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Thermal Physics

The Boyle/Marriotte Law

We now consider varying the pressure and volume of a sample of gas whilst maintaining its temperature constant. The results of such an experiment are shown below. (See experiment 4TP.)

Changes of pressure and volume which take place at constant temperature are called isothermal changes. The curve above is called an isothermal.

This should be no surprise since we know that the pressure exerted by a gas increases as it is compressed into a smaller volume. (Try compressing the air in a bicycle pump with your finger blocking the outlet; it becomes harder to compress as the volume decreases.) If we plot pressure against 1/volume, we obtain the following graph.

These results do not depend on the type of gas.

The conclusion of this experiment is expressed in the Boyle/Marriotte Law, stated as follows.

The pressure of a fixed mass of gas at constant temperature is inversely proportional to the volume.

If a fixed mass of gas has initial pressure p1 and initial volume V1 and final pressure and volume p2 and V2 respectively, then we can write

p1V1 = p2V2

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