The essential connection between them.

The first type of "perpetual motion machine" designed by Galileo

Since then, some scholars have come to the conclusion that "perpetual motion machine is impossible to cause" and used it as an important principle in scientific research. Netherlands Of mathematics Mechanics S· Stevin In 1586, he used this principle to analyze the "Steven Chain" and took the lead in introducing Parallelogram rule 。 Galileo was demonstrating Law of inertia This principle has also been applied.

In 1673, C· Huygens This view is reflected in the book Pendulum Clock. hold Galileo Application of research results on inclined plane movement to Curvilinear motion , which leads to the conclusion that: gravity Under the action, the object rotates around horizontal axis When rotating, its center of mass will not rise above the height it fell. Therefore, it is impossible to make perpetual motion machine by mechanical method.

Historically, using the principle that "perpetual motion machine cannot be made", we have made brilliant achievements in scientific research France youth scientist Kano.

In 1824, Cano launched“ Carnot theorem ”The principle can only be used in mechanical movement and "heat and mass" flow. It is not the law of energy conversion and conservation in the modern sense, but the experience summary of energy conservation in mechanical movement. It is the original form of the law.

"The first type of perpetual motion machine is impossible to cause" is another expression of the first law of thermodynamics. Before the establishment of the first law, many people had fantasized about creating a kind of machine , called the first type of perpetual motion machine. The complete failure of the efforts to manufacture this kind of perpetual motion machine has promoted the establishment of the law of energy conservation and transformation from the opposite side^[6] 。

Thermomechanical

In 1798, American C. Langford found that when boring tools were used to drill the bronze blanks for manufacturing gun barrels, the metal blanks were hot. Longford noticed that as long as the boring and drilling did not stop, the metal kept heating up. The conclusion is that the mechanical motion of the boring tool is transformed into heat, so heat is a form of motion rather than a substance previously thought. Langford tried to calculate the heat generated by a certain amount of mechanical energy, and gave a rough Thermal equivalent Value of. Half a century later, the joule provided the correct value.

In 1712, Britisher T. Newman invented Atmospheric pressure Steam engine. This machine has cylinders and pistons. When working steam Import cylinder At this time, the cylinder stops supplying steam and water enters the cylinder. When the steam is cooled, it condenses into water, which rapidly reduces the air pressure in the cylinder and can suck up water. Later steam Guide the cylinder into the next cycle. The original steam engine went back and forth about ten times per minute and could work automatically, making the pumping work of the mine very convenient.

J· watt The steam engine was improved in the second half of the 18th century. The most important improvement is the invention of condenser Improved steam engine performance efficiency , the other is the invention of Centrifugal governor The steam engine speed can be freely controlled. After Watt improved the steam engine, Industry Shangcai is widely used.

Invention of thermometer

An accurate theory of heat should begin with the manufacture of thermometers. In the 17th century, G. Galileo and others began to make thermometer 。 Due to the adoption of Temperature scale It is inconvenient to use and seldom used by later generations.

In 1714, the practical temperature scale was established by German physicist D. Wallenheim, who began to use mercury as a thermometer and made continuous improvements Fahrenheit scale 。 Scientists officially determine Fahrenheit scale The boiling point of water is 212 degrees, and the freezing point of water is 32 degrees. This regulation is to try to avoid negative values of the normal temperature.

1742-1743, Sweden astronomer A· Anders Celsius Invented Celsius scale , with Standard status The freezing temperature of water is zero, and the boiling point of water is 100 degrees. 1948 Celsius scale Determined by the International Conference on Weights and Measures international standard 。

Experimental findings

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Thermal work equivalent test

J. Joule met in 1835 University of Manchester Professor of Dalton 。 Joule mathematics The research mainly depends on measurement. Energized after several measurements in 1840 conductor , found electric energy It can be converted into internal energy (heat energy) and a law can be obtained: Electrical conductor Resulting quantity of heat And Current intensity Of square , conductor resistance And adopted time Is proportional.

Joule continued to explore the relationship between the conservation and transformation of energy among various forms of motion. Joule declared at the British academic conference: "The energy of nature can not be destroyed. Where mechanical energy is consumed, there is always considerable heat. Heat is only a form of energy."

Joule continuously improved the measurement method and improved the measurement accuracy, and finally obtained the physical constant of "thermal work equivalent". The measured value of Joule is 423.9 kgm/kcal, and the accurate value of this constant is 418.4 kgm/kcal. International System of Units Joule is used as the unit of heat, and 1 card=4.184 Joule.

Discovery of heat

In the 1850s, britain scientist J. Blake mixed 32 ° F ice cubes with 172 ° F water of equal weight, and found that the average temperature was not 102 ° F, but 32 ° F. The effect was that all ice cubes melted into water. Blake concluded that the ice fusion It needs to absorb a lot of heat, which turns ice into water, but does not cause temperature rise. He guessed that ice fusion Hourly absorptive quantity of heat It is certain. Further experiments led Blake to find that various substances are occurring State change （ fusion 、 freezing 、 vaporization 、 coagulation ）When, there is this effect 。

Blake used a simple and intuitive method to determine the heat required for water vaporization. Blake measured that the heat required to melt a certain amount of ice is equal to the heat required to heat 140 ° F of water of the same weight (equivalent to the heat required to heat 77.8 ℃). The correct value is 143 ° F (equivalent to 80 ℃).

Based on the experimental facts, Blake began to realize that heat and temperature are two different concepts, and introduced the concept of "latent heat" (heat).

