First Law of Thermodynamics

The first law of thermodynamics is one of the most fundamental principles in all of physics. It is essentially a statement of the conservation of energy applied to thermal systems: energy cannot be created or destroyed, only transferred from one form to another. In this lesson, we apply it to gases and examine how energy transfers occur in different thermodynamic processes.

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The First Law: Statement and Equation

The Edexcel convention for the first law is:

ΔU = Q + W

where:

• ΔU = change in internal energy of the system (J)
• Q = energy transferred to the system by heating (J) — positive when heat flows into the system
• W = work done on the system (J) — positive when work is done on the system (compression)

Some textbooks use the convention ΔU = Q − W (where W is work done by the system). The Edexcel specification uses the convention above, where W is the work done on the gas, so be careful with signs.

In words: the increase in internal energy equals the heat added to the system plus the work done on the system.

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