Laws of Thermodynamics

Have you ever wondered why you can’t get something for nothing? Why you can’t create energy out of thin air or make a perpetual motion machine? Well, it all comes down to the laws of thermodynamics—the rules that govern how energy works in the universe.

The laws of thermodynamics describe fundamental relationships between energy, work, temperature, and entropy applicable to all nature down to the sub-atomic scale. Although discovered in an effort to create more energy-efficient machines during the 19th century, these laws are central to physics, chemistry, cosmology, and the life sciences. They answer questions such as “Why does everything fall apart?”, “Why do we have to eat?”, “How did the universe begin?” and possibly even “What is time?”

Every practical physics problem is solved by invoking the first law of thermodynamics, most succinctly known as “the conservation of energy.” In its original interpretation, the first law stated that the energy a machine puts out as work can be no more than the amount of energy that goes into it as heat. In other words, energy can only be transformed between heat and work, neither created nor destroyed. Technically, “thermodynamics” describes only systems made of many parts, for which the concept of temperature makes sense, as with gas in a combustion chamber. But we now understand that the conservation of energy applies to everything down to the scale of sub-atomic particles—energy put out must be equal to or less than energy coming in.

The first law of thermodynamics guides all problem-solving in physics; you describe all the forms of energy at work in the problem in such a way that the total energy equals zero. Or you use the principle to find out how much energy must come out or go in. This is how you would calculate the amount of energy required to move a steam-powered locomotive or the amount of food required to sustain a human body through a day’s activities.

After the first law was defined in 1797 by Benjamin Thompson, many people chased the dream of building a perfectly efficient machine—one converting heat into work and vice versa with no loss of energy, which could ideally operate forever without adding fuel: a perpetual motion machine. Realizing this was impossible and why, Sadi Carnot defined the second law of thermodynamics in 1824—the “law of entropy,” which is to say that entropy (the degree of disorder or randomness in the system) always increases. And that is why things fall apart.

The law of entropy can be understood in many ways. It recognizes that there are no truly isolated systems in the universe and no perfect conversion of energy within a system; no matter what you do, some energy will always get lost, expanding into the environment, like heat leaving a frying pan into the air, becoming unavailable to do work. But why does it do that? It has to do with the principle of least action.

Systems naturally change in whatever way requires the least work (nothing non-living works any harder than absolutely necessary!). This means that energy always flows from hot to cold, and systems always become increasingly disordered. It requires energy to put something disordered into order because there are more ways to be disordered than ordered. So, thanks to the laws of probability, systems must change, on average, from order toward disorder.

To move things into greater order takes energy. Life seems to violate the law of entropy, but really what it does is convert food into the energy needed to counteract entropy temporarily. This is why we have to eat and why we die.

In 1905, Walther Nernst made a significant contribution to the development of thermodynamics with his formulation of the heat theorem. This paved the way for the establishment of the third law of thermodynamics. For his groundbreaking work, Nernst was awarded the Nobel Prize in Chemistry in 1920, cementing his place in scientific history.

The third law resolves the problem that entropy can’t increase forever; there is only a finite amount of energy to get lost! The third law states that entropy stops at a temperature of absolute zero—when all parts of the system have stopped moving. This is one theory of how the universe will end; it’s called “heat death.” The third law is also violated by quantum mechanics, which says that nothing in nature can ever become completely still at the sub-atomic scale. This means that even empty space has energy—the “zero-point energy.” This makes it possible for sub-atomic particles to pop in and out of existence at that scale, temporarily violating the first law of thermodynamics during their super-brief existence.

Although the laws of thermodynamics can be violated on short enough timescales, these rules are the most universal and useful laws in all of physics and probably shouldn’t even be referred to as “thermodynamics” (the dynamics of heat) since they have far broader applicability and more profound implications.