The Beginning of Physics: Thermodynamics (Part-12)

(Last Updated On: April 16, 2020)

What is it?

In the words of William Thompson (1824-1907), most know as Lord Kelvin, “Thermo-dynamic is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency“. This was the first definition of thermodynamics and a very simple and elegant one. This is the branch of physics we’ll explore today.

Thermodynamics was crucial on the industrial revolution (you’ll understand why in a minute), in inventions like the steam engine. It was highly developed in the 1700s and 1800s, time which we’ll now explore.

a small scale version of a James Watt steam engine model

The wrong theory of heat

Antoine Lavoisier (1743-1794) was a well-known chemist in the 1700s and made some pretty important discoveries in his field. He discovered hydrogen and oxygen, and the role this last one plays in combustion. He also would formulate the Lavoisier’s Law, or the law of conservation of mass, which states that “Nothing is lost, nothing is created, everything is transformed“. This law would be a predecessor of what would later become the 1st Law of Thermodynamics.

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Antoine Lavoisier and his chemist wife Marie Lavoisier (which will be important up next)

Lavoisier, like many other scientist, tried to answer the important question: How does stuff heats up? Well, Lavoisier tried to answer this with his Caloric theory. This theory explained heat transfer as an ether, or colorless fluid, which migrated from an hotter body to a colder one. Although it seemed elegant and therefore correct at the time, this idea was wrong.

In fact, at this time “ether” was used to explain many unknown phenomena, as you’ll notice in future articles.

The not-so-wrong-theories of heat

As I already said, many scientists of the time were trying to solve the mystery that was heat transfer. Well, Benjamin Thompson tried to understand this phenomena and got it kind of right.

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Portrait of Benjamin Thompson

Benjamin Thompson did some exciting work. He conducted his experiments in the barrels of canons (he would work on designs of war machines most of his life).

Due to his position, Benjamin would work a lot with heat. Benjamin would investigate the insulating properties of various materials. He correctly discovered that air was a great insulator (although he in a somewhat reckless, and incorrect, inference that all gases were perfect non-conductors of heat). Later in 1797, he extended his claim about the non-conductivity of all liquids. This was clearly wrong: water boils, so that was kind of problematic.

Thompson’s most important scientific work took place in Munich, and centered on the nature of heat. He conducted an experiment under water where he would basically drill a hole in a long piece of metal for over two hours. What happened contradicted the caloric theory: the water boiled.

He understood that heat wasn’t some chemical reaction but simple mechanical motion. Benjamin would publish his results in 1798 in “An experimental inquiry concerning the source of the heat which is excited by friction“. This was in accordance with the less accepted theory (at the time) dating from Newton’s time, which explained heat has the motion of the particles of a substance.

Industrial Revolution

You might have heard about this revolution which “happened” in the 19th century in England with the invention of the steam engine. This revolution wasn’t a normal scientific revolution, but a technological one. It didn’t happen because of asking stuff about nature, it did because of the creation of some pretty cool machines. The “whys” only came later.

Carnot – The Father of Thermodynamics

Sadi Carnot was one of the first that tried to understand why did steam engines and other machines related to this worked the way they worked. He his often described as the father of thermodynamics.

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Carnot at the age of 17

Carnot believed that working out the efficiency of steam engines would greatly help France becoming a glorious empire. While trying to understand how to do more efficient engines, Carnot worked a lot with thermodynamics.

You see, in engine cycle parts of the system move through different states of energy until returning to the initial state. And no one could mathematically explain what was going on inside such a engine. Carnot was the first do so, via what is now called the Carnot Cycle.

The Carnot cycle provides an upper limit on the efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work. A system undergoing a Carnot cycle is called a Carnot heat engine, although such a perfect system is only theoretical (however a microscopic Carnot heat engine has been constructed and run).

Sadi Carnot expressed this cycle in his only book called Reflections on the motive power of fire and on machines fitted to develop that power. In this book he also expressed what would later become the 2nd Law of Thermodynamics which states that “the total entropy of a system can never decrease, only stay steady or increase“. This law is a simple way of explaining the state of disorder of a system.

The title page of the first edition

Unfortunately, Carnot didn’t fully understand what he had going: he presented his findings in terms of the caloric theory, which still reigned at the time. In fact he would never understand this idea in its full splendor: he would soon die with the tragic age of 36.

Thermodynamics in the 19th century

In the 1840s the connection between heat and mechanical work was established by Julius Robert von Mayer (1814-1878) and James Prescott Joule (1818-1889). The SI derived unit of energy, the joule, is named after the last one.

James Joule’s experiments

In James Joule’s many experiments with batteries and electromagnets, he tried to find the relation between heat and mechanical work. During his experiments he discovered that the heat needed to increase the temperature of a pound of water by one degree Fahrenheit was equal to 4.14 joule/calorie of work. This meant that with simple mechanical motion you could create heat, which was against the caloric theory! Joule showed his results to the British Association for the Advancement of Science in 1843, but was met by silence.

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James Joule

Undaunted, Joule continued his experiments. This time he forced water through a perforated cylinder and measured the increase in temperature. He found the same results: the mechanical heating-up energy was the same as his electrical heating-up, about four joules. Or in the words of Joule himself: “Whenever mechanical work is expended an exact equivalent of heat is always obtained“.

Although he publish the book “Mechanical equivalent of heat” with all his experiments, caloric theory still reigned.

Julius Von Mayer

Julius Von Mayer did some pretty important work as well. He co-discovered the equivalence of heat energy, although he was only recognized many years later after his death. He also hypothesized that plants convert light into chemical energy or photosynthesize (way ahed of his time).

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Julius Von mayer

When he tried to publish his own results, he was rejected (probably because he was only an amateur scientist). After this, Von Mayer entered a big depression and attempted suicide, but only broke his legs. He was declared insane and locked up on asylum for the rest of his life.


In 1847 William Thompson, more known as Lord Kelvin, would hear James Joule talk at the British Association in 1847, making yet another of his, so far, ineffective attempts to discredit the caloric theory of heat. Kelvin was intrigued but skeptical.

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William Thompson, 1st Baron Kelvin

Kelvin conducted many experiments in the following years, and concluded that Joule was actually right. Due to this, he tried to conciliate Joule’s theory with Carnot’s. In his paper Kelvin supported Joule’s theory of heat and motion, and would collaborate with him for many years to come.

Kelvin also proposed an absolute temperature scale. This would also lead to the postulation of a point that would be reached when no further heat could be transferred, the point of absolute zero. This would be crucial to the 3rd Law of Thermodynamics, that would only appear 50 years later.

Baltas Cruz

Baltas Cruz

A 15-year-old Portuguese who at such a young age has the ambitions of becoming a theoretical physicist. Whilst maintaining his passion of becoming a physicist, he is also a chess player where he has won some local competitors alongside being a runner up in the local team also.

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