Kinetic Theory then developed to explain this movement. The following assumptions are made to help the theory:-

1. There are a very large number of particles (molecules or atoms) involved. This means we can apply statistics to our solutions.
2. The particles are involved in perfectly elastic collisions with their containers and each other. So no E_k is lost.
3. The length of time involved in a collision is negligible compared to the time between collisions (i.e. we can ignore the moments when the potential energy component of the internal energy is not zero).
4. The volume occupied by the particles is negligible compared to the volume of the container. i.e. there are big gaps between particles so they have little or no effect on each other.
5. The particles are involved in random motion i.e. there is no resultant force on them - due to gravity, for example.

Using Kinetic Theory it is possible to derive the equations that describe and explain things like pressure and temperature in terms of the movement of individual molecules.

Where :.

p = pressure

V = volume

N = No of atoms/molecules

m = mass of one atom/molecule

= mean square speed of the atoms

and

Where :.

p = density of the gas

because

In some syllabi you may be required to learn how to derive these from scratch. Check this with your teacher!

The mean square speed, in the equation above is found by taking the average of all the ''squared speeds'' of the molecules. If you take the square root of this you get the root of the average squared speeds or root mean square speed.

This is not the same as the mean speed or average speed of the molecules but it is easily measurable (using ) whereas the average speed isn't directly measurable.