What is thermodynamics?

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Heat
Temperature
Specific heat
Thermal conductivity
Newton's Law of Cooling
Heat transfer
The Carnot cycle
Entropy
The four laws of thermodynamics
Additional resources
Bibliography

Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. In particular, it describes how thermal energy is converted to and from other forms of energy and how thermal energy affects matter.

Thermal energy is the energy a substance or system has due to its temperature — in other words, the energy of moving or vibrating molecules — according to the University of Kentucky. Thermodynamics involves measuring this energy, which can be "exceedingly complicated," David McKee, a professor of physics at Missouri Southern State University told Live Science. "The systems that we study in thermodynamics … consist of very large numbers of atoms or molecules interacting in complicated ways. But, if these systems meet the right criteria, which we call equilibrium, they can be described with a very small number of measurements or numbers. Often this is idealized as the mass of the system, the pressure of the system, and the volume of the system, or some other equivalent set of numbers."

Thermodynamics, then, is concerned with several properties of matter; foremost among these is heat. Heat is energy transferred between substances or systems due to a temperature difference between them, according to Georgia State University. As a form of energy, heat is conserved — it cannot be created or destroyed. It can, however, be transferred from one place to another. Heat can also be converted to and from other forms of energy. For example, a steam turbine can convert heat to kinetic energy to run a generator that converts kinetic energy to electrical energy. A light bulb can convert this electrical energy to electromagnetic radiation (light), which, when absorbed by a surface, is converted back into heat. Throughout this process, a portion of the energy is lost to entropy.

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Temperature is "a measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale," according to the American Heritage Dictionary. The most commonly used temperature scale is Celsius, which is based on the freezing and boiling points of water, assigning respective values of 0 C and 100 C. The Fahrenheit scale is also based on the freezing and boiling points of water, which have assigned values of 32 degrees Fahrenheit and 212 F, respectively.

Scientists worldwide, however, use the Kelvin (K with no degree sign) scale, named after William Thomson, 1st Baron Kelvin, because it's based on total thermal energy rather than the freezing and boiling points of water, according to the National Library of Scotland. This scale uses the same increments as the Celsius scale; for example, a temperature change of 1 C is equal to 1 K. However, the Kelvin scale starts at absolute zero, the temperature at which there is a total absence of heat energy, and all molecular motion stops. A temperature of 0 K is equal to minus 459.67 F or minus 273.15 C.

The specific heat of a metal depends almost entirely on the number of atoms in the sample and the way they bond, not the sample's mass. For instance, a kilogram of aluminum can absorb about seven times more heat than a kilogram of lead. A given mass of water, however, can absorb nearly five times as much heat as an equal mass of aluminum, thanks to the strong forces binding the water molecules, according to Southeastern Louisiana University.

Thermal conductivity (k) is "the rate at which heat passes through a certain cross-section of a specified material," according to Debdatta Ratna's "Thermosets" (Woodhead Publishing limited, 2012). The unit for k is watts (W) per meter (m) per Kelvin (K), according to Swarthmore College. Values of k for metals such as copper and silver are relatively high at 380 and 420 W/m·K, respectively. This property makes these materials useful for automobile radiators and cooling fins for computer chips because they can carry away heat quickly and exchange it with the environment. The highest value of k for any natural substance is diamond at 2,200 W/m·K, according to a 2009 study published in the journal Materials.

Other materials are useful because they are extremely poor conductors of heat; this property is referred to as thermal resistance, or R-value, which describes the rate at which heat is transmitted through the material. These materials, such as fiberglass, goose down, and Styrofoam, are used for insulation in exterior building walls, winter coats, and thermal coffee mugs. R-value is given in units of square feet times degrees Fahrenheit times hours per British thermal unit (ft2·°F·h/Btu), according to OpenStax, an open-source textbook.

In 1701, Sir Isaac Newton first stated his Law of Cooling in a short article titled "Scala graduum Caloris" ("A Scale of the Degrees of Heat") in the Philosophical Transactions of the Royal Society. Newton's statement of the law translates from the original Latin as, "the excess of the degrees of the heat ... were in geometrical progression when the times are in an arithmetical progression." Worcester Polytechnic Institute gives a more modern version of this scientific law as "the rate of change of temperature is proportional to the difference between the temperature of the object and that of the surrounding environment."

