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The first law of thermodynamics states that energy cannot be created or destroyed, only changed in form. The equation ΔU = Q – W or ΔU = Q + W is used to calculate the change in internal energy of a system. The law is often studied in relation to heat engines and involves the study of work, heat, and energy. The standard unit for calculating the law is Joules, but calories or BTUs can also be used. Examples of how the law works are given through calculations involving work done by or on the system.
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 stored in the universe and must end up somewhere, even if it changes shape. It involves the study of the work of the system, 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 start analyzing and calculating 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 minus the work done by the system. Alternatively, the equation ΔU = Q + W is sometimes used. The only difference is that it calculates the work done on the system, rather than the work done by the system. In other words, work is positive when the system works on the surrounding system and negative when the surrounding works 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 works. It can be visualized as a piston pushing down or applying pressure on gases in an internal combustion engine. So, the work is done by the system. Alternatively, when studying chemical processes and reactions, it is typical to study the conditions under which one works on the system.
The standard unit for calculating the first law of thermodynamics is Joules (J); however, many people who study law also do their calculations in terms of calories or British Thermal Units (BTUs). Sometimes it’s helpful to calculate conservation with real numbers, that way people can see how the law works. If a motor does 4,000 J of work on its surroundings, its internal energy decreases by 4,000 J. If it also releases 5,000 J of heat while it is running, its internal energy decreases by another 5,000 J. As a result, the energy internal energy of the system decreases by a total of -9,000 J.
In an alternative calculation, 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 are 5,000 J of energy in and 4,000 J of energy out. Thus, the total internal energy of the system is 1,000 J.
Finally, the negative work or work done on the system by its surroundings can also be exemplified through calculations regarding the first law of thermodynamics. For example, if the system draws 4,000J while the surroundings are simultaneously executing 5,000J or working on the system, you will see another result. As all energies flow into the system, the total internal energy rises to 9,000 J.
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