The first law of thermodynamics states that heat is a form of energy and that it is subject to the principle of conserved energy. This means that heat energy can't be created or destroyed. It can be transferred from one location to another and vice versa.

The relationship between heat and other forms of energy is called thermodynamics. It describes how thermal energy is converted to other forms of energy and how it affects matter. Four laws express the fundamental principles of thermodynamics.

The First Law says that the internal energy of a system has to be equal to the work that is being done on the system, plus or minus the heat that flows in or out of the system and any other work that is done on the system.

The change in internal energy of a system is the sum of all the energy inputs and outputs to and from the system, similar to how all the deposits and withdrawals determine the changes in your bank balance.


The change in the internal energy, Q, and the heat added to the system are expressed in a mathematical way.

According to the American Physical Society, scientists in the late 18th and early 19th centuries followed the theory of calories, which was first proposed byAntoine Lavoisier in 1783. The theory said that heat was a kind of fluid that flowed from hot to cold regions. When this fluid flowed from a hot to a cold region, it could be converted to energy and used to drive a water wheel. The University of Virginia says that The Mechanical Theory of Heat was published in 1867.

Thermodynamic systems

David McKee is a professor of physics at Missouri Southern State University. One is our human-scale contribution, such as pushing a system of gas. We can't keep track of the individual contributions at a very small scale where things happen.

When I put two samples of metal up against each other, and the atoms are rattling around at the boundary, and one of them comes off faster than the other, I can't keep track of it. It happens on a very small time scale and a very small distance, and it happens many, many times per second. The latter is called heat.

The term thermodynamic systems is used to mean open, closed or isolated. An open system is free to exchange energy and matter with its surroundings, a closed system is free to exchange energy and matter with its surroundings, and an isolated system is free to exchange energy and matter with its surroundings. A pot of boiling soup gets energy from the stove, emits heat from the pan, and also carries away heat energy. The system would be open. If we put a tight lid on the pot, it would still emit heat energy, but it would no longer cause steam. The system would be closed. If we poured the soup into a thermos bottle and sealed it, there wouldn't be a thing going into or out of the system. This would be an isolated system.

Perfectly isolated systems are not possible in practice. No matter how well insulated they are, all systems transfer energy to their environment. The soup in the thermos will only stay hot for a few hours and will reach room temperature the next day. White dwarf stars, the hot remnants of burned-out stars that no longer produce energy, can be insulated by light-years of near perfect vacuum in interstellar space, yet they will eventually cool down from several tens of thousands of degrees to near absolute zero due to energy loss through. The process takes longer than the present age of the universe, but there is no stopping it.

Heat engines

The heat engine is the most common practical application of the First Law. Thermal energy is converted into mechanical energy by heat engines. Most heat engines are open systems. Georgia State University says that the basic principle of a heat engine exploits the relationships among heat, volume and pressure of a working fluid. The steam in a steam engine and the hydrofluorocarbons in the refrigeration systems are examples of working fluids.

When gas is heated, it expands, but when it is not, it increases in pressure. If the bottom wall of the confinement chamber is the top of a Piston, the pressure on the surface of the Piston causes it to move downward. This movement can be used to do work that is equal to the total force applied to the top of the piston.

There are many variations on the basic heat engine. steam engines rely on external combustion to heat a boiler tank containing the working fluid. The water is converted to steam, and the pressure is used to drive a Piston that converts heat energy to mechanical energy. The University of Oklahoma says that automobile engines use internal combustion, where liquid fuel is vaporized, mixed with air and ignited inside a cylinder above a moving Piston, driving it downward.

Refrigerators, air conditioners and heat pumps

Refrigerators and heat pumps convert mechanical energy to heat. Most of these are closed systems. The temperature of the working fluid increases when it is compressed. The hot gas can transfer heat to the environment. 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 was removed during the hot cycle. The cold gas can absorb heat from the environment. Boston University says this is the principle behind an air conditioner. Air conditioners do not produce cold; they remove heat.

The fluid is heated by compression when it is transferred outdoors. An electric fan is used to expel heat into the environment from the air-cooled heat exchanger. When the working fluid is brought back indoors, it is allowed to expand and cool so it can absorb heat from the indoor air through another heat exchanger.

An air conditioner run in reverse is a heat pump. The heat from the fluid is used to warm the building. It is transferred outside where it becomes cold, so that it can absorb heat from the outside air, which is usually warmer than the cold working fluid. Even in very low temperatures, the working fluid has a low freezing point.

The U-shaped tubes in deep wells or an array of horizontal tubes buried in a large area through which the working fluid is circulating are used in ground-sourced air conditioning and heat pump systems. Other systems use water from the ocean.

The article was updated on January 28, 2022.

Additional resources

There are other explanations of the first law of thermodynamics.


The first law of thermodynamics was published in June of 2021.

The Science History Institute hasAntoine- Laurent Lavoisier.

The American Society of Mechanical Engineers has a website.

The mechanical theory of heat was written by Rudolfph Clausius.

The American Physical Society has this month's physics history December 1840: Joule's abstract on converting mechanical power into heat.

The Rise and Fall of the Caloric Theory was published in July 2003 by the University of Virginia.

The System and Surrounding was published by the University of Calgary Energy Education.

The University of Georgia Hyperphysics has a heat engine cycle.

The University of Oklahoma has a course called "Thermodynamics - Theory".

The second law and heat engines was published by Boston University.

The US Department of Energy has a page on thermal heat pumps.