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**Charles's law** (also known as the **law of volumes**) is an experimental gas law that describes how gases tend to expand when heated. A modern statement of Charles's law is:

When the pressure on a sample of a dry gas is held constant, the Kelvin temperature and the volume will be in direct proportion.

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This relationship of direct proportion can be written as:

So this means:

- where:

V is the volume of the gas,

T is the temperature of the gas (measured in kelvins),

and k is a non-zero constant.

This law describes how a gas expands as the temperature increases; conversely, a decrease in temperature will lead to a decrease in volume. For comparing the same substance under two different sets of conditions, the law can be written as:

The equation shows that, as absolute temperature increases, the volume of the gas also increases in proportion.

**^**Fullick, P. (1994),*Physics*, Heinemann, pp. 141–42, ISBN 978-0-435-57078-1.