Second law of thermodynamics

1. Second Law ofThermodynamics
Prepared By,
Jay Joshi (17MSE005)
SSE, PDPU

2.  The first law of thermodynamics does not give any
information on whether that change of state or the process
is at all feasible or not.
 It is the second law of thermodynamics which provides the
criterion as to the probability of various processes.

3.  A process can not happen unless it satisfies both the first and
second law of thermodynamics. The first law defines about
the “quantity” of the energy, while the second law defines
the direction which the process can take place and its
“quality”.
 There are two statements defining the second law of
thermodynamics.
 Kelvin-Planck statement of second law and
 Clausius statement of second law

4.  A heat engine is a device which converts heat or thermal
energy to mechanical energy while operating in a cycle.
Heat engine
QH
QL
TH
TL
Net work doneWnet = QH-QL
Thermal efficiency, hth is defined as
hth =Wnet/QH=(QH-QL)/QH
= 1-(QL/QH)

5.  It state that “It is impossible for any device that operates on
a cycle to receive heat from a single reservoir and produce
net work.”
 In other words, it is impossible to build a heat engine with
a 100% efficiency.
Heat engine
QH
TH
Wnet
A heat engine has to reject some energy into a
lower temperature sink in order to complete the
cycle.
The higher the reservoir temperature, the higher
the quality of the energy and more work is
produced.
This statement doesn’t have any proof, yet it hasn’t
been violated yet.

6.  A “heat pump” is defined as a
device that transfers heat from a
low-temperature source to a
high-temperature one. E.g. a
heat pump is used to extract
energy from outside cold
outdoor air into the warm
indoors.
 A refrigerator performs the same
function; the difference between
the two is in the type of heat
transfer that needs to be
optimized.
Heat pump/
Refrigerator
QH
QL
TH
TL
Wnet

7. For a Heat Pump:
COPHP=QH/Wnet=QH/(QH-QL) = 1/(1-QL/QH)
For a Refrigerator:
COPR=QL/Wnet=QL/(QH-QL) = 1/(QH/QL-1)
Note: COPHP = COPR + 1
COPHP>1
The efficiencies of heat pumps and refrigerators are denoted by
the Coefficient of Performance (COP) where

8.  The Clausius statement is another
expression of the second law of
thermodynamics. It states that, It
is impossible to construct a device
that operates in a cycle and
produces no effect other than the
transfer of heat from a lower-
temperature body to a higher-
temperature body.
 In other words, heat can not be
transferred from low temperature
to higher temperature unless
external work is supplied.
Heat pump
QH
QL
TH
TL
Therefore, it is impossible to
build a heat pump or a
refrigerator which can work
without a work input.

9. Sources :
Basics and AppliedThermodynamics by P. K. Nag
Thermodynamics : An EngineeringApproach byY. A. Cengel