PHYSICS

~ /). Q ~ /). Q 2 .
L.J __11 + L.J --' = 0 (12.13)
i jrli i jr2i
In general, we can write this condition in the integral form:
J dQ + J dQ = 0
or (12.14)
AaB T AbB T
fdi = 0 (12.15)
Here the symbol f indicates that the integration is over a
closed path.
The change in entropy (dS) between two equilibrium states is
given by the heat transferred, divided by the absolute temperature
of the system in this interval:
dQ = dS
(12.16)
T
The function S is called entropy. The entropy is a measure of
the amount of energy in a physical system which cannot be used
to do work. It is a measure of the disorder present in a system.
At the end of the cycle, be it reversible or irreversible, there
is no change in the system's entropy because it has returned to its
original state. For irreversible cycles, it means that the system
expels more heat to the exterior. This may be summarized as
follows:
For a reversible state: dS = dQ . t.S = t. dQ = 0 (12.17)
T ' r r T
For an irreversible state: dS > di ; fS = 0, f di < 0 (12.18)
The statement can be made more precise by expressing the
entropy change (dS) as a sum of two parts:
dS = d e S + dS ,
(12.19)
where deS is the change of the system's entropy due to exchange
of energy and matter and d.S is the change in entropy due to
irreversible processes within the system. The quantity deS could be
positive or negative, but dS can only be greater than or equal to
zero.
The main tendencies of the entropy exchange are:
a. For isolated systems, since there is no exchange of energy or
matter:
d e S = 0 and dS , ~ 0 (12.20)
b. For closed systems that exchange energy but not matter:
d S = dQ = dU + pdV and dS ~ 0 (12.21)
e T T ,
c. For open systems that exchange both matter and energy:
dU + pdV
deS = jr + (deS)maller and as ~ 0 (12.22)
Whether we consider isolated, closed or open systems, diS?O.
This is the statement of the Second Law in its most general form.
12.8. THE EQUILIBRIUM STATE
Entropy and the second law of thermodynamics provide the
key to understanding equilibrium. An isolated system may undergo
various spontaneous changes, some of which will increase its entropy.
If the total entropy increases during a process, as it usually
does, the process is irreversible - it is impossible to return to the
starting point, leaving no other traces, since that would require a
decrease in the total entropy, which is impossible. Once entropy
has increased, it cannot decrease again. An isolated system therefore
approaches a state in which entropy has the highest possible
value (S ---7 S max ) - this is a state of equilibrium. In this state, the
entropy of the system cannot increase (because it is already at a
maximum) and it cannot decrease (because that would violate the
second law of thermodynamics (dS ~ 0)). The only changes
allowed are those in which entropy remains constant.
Chapter 13. THERMOREGULATION
'i,",:ilI;i("ltiIW;'t'"'''''"'·''".1~'':;~'l)J;''fjff;i''~if,q".!ic':tW'~\It!5i'i~N@!V};l~t0t~i~'%W1iV;~Ii4')",,"~lilCiiliR~~
13.1. PATTERN OF BODY TEMPERATURE
Temperature of a body directly reflects that of the environment
among cold-blooded (poikilothermic) animals, such as insects,
snakes and lizards. These creatures maintain safe body tempera­
94 95