This article is taken from Les Indispensables de Sciences et Avenir #212 January/March 2023.
Ask your loved ones: “What is warmth?” They will probably answer you “temperature”: hence the measurement of a property associated with heat, but not heat. But what then is its nature? This has been a mystery for a long time. “Unlike what happened in other fields, industrial application preceded understanding,” admits Emanuel Bertrand, ESPCI Paris-PSL lecturer. “At first we wondered about the possibility of converting heat into mechanical energy.”
The first industrial steam engine belongs to the English engineer Thomas Newcomen, who used it in coal mines in 1712 to pump stagnant water from underground galleries. Scottish engineer James Watt improved his performance between 1763 and 1769, paving the way for the Industrial Revolution. Then we know how to use the driving force of heat… without understanding it yet. But knowing the concept will eventually follow.
The French chemist Antoine Lavoisier, in his 1770 treatise, states that heat is due to an invisible and massless fluid, caloric, which circulates between molecules and can pass from one body to another while maintaining itself. On this basis, the French engineer Sadi Carnot, without formalizing it mathematically, in 1824 concluded that the efficiency of a steam engine depends solely on the temperature difference between the hot and cold reservoirs: the higher it is, the greater the heat transfer. significant. He then invented what Rudolf Clausius would call in 1850 the second principle of thermodynamics, a term coined by William Thomson, the future Lord Kelvin, the previous year: “Any transformation of a thermodynamic system occurs with an increase in global entropy. [le degré de désorganisation]”.
Democritus updated … in 1808!
“Carnot achieved the right result thanks to a series of mistakes that compensated for each other,” emphasizes Emanuel Bertrand. “The first was to keep warm.” This was refuted simultaneously and independently by the Englishman James Joule, the German Julius von Mayer, and the Dane Ludwig Kolding between 1842 and 1843. All three experimentally established the principle of equivalence between heat and energy transferred by the system. (called work).
In 1850, Rudolf Clausius went even further: in his opinion, it is not heat that is conserved, as Lavoisier claimed, but the sum of heat and work, hence the total internal energy of the system. To this principle, he adds a hypothesis that could not then be tested experimentally: it is the movement of molecules that produces heat. Ten years later, James Maxwell would publish his kinetic theory of gases, in which he would establish the relationship between, on the one hand, the temperature and pressure of a gas, and, on the other hand, the motion of hundreds of thousands of the billions of billions of molecules that make it up.
The theory that the Austrian physicist Ludwig Boltzmann used in 1872 to describe heat as a manifestation on our scale of molecular motion. “For me, this is a key date. Heat becomes a consequence of the microscopic properties of matter,” says Emanuel Bertrand. Alas, Boltzmann’s proposal is highly controversial because it is based on the atomistic theory of the Greek Democritus, updated in 1808 by the English chemist John Dalton, but did not reach consensus until 1913. the scientific community about it by finding Avogadro’s number in a dozen different experiments. This chemical constant makes it possible to relate the microscopic world (number of atoms) to the macroscopic world (moles) available for experiments.
Once the existence of atoms is recognized, physicists will continue to improve thermodynamic theories throughout the 20th century, especially in non-equilibrium cases where at least one of the variables (temperature, pressure, etc.) in the system is not constant. These studies will earn the American Lars Onsager the Nobel Prize in Chemistry in 1968. The Belgian, born in Russia, Ilya Prigogine, will receive it in 1977 for dissipative structures, systems that organize themselves by exchanging energy with the environment. The topic is still relevant…
Heated by sunlight
If the Sun shines on us, then it does not heat us directly, since heat is transferred by the disturbance of matter, and interplanetary space is almost empty. On the other hand, it warms itself. But what is the source of its heat? At the beginning of the 19th century, scientists imagined that his material was burning like a pile of burning wood. But in this hypothesis, the Sun would have to live only a few thousand years … In 1853, the Englishman John Waterstone thought about the energy released by the gravitational contraction of a star.
Lord Kelvin and Hermann von Helmholtz calculated that the Sun must then have been several million years old. But the sedimentation studies of Charles Lyell, supported by Charles Darwin, indicated the age of the Earth at least several hundred million years … In 1919, the Frenchman Jean Perrin put forward the correct hypothesis: the energy of a star comes from nuclear fusion, which occurs in its heart, turning hydrogen into helium . The reactions described in detail in 1939 by the German exiled to the United States Hans Bethe: every second 620 million tons of hydrogen are converted into 615.7 million tons of helium.
“The mass difference is converted into radiated energy in the form of photons, in accordance with the equivalence between mass and energy shown by the famous Einstein equation,” explains Milan Maksimovic, director of research at the Paris Observatory-Meudon. The emitted photons try to leave the Sun, but are constantly absorbed by the solar matter, which re-radiates others and moves in all directions. As they are deexcited, these molecules emit other photons. On average, after 700,000 years, photons reach the level of the photosphere and go into the interplanetary medium. Then they lost energy, which reduced the agitation of matter and, therefore, heat. From 15 million degrees at the center of the star, the temperature rises to 6051 °C at the level of its photosphere, the visible surface of the Sun. From there, photons enter the Earth’s atmosphere, giving it energy. Excited by this absorption, the nitrogen and oxygen molecules return to their stable state, re-emitting lower energy photons. The Earth reflects this light with thermal radiation towards the atmosphere: at an average temperature of our planet – 15 ° C – it emits infrared photons. This light is partially absorbed by certain gases in the atmosphere, known as greenhouse gases, and re-emitted to the earth. And the Earth is warming up a little more.
Solar flares can disturb the Earth’s magnetic field, but do not affect the temperature of our planet’s atmosphere. 1 credit