AmontonsGesetz

Amontons' law refers to one of two laws named after Guillaume Amontons that describe different physical phenomena. In thermodynamics, Amontons' law states that for a fixed amount of an ideal (or near-ideal) gas at constant volume, the pressure is directly proportional to the absolute temperature: P ∝ T. Equivalently, P1/T1 = P2/T2 when the volume and amount of gas remain unchanged. This relation underpins the linear relationship between pressure and temperature observed when heating a gas in a rigid container and is a precursor to the ideal gas law PV = nRT.

Historically, Amontons observed this proportionality through experiments heating air in sealed volumes in the late 17th century. The law is most accurate for ideal gases at moderate pressures and temperatures; real gases exhibit deviations at high pressures or near phase transitions, requiring more sophisticated equations of state or compressibility corrections.

In practice, Amontons' law is often applied alongside the ideal gas law to estimate how temperature changes affect pressure in rigid vessels. It implies that increasing temperature raises pressure proportionally if volume and quantity remain fixed, and conversely that lowering temperature reduces pressure accordingly.

There is also a separate Amontons' law of friction in tribology, which states that the frictional force between two dry sliding surfaces is proportional to the normal load and largely independent of the apparent contact area (and, within a range, of sliding velocity). The two laws share the name but describe different physical phenomena.