Question

Explain what happens to liquid helium at standard pressure and 2.2 K using BE statistical mechanics.

Answer 1

At low temperatures, the specific heat of liquid helium is very large compared with other materials.

At 1.5 K, the specific heat of 1 g of helium-4 is about 1 J/K. At 1.5 K, the specific heat of 1 g of copper is about 10−5 J/K.

This is important. Imagine a bath of liquid helium in a copper container. Any change in temperature would be entirely due to the helium. Heat gain by the copper would be negligible. This property means that the temperature of the whole experimental setup would rapidly follow any temperature change of the liquid helium bath.

The latent heat of vaporisation of helium is also large compared to the specific heat of other materials. This makes liquid helium effective for cooling other materials by evaporation.

There is a very sudden increase in the specific heat of helium-4 near 2.2 K. This is due to a phase transition. Below 2.2 K, helium-4 changes to a Superfluid with zero viscosity.