In deriving the ideal gas law, Emil Clapeyron combined two earlier laws, one by Robert Boyle and another by Jacques Charles and Joseph Louis Gay-Lussac. Boyle had identified links between pressure and volume while Charles and Gay-Lussac had found a link between volume and temperature. Clapeyron linked all three quantities by stating that in the ideal state of any gas a given number of its particles occupy the same volume and that volume changes are inverse to pressure changes and linear to temperature changes.
Ideal Gas
Simply put, an ideal gas is one that obeys the ideal gas law. A gas does this because the atoms or molecules that make it up are very small compared with the distances between them so when they bounce around they scatter off one another freely. Further, there are no extra forces between particles that could cause them to stick together.
Noble gases such as neon behave as ideal gases made up of individual atoms. Symmetric light molecules such as hydrogen behave almost like ideal gases but heavier gas molecules such as butane are less likely to.
Gases have very low densities and the atoms or molecules in them are not held together but are free to move around. Gases therefore have no boundary but can be held within a container that defines a certain volume. Reducing the size of such a container pushes the molecules closer together and so, according to the gas law, increases both the pressure and temperature.
The pressure of an ideal gas comes about from the forces of the atoms and molecules hitting the walls of the container and each other as they jostle around. According to Newton’s third law the rebounding particles exert an opposite force on the walls of the container.
An increase in temperature increase the particle speeds so that the forces on the walls become even larger. Heat energy is transferred to the molecules, making them move around quicker.
Reducing the volume of the container increases the density of the gas so that there are more collisions and so the pressure rises. The temperature also increases because, as energy is conserved, the molecules speed up when their movement is restricted.
Pressure and Altitude
On Earth, as altitude increases the pressure of the atmosphere drops and such a drop in pressure coincides with a drop in temperature. Similarly, when flying in an aeroplane, the temperature outside the plane drops to well below freezing. Both of these are examples of the ideal gas law.
Sources:
Baker, Joanne (2007) 50 Physics Ideas You Really Need to Know (Quercus Publishing)
Holzner, Steve (2005) Physics for Dummies (John Wiley & Sons)
Kuhn, Karl (1996) Basic Physics: A Self-Teaching Guide (John Wiley & Sons)
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