SECOND LAW OF THERMODYNAMICS
Second law of thermodynamics can be stated in many ways:
i) "It is impossible to construct an engine
which operated in a complete cycle will
absorb heat from a single body and convert it completely to work without leaving some changes in the working
system". This is called as the
Kelvin - Planck statement of II law of thermodynamics.
ii) "It is impossible to transfer heat from a cold
body to a hot body by a machine without
doing some work". This is called as
the clausius statement of II law of thermodynamics.
iii) 'A process accompanied by increase in entropy tends
to be spontaneous". This statement is
called as the entropy statement of II law of thermodynamics. Entropy is a measure of randomness or disorder of the
molecules of a system and it is a
thermodynamic state function. A system always spontaneously changes
from ordered to a disordered state. Therefore entropy of a spontaneous process is constantly increasing.
iv) "Efficiency of a machine can never be cent
percent".
v) The heat Efficiency of any machine is given by the
value of ratio of output to input
energies. Output can be in the form of any measurable energy or temperature change while input can be in the form of
heat energy or fuel amount which can
be converted to heat energy.
thus
% efficiency = (output/ input) x 100
The machine can be a heat engine also. Consider a heat
engine which has an initial
temperature T1 and final temperature as T2, then if T1>T2 then when some
amount of heat is being converted into work, T2 is the lowered temperature.
The efficiency 'h' is given by,
%
efficiency = (T1 -T2 / T1) x 100
According to II law of thermodynamics it is impossible
to have a machine or heat engine
which converts the input energy completely into output energy or output work without any amount of heat or energy being
absorbed by the machine.
Hence, % efficiency can never be achieved as cent
percent.
% efficiency = 1 -(T2 / T1)
x 100
By II law, T2<T1 %
efficiency less than 100. 9.3
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