When we talk about the work done by a thermodynamic system, we often talk about the work done a gas confined to a container with a moving wall (a piston for example). This system is particularly interesting because it is the basis of all heat engines.

In this context, work is the amount of **energy transferred** by a system to its surroundings by a force that causes a displacement.

To calculate the work done by a gas we will start with the expression for the work done by a force and apply it to the situation represented in the following figure:

The left side of the upper figure represents a gas confined to a closed container. The gas exerts a force F on the moving wall, so that it moves a distance dx.

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The work done by **F** on the moving wall between states A and B is given by:

And, remembering the definition of pressure, we get:

**The SI unit of work is the joule** (J).

We can use a PV diagram to represent the process that the gas undergoes when it goes from state A to state B:

The work is the hatched area between states A and B under the curve which represents the process. The geometric interpretation of an integral is precisely the area under the curve, between V_{A} and V_{B}. If the work is done by the surroundings on the gas, a negative sign must be added.

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**Sign criteria: The work done by an expanding gas on its surroundings is positive. When the gas compresses, the work it does is negative.**

In the situation represented in the upper figure, the work done by the gas is positive.

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