Chapter: 11th 12th std standard Class Physics sciense Higher secondary school College Notes
Refrigerator : Coefficient of performance and Inferences
Refrigerator
A refrigerator is a cooling
device. An ideal refrigerator can be regarded as Carnot?s heat engine working in
the reverse direction. Therefore, it is also called a heat pump. In a
refrigerator the working substance would absorb certain quantity of heat from
the sink at lower temperature and reject a large amount of heat to the source
at a higher temperature with the help of an external agency like an electric
motor (Fig.).
In an actual refrigerator
vapours of freon (dichloro difluoro methane CCl2F2) act as the
working substance. Things kept inside the refrigerator act as a sink at a lower
temperature T2. A certain
amount of work W is performed by the
compressor (operated by an electric motor) on the working substance. Therefore,
it absorbs heat energy Q2
from the sink and rejects Q1
amount of heat energy to the source (atmosphere) at a temperature T1.
Since this is a
reversible cyclic process, the change in the internal energy of the working
substance is zero (i.e) dU = 0
According to the first law of
thermodynamics,
dQ = dU + dW
But dQ = Q2 - Q1
dW = -W and dU = 0
dQ=Q2- Q1 = -W
Negative sign with W represents
work is done on the system
(i.e) W = Q1 - Q2
Coefficient of performance
Coefficient of performance (COP)
is defined as the ratio of quantity of
heat Q2 removed per cycle from the contents of the refrigerator to
the energy spent per cycle W to remove this heat.
(i.e) COP = Q2 /W
=Q2 / Q1-Q2
(i.e) COP = T2 / T1-T2
???..(1)
The efficiency of the heat engine
is
η= 1 ?(T2/T1)
1- η = T2/T1
1- η /
η = T2 / T1-T2 ????..(2)
From equations (1) and (2)
COP = 1- η /
η ????.(3)
Inferences
(i) Equation (1) shows that smaller
the value of (T1 - T2) greater is the value of
COP. (i.e.) smaller is the difference in temperature between atmosphere and the
things to be cooled, higher is the COP.
(ii)
As the refrigerator works, T2
goes on decreasing due to the formation of ice. T1 is almost steady. Hence COP decreases. When the
refrigerator is defrosted, T2
increases.
Therefore defrosting is essential
for better working of the refrigerator.
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