Thermal conductivity of air with rise in temperature

The correct answer is option 'A', which states that the thermal conductivity of air increases with a rise in temperature. This can be explained by the following factors:

1. Increase in molecular motion:
As the temperature of a gas (such as air) increases, the average kinetic energy of its molecules also increases. This leads to an increase in molecular motion, resulting in more frequent collisions between the molecules. These collisions facilitate the transfer of thermal energy from one molecule to another, thereby increasing the thermal conductivity of the gas.

2. Increase in collision frequency:
With the increase in temperature, the frequency of collisions between gas molecules also increases. This is due to the higher molecular velocity resulting from increased kinetic energy. As a result, there is a greater probability of energy transfer through collisions, leading to higher thermal conductivity.

3. Decrease in mean free path:
The mean free path is the average distance a molecule travels between successive collisions. With the rise in temperature, the mean free path of air molecules decreases. This is because the increased molecular motion leads to more frequent collisions, reducing the average distance traveled by the molecules. A shorter mean free path implies that the molecules have less distance to travel before they collide and transfer thermal energy. Consequently, the thermal conductivity of air increases.

4. Impact of molecular structure:
The molecular structure of air remains relatively unchanged with temperature variations. Therefore, the increase in thermal conductivity is primarily influenced by the factors mentioned above, such as increased molecular motion and collision frequency.

In summary, the thermal conductivity of air increases with a rise in temperature due to enhanced molecular motion, higher collision frequency, and reduced mean free path. These factors contribute to a more efficient transfer of thermal energy within the gas.