THEORY OF GASES
is basically a macroscopic science. We discussed macroscopic parameters like
pressure, temperature and volume of thermodynamical systems in unit 8. In this
unit we discuss the microscopic origin of pressure and temperature by
considering a thermodynamic system as collection of particles or molecules.
Kinetic theory relates pressure and temperature to molecular motion of sample
of a gas and it is a bridge between Newtonian mechanics and thermodynamics. The
present chapter introduces the kinetic nature of gas molecules.
theory is based on certain assumptions which makes the mathematical treatment
simple. None of these assumptions are strictly true yet the model based on
these assumptions can be applied to all gases.
All the molecules of a gas are identical, elastic spheres.
The molecules of different gases are different.
The number of molecules in a gas is very large and the average separation
between them is larger than size of the gas molecules.
The molecules of a gas are in a state of continuous random motion.
The molecules collide with one another and also with the walls of the
These collisions are perfectly elastic so that there is no loss of kinetic
energy during collisions.
Between two successive collisions, a molecule moves with uniform velocity.
The molecules do not exert any force of attraction or repulsion on each other
except during collision. The molecules do not possess any potential energy and
the energy is wholly kinetic.
The collisions are instantaneous. The time spent by a molecule in each
collision is very small compared to the time elapsed between two consecutive
These molecules obey Newton’s laws of motion even though they move randomly.