Chapter 5
Thermodynamic Temperature
In previous chapters we used T(the
thermodynamic temperature) the kilven
Temperature instead of
the empirical gas
temperature.
Thermodynamic Temperature
V
2
1
Consider 4 different adiabatic
pathes shown in the figure (Black
line).
Let us perform Carnot cycle
between
2
and
1
along adiabatic
path 1 and 2 shown in
Red.
1 2 3 4
2
2 1 1
12
12
( , )
Q
f
Q
Let us perform Carnot cycle
between
2
and
1
along adiabatic
path 1 and 3 shown in
Blue.
2
2 1 1
13
13
( , )
Q
f
Q
14
Thermodynamic Temperature
V
2
1
1 2 3 4
2
2
2
14
1
1
1
13
1
12
12
3
4
1
Note that Q
Q
Q
and
Q
Q
Q
But it is anexperimental that:
13
13
2
2
2
2
1
1
1
1
1
1
14
2
4
2
1
( , )
Q
Q
Q
f
Thermodynamic Temperature
2
i
1
V
a
b
c
e
d
f
This Figure represent Carnot
cycle on a
-V diagram.
Consider the a-b-c-d cycle.
Consider the cycle d-c-e-f.
2
2
(
,
i
)
i
Q
f
Q
1
1
(
,
)
i
i
Q
f
Thermodynamic Temperature
2
2
1
1
(
,
)
(
,
)
i
i
i
i
Q
Q
f
f
Q
Q
2
2
1
1
( , )
i
( , )
i
Q
f
f
Q
2
1
2
1
( ,
)
( , )
i
( ,
i
)
f
f
f
2
2
1
( )
( , )
( )
f
It was proposed by Kilven
T=A ( ) where A is constant.
In this chapter we will introduce the second law of
thermodynamics.
Irreversible and reversible processes:
Entropy
Entropy and the Second Law of Thermodynamics
Irreversible and reversible processes does
not violate 1
st
law .
But why irreversible processes they go one
way only?????
Entropy and the Second Law of Thermodynamics
Allowed
allowed
Not allowed
Reversible Process
Irreversible Process
Example of Irreversible Processes
Imagine that we put our hands around a hot cup of coffee.
Experience tells us that our hands will get warmer.
We would be astonished if our hands got cooler even though
such an event o
Hot Cup Of Coffee
beys the first law of thermodynamics.
Thus, changes in the energy of a closed system
do not set the direction of an irreversible process.
Heat Flow
Example of Irreversible Processes
Free Expansion of a gas
The gas will never go
back by itself to its initial
state.
a to
b is
allo
w
ed
b to
a is
not
allo
w
Example of Irreversible Processes
Heat engine (Carnot cycle)
Impossible
Heat
engine
Re
al
pos
sib
le
He
at
eng
in
Second Law of Thermodynamics
First Law of Thermodynamics –
Review
Review: The first law states that a
change in internal energy in a system
can occur as a result of energy
transfer by heat, by work, or by both
The law makes no distinction
First Law – Missing Pieces
There is an important distinction
between heat and work that is not
evident from the first law
The first law makes no distinction
between processes that occur
spontaneously and those that do not
An example is that it is impossible
to design a device that takes in
The Second Law of Thermodynamics
Establishes which processes do and
which do not occur
Some processes can occur in either
direction according to the first law
They are observed to occur only in
one direction
This directionality is governed by the
Irreversible Processes
An irreversible process is one that
occurs naturally in one direction
only
No irreversible process has been
observed to run backwards
An important engineering
Thermal Efficiency of a Heat Engine
Thermal efficiency is defined as the ratio
of the net work done by the engine during
one cycle to the energy input at the higher
temperature
We can think of the efficiency as the ratio
of what you gain to what you give
eng
1
h
c
c
h
h
h
W
Q
Q
Q
Q
Q
Q
More About Efficiency
In practice, all heat engines expel
only a fraction of the input
energy by mechanical work
Therefore, their efficiency is
always less than 100%
Second Law: Kelvin-Planck Form
It is impossible to construct a heat engine
that, operating in a cycle, produces no
other effect than the absorption of energy
from a reservoir and the performance of
an equal amount of work
Means that
Q
c
cannot equal 0
Some
Q
c
must be expelled to the
environment
Perfect Heat Engine
No energy is expelled
to the cold reservoir
It takes in some
amount of energy and
does an equal amount
of work
= 100%
It is an impossible
Heat Pumps and Refrigerators
Heat engines can run in reverse
This is not a natural direction of energy
transfer
Must put some energy into a device to do this
Devices that do this are called heat pumps or
refrigerators
Examples
A refrigerator is a common type of heat pump
An air conditioner is another example of a
Heat Pump Process
Energy is extracted
from the cold
reservoir,
Q
C
Energy is
transferred to the
hot reservoir,
Q
h
Work must be done
Second Law – Clausius Form
It is impossible to construct a cyclical
machine whose sole effect is to
transfer energy continuously by heat
from one object to another object at
a higher temperature without the
input of energy by work
Or – energy does not transfer
Perfect Heat Pump
Takes energy from the
cold reservoir
Expels an equal
amount of energy to
the hot reservoir
No work is done
This is an impossible