Lecture-Notes (Thermodynamics) - niser

4.9. THIRD LAW OF THERMODYNAMICS (NERNST LAW) 39
2)
But, for T → 0 the ideal gas is not anymore a realistic system because a real gas
undergoes at low temperatures a phase transition - condensation.
lim β = lim
T →0 T →0
3) The absolute T = 0 (zero point) is unattainable.
1
V
∂V
= 0
∂T
In order to show the validity of the last statement, let us analyze what happens when we
are trying to reach low temperatures by subsequently performing adiabatic and isothermal
transformations. If a gas is used as a working substance (Linde method), such a sequence
of transformations looks in the P − V diagram as shown in Fig. 4.9.
Figure 4.9: Linde method: sequence of adiabatic and isothermal transformations in a gas.
Let us consider now the processes involved.
A → B: isothermic compression.
Work is performed on the gas and an amount of heat Q1 < 0 is given to the reservoir
in a reversible process. As a result, ∆S = Q1
T diminishes.
B → C: adiabatic expansion.
The gas performs work. Since δQ = 0, the entropy remains constant and the
temperature diminishes.