Clausius statement : Unaided by an external agency
heat can not be transferred from a body at lower temperature to a body at
higher temperature.
Devices that are used to transfer
heat from a body at lower temperature to a body at higher temperature are known
as refrigerators (or) heat pumps. If the high temperature side is atmosphere it
is a refrigerator. If the low temperature side is atmosphere it is known as a
heat pump. The performance index here is called coefficient of performance (Cop). In refrigerator (and heat pumps)
the performance is the ratio of two independent parameters and hence the
possibility of getting the value more than unity is always there. But the term
efficiency is restricted to a maximum of unity. Hence the term efficiency is
not used here.
Taking work
as external agency, for refrigerators (Figure 4.2)
...(4.3)
From first
law
SQ = SW
Q1 -
Q2 = W
Ir-reversibility and Causes of Ir-reversibility
The factors that make a process irreversible are known as
irreversibilities. Various forms of irreversibilities are listed below.
a) Friction : Friction occurs at the interface of two
bodies moving relative to each other. It is the main cause of irreversibility
in many processes. Energy spent in overcoming friction is dissipated in the
form of heat which can never be restored.
b) Heat
transfer: Once heat is
transferred from a body at higher temperature to a body at lower temperature, it can never be
reversed without the aid of an external agency.
through finite
temperature
difference
c) Unregistered expansion :
Consider a vessel with two chambers as given
in the arrangement as shown in Fig. 4.6. If the members separating the gas from
vacuum is removed, gas will expand and occupy the entire space. Since the
expansion has no influence on the surroundings, there is no work output in this
process. But to restore the initial arrangement, a definite work input is
required.
d) Mixing of two gases :
Consider a vessel with two chambers, one with O2 and the other with N2. When the
member separating O2 & N2 is removed, uniform mixing is taking place
without any work output. But such a process can not be reversed without any
work input.
e) Throttling : It is a totally irreversible process. Gas or
vapour will expand through a restricted passage with its pressure decreasing
rapidly without any work output. Such an expansion can not be reversed.
