**NEET Physics Notes Mechanics-Behaviour of Perfect Gas and Kinetic-Theory Concept of Pressure**

**Concept of Pressure**

**Concept of Pressure**

Pressure p exerted by a perfect gas on the walls of container is given by

Here, m = mass of each molecule,= root mean square velocity of molecules, ρ= density of gas, M = mass of gas enclosed in volume V of container, and E = Total KE of the ideal gas.

**Various Speeds of Gas Molecules**

**Root mean square speed**It is defined as the square root of mean of squares of the speed of different molecules

**Most probable speed**It is defined as the speed which is possessed by maximum fraction of total number of molecules of the gas.

**Average speed**It is the arithmetic mean of the speeds of molecules in a gas at given temperature.

**Degree of Freedom ( f)**

The term degree of freedom of a system refers to the possible independent motions, system can have

- for monoatomic gas, (
*f*) = 3 - for diatomic gas, (
*f*) = 5 - for triatomic gas, (f ) = 6(non-linear)
- for triatomic (linear) gas, (
*f*) = 7 - for N-atomic molecule (
*f*) = 6N – 3 - for N-atomic linear molecule (
*f*) = 6N – 5