What is the First Law of Thermodynamics?

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  • Written By: Dee S.
  • Edited By: Bronwyn Harris
  • Last Modified Date: 11 November 2019
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The first law of thermodynamics is also known as the Law of Conservation of Energy. It states that energy cannot be destroyed or created; it is conserved in the universe and must end up somewhere, even if it changes forms. It involves the study of system work, heat, and energy. Heat engines often prompt a discussion of the first law of thermodynamics; however, it is considered one of the most fundamental laws of nature.

Once people delve into the study of the first law of thermodynamics, they immediately begin to analyze and compute the equation associated with the law: ΔU = Q – W. This equation means that the change in internal energy of the system is equal to the heat added to the system less the work done by the system. In the alternative, sometimes the equation ΔU = Q + W is used. The only difference is that is computes the work done on the system, instead of the work done by the system. In other words, work is positive when the system does work on its surrounding system and negative when the surroundings do work on the system.


When studying physics, there is a common example that involves adding heat to a gas in a closed system. The example continues by expanding that gas so that it does work. It can be visualized as a piston pushing down or applying pressure on gases in an internal combustion engine. Thus, work is done by the system. In the alternative, when studying chemical processes and reactions, it is typical to study conditions where work is done on the system.

The standard unit for computing the first law of thermodynamics is Joules (J); however, many people studying the law also make their computations in terms of the calorie or the British Thermal Unit (BTU). It is sometimes helpful to calculate conservation with actual numbers, doing so allows people to see how the law works. If a motor does 4,000 J of work on its surrounding, the internal energy decreases by 4,000 J. If it also releases 5,000 J of heat while it is working, then the internal energy decreases by an additional 5,000 J. As a result, the internal energy of the system decreases by a total of -9,000 J.

In an alternative computation, if a system does 4,000 J of work on its surroundings and then absorbs 5,000 J of heat from its surroundings, the result is different. In that case, there is 5,000 J of energy going in and 4,000 J of energy going out. Thus, the system’s total internal energy is 1,000 J.

Lastly, negative work or work done on the system by the surroundings can be exemplified through calculations regarding the first law of thermodynamics, as well. For example, if the system absorbs 4,000 J as the surroundings simultaneously perform 5,000 J or work on the system, another result will be seen. Since all the energies are flowing into the system, the total internal energy jumps up to 9,000 J.


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