# Gay Lussac's Law

Gay Lussac's Law, named after Joseph Louis Gay Lussac, says that for a gas confined at a constant volume, the temperature and pressure are directly related. In English, that means that when the temperature of a gas increases, the pressure also increases.

That can be stated mathematically:

It can also be stated graphically:

Why are temperature and pressure directly related? The answer depends on our understanding of temperature. As we have said before, temperature is a measure of average kinetic energy, that I, as temperature increases, particles move faster. That means that as the temperature of a gas increases the particles will hit the walls with more force. It also means that they will hit the walls more often, since it will take less time to get from one wall to another when the particles are moving faster. Both of these (more collisions and harder collisions) increase the force exerted on the wall and therefore increase the pressure.

Before we leave Gay Lussac's Law there is one other thing worth noting, which requires us to go back to the graphical representation we saw before. This graph can do more than just illustrate the law for us. If you extrapolate the graph to lower and lower temperatures, there is a temperature at which the pressure should be zero.

This would be the temperature at which the particles actually stop moving and therefore stop hitting the walls. This temperature is defined as absolute zero, and it occurs at -273.15 oC. It is this temperature that is the bottom of the Kelvin scale.