Question:

The plot that depicts the behavior of the mean free time \(t\) (time between two successive collisions) for the molecules of an ideal gas, as a function of temperature (\(T\)), qualitatively, is:
(Graphs are schematic and not drawn to scale)

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Mean free time \(=\) mean free path \(/\) average speed, and the average speed varies as \(\sqrt{T}\).
Updated On: Nov 5, 2026
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The Correct Option is B

Solution and Explanation

Mean free time \(t=\dfrac{\lambda}{v_{avg}}\) where \(\lambda=\dfrac{1}{\sqrt{2}\,\pi d^2 n}\) (for a fixed gas sample, fixed number density) and \(v_{avg}=\sqrt{\dfrac{8k_BT}{\pi m}}\propto\sqrt{T}\).
Hence \(t\propto\dfrac{1}{\sqrt{T}}\).
So a plot of \(t\) against \(\dfrac{1}{\sqrt{T}}\) is a straight line through the origin with positive slope. This is the graph in option (2) (option (1) is t vs 1/T, option (3) is a falling line against \(\sqrt{T}\), option (4) is t vs T).
Answer: (2) t versus \(1/\sqrt{T}\), a straight line with positive slope
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