First Law of Thermodynamics

What is the First Law of Thermodynamics?

The first law of thermodynamics is simply an expression of the conservation of energy principle, and it asserts that energy is a thermodynamic property.
The first law of thermodynamics defines the internal energy (E) as equal to the difference of the heat transfer (Q) into a system and the work (W) done by the system.

E2 - E1 = Q - W

Here the words heat transfer into a system and work done by the system are important as they define the sign convention. Heat removed from a system would be assigned a negative sign in the equation. Similarly work done on the system is assigned a negative sign.


The First Law of Thermodynamics. The energy change of a system during a process is equal to the net work and heat transfer between the system and its surroundings.


Applications of the First Law of Thermodynamics

As mentioned above, the first law of thermodynamics is a generalization of the law of conservation of energy, which states that energy can neither be created nor destroyed.

This is a particularly important law when discussing heat engines (like car engines), because it helps to understand that heat must go somewhere—the basis for cogeneration. As an engine creates heat, if the heat is not vented, the engine will become hotter and hotter, which will eventually lead to melting. Therefore, because it is impossible to achieve 100% efficiency, heat must be vented. This can either be treated as waste heat, or used for cogeneration, which is applying the waste heat to another cause, like heating a car, or heating houses.

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