The upper cold denser air goes down while the lower warmer air goes up. The convection currents so set up quickly cool the chamber of the refrigerator.

P, T and V are thermodynamic variables while the gas constant R is not thermodynamics coordinate.

No work is done during the free expansion of a gas. There is no change in internal energy or temperature of the gas.

As work done on a system is negative, so ∆U=-∆W or ∆U+∆W=0 or ∆Q=0. Hence the system undergoes an adiabatic process.

Efficiency,
Ƞ=1-T₂/T₁
When the temperature T₁ of the source increases, the efficiency increases.