In 1780, France Scientist A. Lavoisier and P· Laplace Jointly proposed the correct measurement of substances Heat capacity Method. Due to the accuracy of heat, in 1822, the French scholar J. Fourier published his concluding work Analytical Theory of Heat.

The Precursors

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The battle between vitality and death

joule

1644 R· Descartes Discussed in Philosophical Principles Collision problems Introduced at momentum To measure motion. 1687 Newton On《 Mathematical Principles of Natural Philosophy 》The change of momentum is measured by force. In contrast, G. Leibniz criticized Descartes in a 1686 paper, claiming that motion is measured by multiplying mass by the square of speed, which Leibniz called vitality. Newton's force measured by momentum is called dead force. Leibniz's Proposition and Huygens' collision The conclusion of the problem study is consistent, which says that "when two objects collide with each other, the sum of the product of their mass and the square of their velocity remains unchanged before and after the collision."

Since Leibniz provoked the controversy, there has been a controversy between Descartes and Leibniz. This debate has lasted for nearly half a century, and many scholars have participated in the debate, each with experimental evidence. 1743 French scholar J· Darumbel In On Dynamics, he said: "For measuring a force, it is also reasonable to use it to give vitality to an object that is affected by it and passes a certain distance, or to use it to give momentum to an object that is affected by it for a certain time." Dalanberg revealed that vitality is measured by force at the action distance, while momentum is measured by force at the action time. The dispute finally settled. As a formal mechanical term, vitality is generally accepted.

Although the concept of vitality has been accepted, the relationship between vitality and force has not been clarified. In 1807, the British scholar T. Yang introduced the concept of energy, and in 1831, the French scholar G· Coriolis The concept of force doing work is introduced, which means that the work done by force is converted into the kinetic energy of the object, namely nature Of Conservation of mechanical energy 。

Meier's discovery

Mayer

J. Mayer (1814-1878) was a German physicist. In 1840 Java During the voyage of physics Be interested. When he was bleeding the sick sailor (the popular therapy at that time), he found that the blood in the vein was bright. He thought that the blood was bright red in the tropics, and the body needed more oxygen than in the temperate zone burning To maintain body temperature. This phenomenon prompted Mayer to think about the fact that food in the body is converted into heat and the body can do work. It is concluded that heat and work can be transformed into each other.

He noticed that many people's experiments on perpetual motion machines ended in failure, which made him guess that“ Mechanical work It can't come from nothing ".

Mayer

On September 12, 1841, he first mentioned the thermal equivalent in his letter to a friend: "It is still extremely important to solve the following problem: how high a weight (such as 100 pounds) must be lifted on the ground, so that the amount of movement corresponding to this height and the amount of movement obtained by putting the weight down is exactly equal to the heat necessary to convert a pound of 0 ℃ ice into 0 ℃ water. ”

In 1840, Mayer began to think about the source of heat in people. The movement of the heart could not produce so much heat and could not maintain human temperature. The body temperature is maintained by the whole body, which comes from food and ultimately from plants, which grow by absorbing the light and heat of the sun. Finally, it comes down to how the energy is transformed (transferred).

Mayer wrote an article "On the Force of Inorganic World", and measured the thermal work equivalent of 365 kg m/kcal. The paper was submitted to the Physical Yearbook, but could not be published. Not only was he not understood academically, but he also experienced a major blow in life. In 1858, world Rediscover Mayer, Switzerland Basel The Academy of Natural Sciences awarded him an honorary doctor. Obtained from the Royal Society Copley Medal 、 university of tubingen Honors of Doctor of Philosophy , Bavaria and Italy Academician of the Turin Academy of Sciences.

Mayer was the first scholar to carry out the thermal work equivalent experiment, although his experiment was rougher than that of Joule. He first expressed the law of conservation of energy: "What shows the absolute truth of my law is the opposite proof: that is, a universally recognized theorem in science: the design of perpetual motion machines is absolutely impossible in theory."

Mayer demonstrated that the sun is the ultimate source of all living and nonliving energy on the earth.

Later, the papers of Helmholtz and Joule were published one after another. People attributed the inventor of the law of conservation of energy to Helmholtz and Joule, and did not recognize Mayer.

In 1858, Helmholtz read Mayer's 1852 paper and admitted that Mayer's thoughts predate his influential paper. Clausius also thinks that Maier is conservation law The discoverer of. In 1862, Tindall Royal Society of London The work of Meier was systematically introduced on the website, and his achievements were finally recognized by the society.

Helmholtz's discovery

On July 23, 1847, H· Helmholtz (1821-1894) made a report entitled "On the Conservation of Force" to the Physics Society, and submitted the article to the editor of the Annals of Physics. Unexpectedly, the manuscript of Mayer in 1841 met the same fate, and the editor refused to publish it because there was no experimental fact. He published his paper as a pamphlet in a famous publishing house. The conclusion of this paper is completely consistent with the Joule experiment in 1843, and soon became known as "the highest and most important principle in nature". Due to the publication of a famous publishing house, Helmholtz and Mayer have completely different destinies. British scholar Kelvin The concept of energy proposed by T. Yang is adopted“ potential energy ”Substitution“ elastic force ”, with“ kinetic energy ”Instead of "vitality", the ambiguous concept that has lasted for nearly 200 years in mechanics has been changed.

The law of conservation of energy is a universal basic law of nature and a powerful weapon for people to understand and use nature.

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