This results in an exponential decay in the temperature difference. For example, if a warm object is placed in a cold bath, within a certain length of time, the difference between the two temperatures will decrease by half. Then in that same length of time, the remaining difference will again decrease by half. This repeated halving of the temperature difference will continue at equal time intervals until it becomes too small to measure. At that point, the system is in thermal equilibrium.

The first step in the Carnot cycle is isothermal expansion, where a gas in a cylinder under pressure from a piston is kept at a constant temperature, but a heat source is brought in contact with the cylinder. To maintain the same temperature, the gas must expand. Next, adiabatic expansion, or expansion without heat transfer to the surrounding environment, removes weight from the piston to allow the gas to expand further, which helps it push on a cylinder to do work. Next, the gas is cooled at a constant temperature and compressed by adding weight to the piston to raise the pressure in the gas, at which point heat transfers from the gas back to the heat source. And finally, adiabatic compression adds more weight to the piston to further increase the pressure of the gas and, therefore, increase its temperature. Then the cycle repeats, according to NASA. This is the basic principle behind heat pumps used for heating, air conditioning, and refrigeration, according to Georgia State University.

All thermodynamic systems generate waste heat. This waste results in an increase in entropy, which is a measure of the disorder of a system. Because work comes from ordered molecular motion, entropy is a measure of the energy that's not available to do work, according to Britannica. Entropy in any closed system always increases; it never decreases. Additionally, moving parts produce waste heat due to friction, and radiative heat inevitably leaks from the system.

This makes so-called perpetual motion machines impossible. Siabal Mitra, a professor of physics at Missouri State University, told Live Science "You cannot build an engine that is 100% efficient, which means you cannot build a perpetual motion machine. However, there are a lot of folks out there who still don't believe it, and there are people who are still trying to build perpetual motion machines."

Entropy is also defined as "a measure of the disorder or randomness in a closed system," which also inexorably increases. You can mix hot and cold water, but because a large cup of warm water is more disordered than two smaller cups containing hot and cold water, you can never separate it back into hot and cold without adding energy to the system. Put another way, you can't unscramble an egg or remove cream from your coffee. Entropy, therefore, provides us with an arrow of time: forward is the direction of increasing entropy.

The fundamental principles of thermodynamics were originally expressed in three laws. Later, scientists figured out that a more fundamental law had been neglected, apparently because it had seemed so obvious that it did not need to be stated explicitly. To form a complete set of rules, scientists decided this most fundamental law needed to be included. The problem, though, was that the first three laws had already been established and were well known by their assigned numbers. When faced with the prospect of renumbering the existing laws, which would cause considerable confusion, or placing the pre-eminent law at the end of the list, which would make no logical sense, British physicist Ralph H. Fowler came up with an alternative that solved the dilemma: he called the new law the "Zeroth Law," according to Saint Anselm College. Here are all four laws in brief:

The Third Law of Thermodynamics states that the entropy of a perfect crystal — that is, a substance made up of atoms arranged in a perfectly ordered, symmetrical pattern — at absolute zero is zero. As explained above, entropy is sometimes called "waste energy," or the energy that is unable to do work; and since there is no heat energy whatsoever at absolute zero, there can be no waste energy. Entropy is also a measure of the disorder in a system, and while a perfect crystal is by definition perfectly ordered, any positive value of temperature means there is motion within the crystal, which causes disorder. For these reasons, there can be no physical system with lower entropy, so entropy always has a positive value.

The field of thermodynamics has a long history, and this video from CrashCourse provides a tour through its most notable moments and figures.
Want to see thermodynamics in action? This resource from West Virginia University links to a number of fascinating video examples. Just because thermodynamics is fundamental to science doesn't mean it doesn't have its issues. This article in Physics Today explores why it's been called the "village witch" of physics.

Jim Lucas is a contributing writer for Live Science. He covers physics, astronomy and engineering. Jim graduated from Missouri State University, where he earned a bachelor of science degree in physics with minors in astronomy and technical writing. After graduation he worked at Los Alamos National Laboratory as a network systems administrator, a technical writer-editor and a nuclear security specialist. In addition to writing, he edits scientific journal articles in a variety of topical areas.

With contributions from

Ashley Hamer PritchardLive Science Contributor

Heat

Temperature

Specific heat

Thermal conductivity

Newton's Law of Cooling

Heat transfer

The Carnot cycle

Entropy

The four laws of thermodynamics

Additional resources

Bibliography