# How would you use the kinetic theory to explain why a helium filled balloon "shrinks" when it is taken from a warm room to the outside on a cold day?

Jun 6, 2017

From kinetic theory, the pressure can be interpreted as, 2/3 the kinetic energy of molecules per unit volume.

$p = \frac{2 N E}{3} V$

Also, from kinetic interpretation of temperature, the ideal gas molecules have their energy proportional to the absolute temperature.

Thus, $E = \frac{3 k T}{2}$

Combining the two relations,

$p V$ is proportional to $T$.

Then for a decrease in temperature $T$, then $V$ and $p$ decrease.

This explains why the balloon shrinks when the temperature decreases.