# What will be the average value of energy for a monoatomic gas in thermal equilibrium at temperature T?

Question:

What will be the average value of energy for a monoatomic gas in thermal equilibrium at temperature T?

1. $\frac{2}{3} \mathrm{k}_{\mathrm{B}} \mathrm{T}$

2. $\mathrm{k}_{\mathrm{B}} \mathrm{T}$

3. $\frac{3}{2} \mathrm{k}_{\mathrm{B}} \mathrm{T}$

4. $\frac{1}{2} \mathrm{k}_{\mathrm{B}} \mathrm{T}$

Correct Option: , 3

Solution:

As per Equi-partition law :

Each degree of freedom contributes

$\frac{1}{2} \mathrm{k}_{\mathrm{B}} \mathrm{T}$ Average Energy

In monoatomic gas D.O.F. $=3$

$\Rightarrow$ Average energy $=3 \times \frac{1}{2} k_{B} T=\frac{3}{2} k_{B} T$