First Law of Thermodynamics Definition

Chemistry Glossary Definition of First Law of Thermodynamics

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The first law of thermodynamics is the physical law which states that the total energy of a system and its surroundings remain constant. The law is also known as the law of conservation of energy, which states energy can transform from one form into another, but can neither be created nor destroyed within an isolated system. Perpetual motion machines of the first kind are impossible, according to the first law of thermodynamics. In other words, it is not possible to construct an engine that will cycle and produce work continuously from nothing.

First Law of Thermodynamics Equation

The equation for the first law can be confusing because there are two different sign conventions in use.

In physics, particularly when discussing heat engines, the change in the energy of a system equals the heat flow in the system from the surroundings minus the work done by the system on the surroundings. The equation for the law may be written:

ΔU = Q - W

Here, ΔU is the change in the internal energy of a closed system, Q is the heat supplied to the system, and W is the amount of work done by the system on the surroundings. This version of the law follows the sign convention of Clausius.

However, the IUPAC uses the sign convention proposed by Max Planck. Here, net energy transfer to a system is positive and net energy transfer from a system are negative. The equation then becomes:

ΔU = Q + W


  • Adkins, C. J. (1983). Equilibrium Thermodynamics (3rd ed.). Cambridge University Press. ISBN 0-521-25445-0.
  • Bailyn, M. (1994). A Survey of Thermodynamics. American Institute of Physics Press. New York. ISBN 0-88318-797-3.
  • Denbigh, K. (1981). The Principles of Chemical Equilibrium With Applications in Chemistry and Chemical Engineering (4th ed.). Cambridge University Press. Cambridge UK. ISBN 0-521-23682-7.
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Helmenstine, Anne Marie, Ph.D. "First Law of Thermodynamics Definition." ThoughtCo, Apr. 5, 2023, Helmenstine, Anne Marie, Ph.D. (2023, April 5). First Law of Thermodynamics Definition. Retrieved from Helmenstine, Anne Marie, Ph.D. "First Law of Thermodynamics Definition." ThoughtCo. (accessed June 9, 2023).