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Basics of Thermodynamics

Updated: Jan 11

A very basic primer on one of the most important branches of science.

Thermodynamics is a branch of physics that deals with heat, work, and temperature. Such a seemingly simple science governs so much of our universe, our planet, engineering, chemistry, biology, the economy, and even life itself. The laws of thermodynamics are some of the most robust concepts in science and, therefore, can serve as a concrete source of truth in the world.

The Laws of Thermodynamics:

First Law: In a process without transfer of matter, the change in internal energy, ΔU, of a thermodynamic system is equal to the energy gained as heat, Q, less the thermodynamic work, W, done by the system on its surroundings. This is basically saying that energy can be transferred from one form to another but the total quantity of energy remains the same.

ΔU = Q - W

Second Law: Any spontaneous process is necessarily accompanied by an increase in entropy of the universe. Although the total quantity of energy is always conserved in any process, the distribution of that energy changed in an irreversible manner. The Second Law is concerned with the natural direction of change of the distribution of energy.

Third Law: Matter cannot be brought to absolute zero in a finite number of steps. As the temperature of a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

Zeroth Law: If two systems are each in thermal equilibrium with a third, they are also in thermal equilibrium with each other. This basically establishes the existence of temperature.


Early pioneers in the field of thermodynamics include Sadi Carnot, James Prescott Joule, Rudolf Clausius, William Thompson (Lord Kelvin), James Clerk Maxwell, Ludwig Boltzman, and Josiah Willard Gibbs.

Branches of Thermodynamics:

Several branches of thermodynamics exist, each using a different fundamental model as a theoretical or experimental basis, or applying the principles to varying types of systems. These include:

  • Classical Thermodynamics

  • Statistical Mechanics

  • Chemical Thermodynamics

  • Equilibrium Thermodynamics

  • Non-Equilibrium Thermodynamics

Other Important Concepts:

There are a few key concepts that are necessary for understanding thermodynamics. One is the idea of a system and surroundings. A system is the defined region being studied. The surroundings are everything else in the universe besides the system. The boundary is the interface between the two.

Temperature is the average kinetic energy of the particles in a system. Heat is the thermal energy transferred between two systems due to a temperature difference. Work is a transfer of energy that results in measurable macroscopic forces on the system's surroundings.

A notable difference between heat and work has to do with coherence. When all the particles in a system are moving in random chaotic directions, their motion is incoherent, and this is called thermal motion. When the particles in a system are moving in the same direction, their motion is coherent (think of an object moving in a given direction). When we do work on a system, we are stimulating its particles with coherent motion. When we heat a system, we are stimulating its particles with incoherent motion. Due to the Second Law, it is impossible to completely convert heat into work. However, work can be completely converted into heat.

Energy's quality describes its ability to do work. Higher-quality energy has a greater availability to do work, and lower-quality energy has less of an ability to do work. A similar concept is entropy. Entropy can be described in many ways, but it describes the inability of energy to do work. Entropy is colloquially defined as disorder or chaos. This is not quite accurate. Entropy defines the manner in which energy is stored. If energy is stored at a very high temperature, it has high quality and low entropy. If that same energy is stored at a temperature close to the surroundings' temperature, it has high entropy. The natural direction of change is the one that causes entropy to increase overall, although pockets of low entropy can form provided an even greater increase in entropy is formed elsewhere in the universe.

Free energy, also called Gibbs free energy, is one of the most important concepts in thermodynamics. It is the energy available for doing work and is given by:

Free energy = total energy – (temperature x entropy change)

G = U + pV – TS

Enthalpy is the total heat content of a system and is given by the sum of internal energy and the product of pressure and volume.

H = U + pV

Common Thermodynamic Processes:

  • Adiabatic process: occurs without loss or gain of energy by heat

  • Isenthalpic process: occurs at a constant enthalpy

  • Isentropic process: a reversible adiabatic process, occurs at a constant entropy

  • Isobaric process: occurs at constant pressure

  • Isochoric process: occurs at constant volume (also called isometric/isovolumetric)

  • Isothermal process: occurs at a constant temperature

  • Steady state process: occurs without a change in the internal energy

Final Thoughts:

Life is arguably an emergent phenomenon from the laws of thermodynamics. The details are well beyond the scope of this introduction. Complexity can arise from simple physical laws, even as the universe's entropy increases overall. This includes everything from cell division to protein synthesis.

Free energy is the capital consumed by all creatures of all kinds, and by its conversion, everything is done. – Wilhelm Ostwald, 1853–1932

The low-entropy energy from the sun powers nearly all the natural processes we observe on Earth, from the hydrologic cycle to plant growth. If we want to understand and improve our world, a great way to start is with thermodynamics. Thermodynamics will tell us what can be true and what cannot. It serves as the foundational framework for evaluating energy and resource challenges and solutions.

If you’re interested in learning more, these are some of my favorite books on thermodynamics:

  • The Second Law, by Peter W. Atkins

  • Engines, Energy, and Entropy: A Thermodynamics Primer, by John B. Fenn

  • Entropy Demystified: The Second Law Reduced to Plain Common Sense, by Arieh Ben-Naim

  • Every Life Is on Fire: How Thermodynamics Explains the Origins of Living Things, by Jeremy England

  • Einstein's Fridge: How the Difference Between Hot and Cold Explains the Universe, by Paul Sen


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