December 8, 2022
  • December 8, 2022

What is the first law of thermodynamics?

By on February 28, 2022 0

The first law of thermodynamics states that heat is a form of energy and therefore thermodynamic processes are subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed, depending on British. It can, however, be transferred from place to place and converted to and from other forms of energy.

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 it affects matter. The fundamental principles of thermodynamics are expressed in four laws.

“The first law says that the internal energy of a system must equal the work done on the system plus or minus the heat that enters or leaves the system and any other work done on the system,” said Saibal Mitra , professor of physics at Missouri State University, told Live Science “So this is a reformulation of the conservation of energy.”

“A system’s internal energy change is the sum of all energy inflows and outflows to and from the system, in the same way that all deposits and withdrawals you make determine changes in your bank balance” , said Mitra.

This is expressed mathematically by: Δyou = QOwhere Δyou is the change in internal energy, Q is the heat added to the system, and O is the work done by the system, according to Britannica.

History of the first law of thermodynamics

Scientists in the late 18th and early 19th centuries adhered to the caloric theory, first proposed by Antoine Lavoisier in 1783 and later reinforced by the work of Sadi Carnot in 1824, according to the American Physical Society. This scientific theory treated heat as a kind of fluid that naturally flowed from warm regions to cold regions, much like water flows from high to low. When this caloric fluid flowed from a hot region to a cold region, it could be converted into kinetic energy and function in the same way that falling water could drive a paddle wheel. It wasn’t until Rudolf Clausius published “The Mechanical Theory of Heat” in 1867 that the caloric theory was finally abandoned, according to the University of Virginia.

Thermodynamic systems

Energy can be split into two parts, said David McKee, professor of physics at Missouri Southern State University. One is our macroscopic contribution on a human scale, like a piston moving and pushing on a gas system. The rest is made up of things that happen on a very small scale where we can’t track individual contributions.

“When I put two samples of metal against each other, and the atoms click at the boundary, and two atoms bounce against each other, and one of them breaks off faster than the other, I can’t follow it. . It happens on a very small time scale and over a very small distance, and it happens many, many times per second,” McKee told Live Science. So we simply divide all energy transfers into two groups: the things we are going to follow and the things we are not going to follow. The latter is what we call heat.

Thermodynamic systems are generally considered to be open, closed or isolated. According to University of Calgary, an open system freely exchanges energy and matter with its environment; a closed system exchanges energy, but not matter, with its environment; and an isolated system does not exchange energy or matter with its surroundings. For example, a pot of boiling soup receives energy from the stove, releases heat from the pot, and emits matter in the form of vapor, which also carries away heat energy. It would be an open system. If we put a tight lid on the pot, it would still radiate thermal energy, but ideally would no longer emit matter in the form of vapor. It would be a closed system. However, if we were to pour the soup into a perfectly insulated thermos bottle and seal the lid, there would be no energy or matter entering or leaving the system. It would be an isolated system.

In practice, however, perfectly isolated systems cannot exist. All systems transfer energy to their surroundings, even if they are well insulated. The soup in the thermos will only stay hot for a few hours and will reach room temperature the next day. In another example, white dwarf stars, the hot remnants of scorched stars that no longer produce energy, may be isolated by light years of near-perfect vacuum in interstellar space, but they will eventually cool by several tens of thousands of degrees. close to absolute zero due to the loss of energy by radiation. Although this process is taking longer than the current age of the universe, there is no stopping it.

Heat engines

The most common practical application of the first law is the heat engine. Heat engines convert thermal energy into mechanical energy and vice versa. Most heat engines fall into the category of open systems. The basic principle of a heat engine exploits the relationships between heat, volume and pressure of a working fluid (any substance that flows), usually a gas, according to Georgia State University. Examples of working fluids include steam in a steam engine and hydrofluorocarbons in refrigeration systems.

When gas is heated, it expands; however, when this gas is prevented from expanding, its pressure increases. If the bottom wall of the containment is the top of a moving piston, this pressure exerts a force on the surface of the piston causing it to move downward. This motion can then be harnessed to do work equal to the total force applied to the top of the piston times the distance the piston has traveled.

There are many variations of the basic heat engine. For example, steam engines rely on external combustion to heat a boiler tank containing the working fluid, usually water. The water is converted to steam, and the pressure is then used to drive a piston which converts thermal energy into mechanical energy. Automotive engines, however, use internal combustion, where liquid fuel is vaporized, mixed with air, and ignited inside a cylinder above a moving piston, driving it downward, according to The University of Oklahoma.

Refrigerators, air conditioners and heat pumps

Refrigerators and heat pumps are heat engines that convert mechanical energy into heat. Most of them fall into the category of closed systems. When the working fluid, or gas, is compressed, its temperature increases. This hot gas can then transfer heat to its surrounding environment. Then, when the compressed gas is allowed to expand, its temperature becomes colder than it was before it was compressed because some of its heat energy has been removed during the hot cycle. This cold gas can then absorb heat energy from its surroundings. This is the principle of operation of an air conditioner, according to Boston University. Air conditioners do not actually produce cold; they evacuate the heat.

A mechanical pump transfers the working fluid outside, where it is heated by compression. Then the heat is transferred to the outside environment, usually via an air-cooled heat exchanger, which often uses an electric fan to expel the heat to the environment. Then the working fluid is brought back inside, where it is allowed to expand and cool so that it can absorb heat from the indoor air through another heat exchanger.

A heat pump is simply an air conditioner running in reverse. The heat from the compressed working fluid is used to heat the building. It is then transferred outside where it expands and becomes cold, allowing it to absorb heat from the outside air, which, even in winter, is generally warmer than the cold working fluid. The working fluid usually has a freezing point low enough to continue flowing even at very low temperatures.

Geothermal or geothermal air conditioning and heat pump systems use long U-shaped tubes in deep wells or a network of horizontal tubes buried in a large area through which the working fluid circulates and heat is transferred to or from the earth, according to the US Department of Energy. Other systems use rivers or sea water to heat or cool the working fluid.

Live Science contributor Ashley Hamer updated this article on January 28, 2022.

Additional Resources

Here are three other explanations of the first law of thermodynamics:


Britannica, “The First Law of Thermodynamics”, June 1, 2021.

Institute for the History of Science, “Antoine-Laurent Lavoisier”, December 11, 2017.

The American Society of Mechanical Engineers, “Nicolas Léonard Sadi Carnot”, April 10, 2012,

Rudolfph Clausius, “The Mechanical Theory of Heat.” John Van Voorst, 1867.

American Physical Society, “This Month Physics History December 1840: Joule’s abstract on converting chemical power into heat”, December 2009.

University of Virginia, “Teaching Heat: the Rise and Fall of the Caloric Theory”, July 2003.

University of Calgary Energy Education, “System and Surrounding,” September 27, 2021.

University of Georgia Hyperphysics, “Heat Engine Cycle”,

Online course from the University of Oklahoma, “Thermodynamics – Theory”.

Boston University, “Heat Engines and the Second Law”, December 10, 1999.

US Department of Energy, “Ground Source Heat Pumps”.