Discovery process
/kloc-The law of conservation of energy discovered in the middle of 0/9th century is a very important law in natural science. Its discovery is an inevitable event that human understanding of the laws of natural science has gradually accumulated to a certain extent. Nevertheless, its discovery is still tortuous, arduous and exciting. Understanding the discovery process of the law of conservation of energy is conducive to understanding the accumulation and formation of theories in the development of natural science. This paper briefly describes the discovery process of the law of conservation of energy.
1. Prepare for discovering the law of conservation of energy
The law of conservation of energy is a law about mechanical energy and thermal energy. It goes without saying that before it is discovered, people must have a deeper study of mechanical energy and thermal energy. We will now describe these two aspects.
Debate between vitality and death
Rene descartes (1596- 1650) introduced the concept of momentum to measure motion when discussing collision in Principles of Philosophy. Isaac newton (1687) measured force through the change of momentum in his Mathematical Principles of Natural Philosophy. On the other hand, Gottfried Wilhelm Leibniz (1646- 17 16) criticized Descartes in a paper in 1686, and advocated that the movement should be measured by multiplying the quality by the square of the speed, which Leibniz called vitality. Newton's force measured by momentum is also called death force. Leibniz's proposition coincides with Huygens' conclusion about collision in 1669. The conclusion is that "when two objects collide with each other, the sum of the square products of their mass and velocity remains unchanged before and after the collision."
Descartes and Leibniz have been arguing since Leibniz provoked the argument. This debate lasted for nearly half a century, and many scholars participated in it, each with its own experimental evidence. Until 1743, French scholar D'Alembert (17 17- 1783) said in his "Dynamics": "For measuring a force, use it to give vitality to an object affected by it and passing a certain distance, or use it to give it vitality. Here, D'Alembert reveals that vitality measures force according to action distance, while momentum measures force according to action time. The argument was finally settled. As a formal mechanical term, vitality is generally accepted by mechanics.
Although vitality is accepted by mechanics, its relationship with force is not clear. Until 1807, British scholar Thomas Young (Thomas Young, 1773, 5 10- 1829, 5 10) introduced the concept of energy, and French scholar Coriolis (183).
The formula F= 1/2mv2 indicates that the force does work and is converted into the kinetic energy of the object. That is to say, the mechanical energy of nature is conserved.
The invention of thermometer and the discovery of latent heat
An accurate thermal theory should start with the manufacture of thermometers. Starting from17th century, Galileo (1564- 1642) and others began to make thermometers in Italy. However, due to the inconvenience of temperature scale, it is rarely used by future generations.
The earlier practical thermometer was German physicist Daniel Gabriel Fahrenheit (1686- 1736). From 17 14, mercury was used as a thermometer, and it was continuously improved until 17 17 basically determined the Fahrenheit thermometer. It was not until the death of Warren Hai that scientists officially determined the Fahrenheit scale as follows: the boiling point of water is 2 12 degrees, and 32 degrees is the freezing point of water. Therefore, this regulation is to try to avoid taking negative values for the usual temperature.
Like the centigrade scale from 1742 to 1743 invented by Swedish astronomer anders celsius (1701744), the freezing point of water in the standard state is zero, and the boiling point of water is 100 degrees. The centigrade scale was designated as an international standard by the International Conference on Weights and Measures in 1948.
The invention of thermometer prepared the necessary conditions for the accuracy of heat, and people can use it to measure the temperature changes of substances under various conditions. At the earliest time, people did not distinguish between temperature and heat, and thought that temperature was heat.
In the 1950s, British scientist Joseph. Blake (1728- 1799) mixed ice cubes with the same weight of172 f water, and found that the average temperature was 32 instead of102 f. ..
