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The Laws of Thermodynamics Explained

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What Is Thermodynamics?

Thermodynamics is a branch of physics and chemistry that is the study of conversions between thermal energy and other forms of energy. The term thermodynamics comes from the Greek words therme, or “heat,” and dynamis, or “force.” The foundation of thermodynamics is the conservation of energy and the fact that heat always flows from hot to cold. The science of thermodynamics bypasses the molecular details of systems and focuses on the macroscopic level—mechanical work, pressure, temperature, and how these types of energy are transformed.

The serious study of the movement of heat, which led to the discipline of thermodynamics, goes back to the early 19th century. The invention of the steam engine and its widespread adoption was one of the prime drivers for the study of heat. Engineers and mechanics wanted to build more powerful and efficient steam engines to power factories. The early studies revealed several underlying properties of the nature of heat, the most important of which are the first and second laws of thermodynamics.

The study of thermodynamics allows us to understand how heat engines work: the gasoline motor in your car, the steam turbine from a coal burning or nuclear power plant that powers your house, and the basic flow of heat and energy within the human body.

Zeroth Law of Thermodynamics

The zeroth law of thermodynamics, which is the most recent law developed, states:

If two systems are both in thermal equilibrium with a third system, then the two systems are in thermal equilibrium with each other.

Though this law may seem trivial at first glance, it is necessary to define temperature scales, such as Fahrenheit, Celsius, and Kelvin.

Three systems in thermal equilibrium. All have the same temperature.

Three systems in thermal equilibrium. All have the same temperature.

First Law of Thermodynamics

The First Law of Thermodynamics states that whenever heat is added to a system, it is transformed into an equal amount of some other form of energy.

The term system is any group of atoms, molecules, particles, or objects. The system can be something as large as the entire earth’s atmosphere, or as small as a cell that makes up a living organism. When we add heat energy to the earth’s atmosphere, it manifests itself in the movement of air and water vapor; thus, winds and storms. If we add heat energy to a steam engine, the liquid water turns to steam and drives the pistons within the steam engine; thus, mechanical power is produced. With this energy added to a system, one of two things or both happens: (1) the energy increases the internal energy of the system, and it remains in the system or (2) it is converted to external work if it leaves the system. The first law can be thought of as:

Heat added to a system = increase in internal energy + external work done by the system.

The first law of thermodynamics tells us that the energy output in any form from a system can never be greater than the energy input; in other words, the first law is simply the thermal version of the law of conservation of energy. The first law of thermodynamics serves as the basis for establishing all energy equations. It is truly the foundation of the science of thermodynamics.

Formulation of the First Law of Thermodynamics

The first law can be formulated in terms of heat, energy, and work in a closed system:

Q = ΔU + W

Where Q is the heat supplied to the system, W is the work done by the system on its surroundings, and ΔU is the change in internal energy of the system.

Illustration of a system in two different states. State 1 of the system shows heat being added. In State 2, the piston has moved up.

Illustration of a system in two different states. State 1 of the system shows heat being added. In State 2, the piston has moved up.

Example of the First Law of Thermodynamics

In the figure about, State 1 represents a cylinder and piston with trapped air which has an internal energy of U1. As the gas inside the cylinder is heated, heat energy Q is added. The expanding gas inside the cylinder does work W to lift the piston and increase the internal energy of the gas to U2.

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Second Law of Thermodynamics

The second law of thermodynamics can be stated in various ways; of them the simplest is:

Heat will never of itself flow from a cold object to hot object.

Heat always flows from hot to cold.

Heat always flows from hot to cold.


To understand the laws of thermodynamics, one must understand the concept of entropy. The thought of ordered energy tending to a state of greater disorder is embodied in the concept of entropy. Entropy is the measure of the amount of disorder in a system—as disorder increases so does the entropy of a system. Another way to conceive of the second law of thermodynamics for a natural process is over time the entropy always increases. Whenever a physical system is allowed to distribute its energy without hindrance, it always does so in a manner such that the entropy increases while at the same time the available energy of the system for doing work decreases.

Example of Entropy

An example of a change of entropy in a system is when ice melts. The process of going from solid ice to liquid water is an increase in disorder or entropy of the system. When more heat is added to the water it begins to boil and thus there is another increase in entropy.

A melting iceberg illustrates the change of ice to liquid, an increase in entropy.

A melting iceberg illustrates the change of ice to liquid, an increase in entropy.

History of Entropy

Entropy owes its origin to a paradox first pointed out by William Thomson (Lord Kelvin) in 1847: energy cannot be destroyed or created, yet heat energy loses its capacity to do work when it is transferred from a warm body to a cold one. He suggested that in processes like heat conduction, energy is not lost but becomes “dissipated” or unavailable. Furthermore, according to Thompson, the dissipation amounts to a general law of nature, expressing the “directionality” of natural processes.

Both Scottish engineer Macquorn Rankine and German physicist Rudolf Clausius proposed a new concept, which represents the same tendency of energy towards dissipation. Initially it was called “thermodynamic function” by Rankine and “disgregation” by Clausius. In 1865 Clausius gave the concept its definitive name, “entropy,” after the Greek word for transformation. Every process that occurs in an isolated system increases the system’s entropy. Clausius thus formulated the first and second laws of thermodynamics in the statement: “The energy of the universe is constant, its entropy tends to a maximum.”

Thermodynamics: Crash Course Physics #23

The Third Law of Thermodynamics

The third law of thermodynamics states:

There is a minimum temperature, called absolute zero, where matter has its minimum heat energy and cannot become colder.

However, it is impossible for matter to reach the temperature of absolute zero because it would immediately absorb heat from its surroundings. Though exact absolute zero is not obtainable, scientists have been able to get very close to this coldest temperature with experimental apparatus. In the three different temperature systems, absolute zero is defined as:

  • 0 K, in the Kelvin system. In this system, ice melts at 273K and water boils at 373K. The Kelvin temperature scale was named after the British physicist Lord Kelvin, who coined the word thermodynamics and first suggested the temperature scale.
  • -273.15˚ C, in the Celsius system. In this system ice melts at 0˚ C and water boils at 100˚ C.
  • -459.7˚ F, in the Fahrenheit system. This is the temperature scale commonly used in the United States in which ice melts at 32˚ F and water boils at 212˚ F.

The third law can also be written in terms of entropy:

A system’s entropy approaches a constant value as its temperature approaches absolute zero.

At zero temperature, a system must be in a state with minimal thermal energy or the ground state. At this temperature, the entropy of the system is not necessarily zero, just a minimum.

Illustration of the third law of thermodynamics.

Illustration of the third law of thermodynamics.


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  • Heilbron, J.L. (Editor). The Oxford Guide to the History of Physics and Astronomy. Oxford: Oxford University Press, 2005.
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  • The Kingfisher Science Encyclopedia. New York: Kingfisher, 2011.
  • The New Encyclopedia Britannica. Chicago: Encyclopedia Britannica, Inc., 1994.
  • Wysession, Michael, David Frank, and Sophia Yancopoulos. Prentice Hall Physical Science: Concepts in Action. Upper Saddle, New Jersey: Pearson Education, Inc., 2009.

© 2022 Doug West

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