New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
Ltherrna I
insule ition d
Figure 3. Irreversible expansion of a gas into a vacuum.
Figure 4. Reversible expansion of a gas into a vacuum.
mixed. When the wall separating the two gases is removed, N bit of information are lost since the
question concerning the type of molecule (gas A or gas B) can no longer be answered by simple
macroscopic means.
These two examples indicate that irreversible processes are always connected with a loss of
information which is described thermodynamically by the increase of entropy.
Finally we have to give the reasons for the choice of the special factor of proportionality,
0.7 k, in the definition of entropy. For this purpose we consider a gas in contact with a heat
reservoir with temperature T. This gas can be expanded to twice .its volume by slowly moving
a piston outward (figure 4). The initial and the final conditions of this reversible process are the
same as for the irreversible process (figure 3) considered before. In both cases the volume of the
gas doubles while the temperature remains constant. The increase of entropyAS = 0.7 X k X N is
thus the same in both cases.
During the reversible process, the total entropy of the gas and the heat reservoir has to remain
constant. Thus not only the heat AQ but also the entropy AS = 0.7 k N flows from the heat
reservoir to the gas during the expansion. Reversing this process, heat and entropy return again
to the reservoir. This indicates a connection between heat and increase of entropy. The elementary
calculation of AQ leads to AQ = 0.7 p V = 0.7 k X N X T. Thus at least in our special example
we see:
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