Blake came to the conclusion that when ice melts, it needs to absorb a lot of heat to turn ice into water, but it can't cause the temperature to rise. He also guessed that the amount of heat absorbed by ice when it melted was certain. In order to find out this problem, he carried out a reverse experiment, that is, to observe whether water will release a certain amount of heat when it solidifies. He kept shaking the supercooled water at MINUS 4 degrees Celsius, so that part of the supercooled water solidified into ice, and the temperature rose as a result; When the supercooled water is completely solidified, the temperature rises to zero degrees Celsius, indicating that water does release heat when it is solidified. A large number of further experiments made Blake discover that all kinds of substances have this effect when their states change (melting, solidification, vaporization and condensation). He used to cover a vessel containing alcohol with a glass cover, and the air in the glass cover was taken away, so the alcohol in the vessel evaporated rapidly, and as a result, many small water droplets condensed on the outer wall of the glass cover. This shows that liquid (alcohol) absorbs a lot of heat when it evaporates, which cools the glass cover and condenses water droplets on the outer wall.
Blake used a very simple and intuitive method to measure the heat required for water vaporization. He used a stable fire to burn one kilogram of water at zero degrees Celsius to make the water boil, and then continued to burn the fire until the water completely evaporated. He measured that the time for boiling water to completely evaporate was 4.5 times that for water to rise from 0℃ to boiling, indicating that the heating ratio was 100: 450. This experiment is of course very rough, and the measured values also have great errors; Current measurements show that the ratio is 100: 539. Blake also measured by a similar method that the heat required to melt a certain amount of ice is equal to the heat required to heat the same weight of water at140 F (equivalent to the heat required to heat 77.8℃), which is a little too small. The correct value is 143 F (equivalent to 80℃), but the measurement result was the same at that time.
Based on these experimental facts, Blake began to realize that heat and temperature are two different concepts in 1760, and then he introduced the concept of "latent heat" in 176 1.
Subsequently, French scientists lavoisier (1743- 1794) and Laplacian (1749- 1827) cooperated and put forward the correct method for measuring the heat capacity of materials in 1780. Due to the maturity of accurate heat, in 1822, the French scholar jean baptiste joseph fourier (1768 ~ 1830) published his summary work "Analysis Theory of Heat" about years of heat research.
The invention of heat engine
Since ancient times, human beings have realized that mechanical movement can generate heat. Whether in the East or the West, there are ancient records of drilling wood for fire, which is an early practice of converting mechanical motion into heat energy. However, no one has thought about the quantitative conversion of mechanical energy and thermal energy for thousands of years. Until American langford (Benjamin Thompson rumford, Earl,1753-1814)1798 noticed in Munich that when boring the bronze blank of the gun barrel, the metal blank was as hot as fire and had to be continuously cooled with water. Langford noticed that as long as the drilling didn't stop, the metal would keep heating; If all this heat is transferred to the original metal, it is enough to melt it. Langford's conclusion is that the mechanical motion of boring cutter is converted into heat, so heat is a form of motion, not a substance as previously thought. Langford also tried to calculate the heat generated by a certain amount of mechanical energy. Therefore, langford first gave a value that we now call the mechanical equivalent of heat. But his number is too high. Half a century later, Joule provided the correct value.
When it comes to the conversion of thermal energy into mechanical energy, the earliest thing that should be mentioned is the steam engine invented by Hiro of Alexandria (about AD 62) in Alexandria. The invention is a hollow ball connected with two elbows. When the water in the sphere boils, steam is ejected through the tube, and the sphere rotates rapidly. This is the earliest steam engine. But at that time, it was only used for offering sacrifices to gods and playing, and it had no practical application.
17 12 years, an Englishman named Thomas Newcomen (1663- 1729) invented the atmospheric steam engine. This machine has a cylinder and a piston. When working, steam first enters the cylinder. At this point, the cylinder stops supplying steam and water enters the cylinder. When steam condenses into water, the air pressure in the cylinder drops rapidly and water can be sucked up. The steam is then introduced into the cylinder for the next cycle. At first, this kind of steam engine went back and forth about ten times per minute, which could work automatically, greatly facilitating the pumping work in the mine, so it was used not only by the British, but also by Germany and France.
James watt (1736- 18 19) improved the steam engine in the second half of 18th century. Among them, there are two most important improvements, one is the invention of condenser, which greatly improves the efficiency of steam engine, and the other is the invention of centrifugal governor, which enables the speed of steam engine to be controlled freely. After Watt's improvement, the steam engine was really widely used in industry.
The impossibility of perpetual motion machine
It is said that the concept of perpetual motion machine originated in India and was introduced to Europe in the12nd century.
