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Trang 1CHAPTER 22 HEAT ENGINES,
ENTROPY, AND THE SECOND LAW
OF THERMODYNAMICS
Lecturer: Tran Thi Ngoc Dung
Trang 2heat engine
• A heat engine is a device that takes
in energy by heat and, operating in a cyclic process, expels a fraction of that energy by means of work
• A heat engine carries some working substance through a cyclic process during which
• (1) the working substance absorbs
energy by heat from a high-temperature energy reservoir,
• (2) work is done by the engine,
• (3) energy is expelled by heat to a
lower-temperature reservoir
Trang 3The Efficiency of an Engine
h
Q
' W reservoir
_ hot _ from _
received _
Heat
engine _
the _ by _ done _
Work
The Efficiency of an Engine
Engine operates in a cycle process, the change in internal energy is 0:
' c h
c
Q W
'
Work done by the
engine:
0 W
Q Q
h
' c h
' c h
Q 1
Q
Q Q
Q
' W
Trang 4Example
• An engine transfers 2.00 x 10 3 J of energy from a hot reservoir during a cycle and transfers 1.50 x 10 3 J as exhaust to a cold reservoir
(A) Find the efficiency of the engine
(B) How much work does this engine do in one cycle?
J 10 5
0 )
10 5
1 ( )
10 2
( Q
Q '
W
%
25 10
2
10 5
.
1 1
Q
Q 1
e
3 3
3
' c h
3
3
h
' c
Trang 5The Carnot Engine
• The engine operates in a cyclic process consisting of 2 isothermal processes and 2 adiabatic processes
) V
V ln(
T
) V
V ln(
T 1 Q
' Q 1
e
) V
V ln(
nRT Q
'
Q
) V
V ln(
nRT Q
Q
0 Q
: adiabatic
:
DA
0
) V
V ln(
nRT Q
: isothermal
:
CD
0 Q
: adiabatic
:
BC
0
) V
V ln(
nRT Q
: isothermal
:
AB
A
B h
D
C c
h c
D
C c
CD c
A
B h
AB h
DA
C
D c
CD BC
A
B h
AB
Trang 6The Carnot Engine (cont.)
D
C A
B
1 D c
1 A h 1
1 C c
1 B h 1
A
B h
D
C c
h c
V
V
V
V
V T V
T const
TV : adiabatic
:
DA
V T V
T const
TV : adiabatic
:
BC
) V
V ln(
T
) V
V ln(
T 1 Q
' Q
1
e
h
c
T
T 1
e
Trang 7Carnot Cycle
In process D -A , (Active
Fig 22.9d), the base of the
cylinder is replaced by a
nonconducting wall and the
gas is compressed
adiabatically
The temperature of the gas
increases to T h, and the
work done by the piston
on the gas is W DA
In process C S D (Active Fig 22.9c), the
gas is placed in thermal contact
with an energy reservoir at temperature T c
and is compressed isothermally
at temperature Tc
During this time, the gas expels energy
|Qc| to the reservoir and the work done by the piston on the gas is W CD
Process A B is an isothermal expansion at temperature T h The gas is placed in thermal contact with an energy reservoir at
temperature T h During the expansion, the gas absorbs
energy |Q h| from the reservoir through the base of the cylinder
and does work W AB in raising the
piston
In process B C (Active Fig 22.9b), the
base of the cylinder is replaced by a thermally nonconducting wall and the gas expands adiabatically; that is, no energy enters or leaves the system by heat During the expansion, the temperature of the gas
decreases from T h to T c and the gas does
work W BC
in raising the piston
Trang 822.2 Heat Pumps and Refrigerators
In a refrigerator or a heat
pump, the engine takes in
energy |Q c | from a cold
reservoir and expels energy
|Q h | to a hot reservoir (Active
Fig 22.4), which can be
accomplished only if work is
done on the engine
Trang 9Refrigerator
c h
c
c h
c
T T
T )
e mod cooling
( COP :
cycle _
Carnot
Q '
Q
Q )
e mod cooling
( COP
c h
c h
c h
c
Q '
Q Q
Q W
0 W Q
Q
U
W
Q or
refrigerat _
the _ on _ done _
Work
reservoir _
cold _
from _
received _
heat )
e mod cooling
(
COP
The effectiveness of a heat pump /refrigerators is described in terms of a
number called the coefficient of performance
Trang 10Heat Pump
c h
h
c h
h
c h
c h
c h
h
T T
T )
e mod heating
( COP :
cycle _
Carnot
Q '
Q
'
Q )
e mod heating
(
COP
Q '
Q Q
Q W
0 W
Q Q
U
W
' Q or
refrigerat _
the _
on _ done _
Work
reservoir _
hot _
to _ delivered _
heat )
e mod heating
(
COP
Trang 11Example 22.5 Efficiency of the Otto Cycle
Find the thermal efficiency of an engine operating in an idealized Otto cycle Treat the working substance as an ideal gas
1
1
2 h
c
A D
B C
D
C A
B 1
2
1
1
1 2 C
1 1
D
1
1 2 B
1 1
A
C
A D
h c
A D
V DA
c
B C
V BC
h
D A
V DA
CD
B C
V BC
AB
V
V 1
Q
' Q 1
e
T T
T T
T
T T
T V
V
V T V
T
const TV
: adiabatic
:
CD
V T V
T
const TV
: adiabatic
:
AB
T T
T T
1 Q
' Q 1
e
) T T
( nC Q
'
Q
) T T
( nC Q
Q
0 ) T T
( nC Q
: ric isovolumet
:
DA
0 Q
: adiabatic
:
CD
0 ) T T
( nC Q
: ric isovolumet
:
BC
0 Q
: adiabatic
:
AB