Question

Explain how good thermal contact with liquid nitrogen can keep objects at a temperature of$77\mathrm{K}$(liquid nitrogen’s boiling point at atmospheric pressure).

Short answer

The phase shift of nitrogen absorbs the heat, causing all of the heated nitrogen to evaporate, leaving only liquid nitrogen (below $77\mathrm{K}$) in contact with the object.

Step-by-step solution

Define Superconductor

Superconductivity is a collection of physical phenomena that occur in certain materials when electrical resistance disappears and magnetic flux fields are ejected. A superconductor is any substance that has these characteristics.

Explanation

Until a thermal equilibrium is attained, a body with a temperature greater than $77\mathrm{K}$ will transmit its heat to liquid nitrogen. Because liquid nitrogen has a boiling point of $77\mathrm{K}$, it will evaporate, using up the transferred heat as latent heat for the phase transition. Only the liquid (below $77\mathrm{K}$) nitrogen remains in touch with the item as the now heated nitrogen gas evaporates and leaves the thing.

Therefore, the heat released is used up by the phase shift of nitrogen, which causes all of the heated nitrogen to evaporate, leaving only liquid nitrogen (below $77\mathrm{K}$) in contact with the item.