According to records, the earliest and most famous design scheme of perpetual motion machine in Europe was put forward by a Frenchman named Vilander de Honnecott in the 3rd century. As shown in the figure: there is a rotating shaft in the center of the wheel, and 12 movable short rods are installed at the edge of the wheel, and one end of each short rod is equipped with an iron ball.
Subsequently, people who studied and invented perpetual motion machines constantly emerged. Although many scholars have pointed out that perpetual motion machine is impossible, people who study perpetual motion machine are still moving forward wave after wave.
Leonardo da Vinci (1452- 15 19), a great Italian scholar in the Renaissance, once studied perpetual motion. What is commendable is that he finally came to the conclusion that perpetual motion machine is impossible.
At the same time as Leonardo da Vinci, there was an Italian named Cardin (Jerome Cardin,1501-kloc-0/576). He was first famous for giving the roots for solving cubic equations, and he also thought perpetual motion machine was impossible.
Regarding the impossibility of perpetual motion machine, we should also mention the Dutch physicist simon stevin (1548 1620). /kloc-before the 6th century, in statics, people only dealt with the problem of finding the resultant force of parallel force systems and its balance, and the problem of decomposing a force into parallel force systems, but not the balance of intersecting force systems. In order to solve this problem, people boil it down to solving the balance problem of three intersecting forces. The problem was solved by clever debate. Suppose you put a uniform chain ABC on an asymmetric upright (frictionless) wedge, as shown in the figure. At this time, the chain is affected by the reaction force on the two contact surfaces and its own gravity. It happens to be three converging forces. Will the chain slide here or there? If so, which way? Stephen imagined stopping the wedge in the air and connecting the chain at the bottom with CDA, as shown in the figure, and finally solved the problem. The chain hanging at the bottom is self-balancing. Connect the suspension parts with the upper chain. Stephen said, "If you think the chain on the wedge is unbalanced, I can be a perpetual motion machine." In fact, if the chain will slide, then you will inevitably introduce that the closed chain will slide forever; This is obviously absurd, and the answer must be that the chain does not move. He got three conditions of balance of power. He thought this proof was wonderful, so he put Figure 2 on the title page of his book The Essence of Mathematics, and his colleagues carved it on their tombstones to show their admiration. The solution of the balance problem of cross force system also marks the maturity of statics.
With the impossibility of perpetual motion machine, some countries have imposed restrictions on perpetual motion machine. For example, as early as 1775, the French Academy of Sciences decided not to publish a newsletter about perpetual motion machines. 19 17, the us patent office decided not to accept the patent application of perpetual motion machine.
According to F. Charlesworth, assistant evaluator of British Patent Office, the first patent of perpetual motion machine in Britain was 1635. Between 16 17 and 1903, the British Patent Office received about 600 patent applications for perpetual motion machines. This does not include the patent application for perpetual motion machine using the principle of gravity. However, in the United States, after 19 17, there are still many perpetual motion machine schemes that can't see the mystery at the moment and are accepted by the patent office.
2. Meyer's discovery and experience
On the basis of previous scientific research, the measurement and conservation of mechanical energy, the measurement of thermal energy, the mutual transformation of mechanical energy and thermal energy, and a large number of practices of perpetual motion machines were declared impossible. The conditions for discovering the law of conservation of energy are gradually maturing. So this discovery was first started by Mayer.
Julius Robert Meyer (18 14- 1878) is a German physicist. He studied medicine in university, but he didn't like being a doctor. He is a ship doctor, and his work is relatively leisure.
In the west, there has been a large-scale bloodletting therapy since about the 4th century. About 12 to 13 ounces (about 340-370 grams, as much as a cup) of blood will be discharged at a time, and the rest will be discharged until the patient feels dizzy. The basis of this therapy is a so-called "liquid pathology" theory in the ancient west, which holds that the human body contains many kinds of liquids, such as blood, phlegm and bile. Too much or not enough of these liquids will cause diseases. The function of bloodletting is to remove excess fluid. In the Middle Ages, the rich in the West, especially those noble elites and gentlemen, had to bleed regularly all year round, usually once in spring and autumn. Another function of bloodletting is to make women look better, which is related to the western aesthetic standards at that time, making them look white and not ashamed. So western ladies often bleed. As a doctor, it goes without saying that Meyer often uses bloodletting therapy to treat people.
About 1840, during the voyage to Java, I became interested in physics because of the consideration of animal body temperature. In Surabaya, when he bled some sick sailors, he found that the blood in the veins was bright. At first, he mistakenly thought that he had cut the wrong artery. So, he thought, blood is redder in the tropics, and the body doesn't need to burn more oxygen to keep its body temperature as it does in temperate regions. This phenomenon prompted Meyer 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 also noticed that many people's experiments with perpetual motion machines ended in failure at that time, which left a profound influence on him since he was a child. These made him guess that "mechanical work can't be created from scratch."
The mechanical equivalent of heat was first mentioned in his letter to a friend on September 184 1. He said: "For my theory that can be explained by mathematical reliability, it is still extremely important to solve the following problem: How high must a heavy object (for example, 100 pounds) be lifted to the ground, so that the amount of exercise corresponding to this height and the amount of exercise obtained by putting down the heavy object are exactly equal to the heat required to convert a pound of ice at 0℃ into water at 0℃."
1842 In March, Meyer wrote a short article "Views on Force in Inorganic Field" and sent it to Justus von Liebig, editor-in-chief of Chronicle of Pharmacy and Chemistry (Justus von Liebig, 1803- 1873). Justus von Liebig immediately agreed to use this article. This paper explains the mechanical equivalence of heat for the first time. It is found that the work done by a heavy object falling from a height of about 365 meters is equivalent to the heat required to raise the same weight of water from 0℃ to 1℃. His article was published in May 1842.
Meyer was the first scholar to carry out mechanical equivalent thermal experiments. 1842, he used a horse-drawn mechanical device to stir the pulp in the pot, compared the work done by the horse with the temperature rise of the pulp, and gave the mechanical equivalent of the calorific value. His experiment was rougher than Joule's later experiments, but he deeply realized the great significance of this problem and expressed the law of conservation of energy for the first time. In his letter to a friend at the end of 1842, he said: "I think subjectively, it is this opposite proof that shows the absolute truth of my law: that is, a universally recognized theorem in science: the design of perpetual motion machine is absolutely impossible in theory (that is, even if people do not consider mechanical difficulties, such as friction, etc., it is impossible for people to design it successfully ideologically). And all my assertions can be regarded as pure conclusions drawn from this impossible principle. If someone denies my theorem, then I can build a perpetual motion machine at once. "
Meyer's paper did not attract social attention. In order to make up for the shortcomings of the first paper, he wrote a second paper, which was not adopted. He proved that the sun is the ultimate source of all living and abiotic energy on earth.
Later, Helmholtz and Joule's papers were published one after another, and people attributed the inventor of the energy conservation theorem to Helmholtz and Joule. However, although his paper was early and systematic, it was not only not recognized, but also attracted some attack articles. Add 1848, it never rains but it pours. Two children died and his younger brother was involved in revolutionary activities. 1849, Meyer jumped from the third floor and became severely disabled. Later, he was diagnosed with schizophrenia and sent to a mental hospital. Doctors think that the new discovery he often talks about is the mental symptom of megalomania.
1858 Helmholtz read Meyer's 1852 paper and admitted that Meyer was earlier than his influential paper. Clausius also thinks that Meyer is the discoverer of conservation law. Clausius told this fact to the British vocalist john tyndall (1820- 1893). It was not until 1862 that Tindal systematically introduced his work at the Royal Society of London, and his achievements were recognized by the Society. 1860, Meyer's early papers were translated into English and published. 1870, Meyer was elected as a member of the Communication Academy of Paris Academy of Sciences and won the Pang Si Prize. After that, Meyer's fate improved greatly.
3. The work of Helmholtz and Joule
Helmholtz and his law of conservation of force
Herman Helmholtz (182 1- 1894) was born in a poor German teacher's family. After graduating from high school, he served in the army for 8 years and was admitted to the Royal Academy of Medical Sciences in Berlin at public expense. 1842 Helmholtz received his doctorate. From 65438 to 0845, he joined the Berlin Physics Association organized by young scholars. After that, he often participated in the activities of the association. Besides being a military doctor, he also studies all the problems he is interested in.
1847 On July 23rd, he gave a famous report entitled "On the Conservation of Force" to the Physical Society. After the report, he handed the article to the editor of the Chronicle of Physics, only to find that this article had the same fate as Meyer's manuscript six years ago, and the editor refused to publish it on the grounds that there were no experimental facts. Later, he published this paper as a pamphlet in another famous publishing house. The conclusion of this paper is completely consistent with Joule's experiment in 1843, and soon it is called "the highest and most important principle in nature". Time is only a few years, and because of the publication of a famous publishing house, his fate is completely different from Meyer's. Later, Kelvin, a British scholar, adopted the concept of energy proposed by Yang, replacing "elasticity" with "potential energy" and "vitality" with "kinetic energy", which changed the conceptual ambiguity in mechanics that lasted for nearly 200 years.
About Helmholtz, it is worth introducing his organizational role in the development of German scientists. 1870, his teacher Heinrich Gustav Magnus (1802- 1870), the earliest director of the German Institute of Physics, died. Helmholtz, then an associate professor, succeeded him as director. At that time, Germany's scientific research level was far behind Britain and France. Shortly after the Franco-Prussian War, Germany received a large sum of compensation from France, and its economic situation improved. Helmholtz got a fund of 3 million marks to build a new research institute, which was built after five years' efforts. This institute later attracted a large number of outstanding young scholars, whose research topics were closely related to the development of industry, and later formed a very good scientific research tradition in Germany. Among the supporters of the Institute are Sir William Siemens, a great German entrepreneur and inventor (1823- 1883). He and Helmholtz are the first members of the Berlin Physical Society and are old friends. Helmholtz served as the German Physical Society for decades. Known as the "Chancellor of German Physics".
Mechanical equivalent of Joule thermal experiment
James Prescott Joule (18 18- 1889) is the son of a wealthy British brewer, and his economic conditions can provide him with a lifetime of research work. Joule was weak from childhood and suffered a spinal injury, so he concentrated on reading and studying, and his father provided him with a home laboratory. 1835, he met Dalton, a professor at Manchester University, and got the guidance of the latter. Joule's success mainly depends on self-study. Joule knows little about mathematics, and his research mainly depends on measurement. 1840, he made many measurements on charged conductors, found that electric energy can be converted into heat energy, and came to a law that the heat generated by electric conductors is directly proportional to the square of current intensity, the resistance of conductors and the passing time. He wrote this law into a paper "On the Heating of Voltaic Electricity".
Later, Joule continued to explore the relationship between energy conservation and transformation among various forms of motion. In 1843, he published a paper on the heat generated by water electrolysis and the mechanical value of electromagnetic thermal effect and heat. Especially in the latter paper, Joule declared at the British academic conference: "Energy in nature cannot be destroyed, where mechanical energy is consumed and considerable heat can always be obtained, and heat is only one form of energy."
Since then, Joule has continuously improved the measurement method and improved the measurement accuracy, and finally got a physical constant called "mechanical equivalent of heat". At that time, the value measured by Joule was 423.9 kg m/kcal. Now the value of this constant is 4 18.4. In order to commemorate him, later generations adopted joule as the unit of heat in the international system of units, taking 1 calorie =4. 184 joule.
4. Summary
Only when the concepts of work and energy become clear, can heat be distinguished from temperature, and they can be accurately measured. Only when the trend of heat engine is practical and familiar to people, the conditions discovered by the law of conservation of energy tend to mature when a large number of perpetual motion machines fail.
Even so, people's understanding of the prophet is relatively slow. Meyer's experience illustrates this point.
Importance of the Law of Conservation of Energy
The law of conservation of energy is still an important law of mechanics and even the whole natural science. But it will still develop. 1905, Albert Einstein (1879- 1955) published a famous paper on the special theory of relativity, an enlightening view on the generation and transformation of light, which revealed the law of conservation of mass and energy, that is, in an isolated system, the sum of relativistic kinetic energy and static energy of all particles remained unchanged during the interaction.