The Laws of Thermodynamics
Physics Lecture
Notes
The Laws of Thermodynamics (01 of 38)
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FOUNDRY: It requires about 289 Joules of heat to
melt one gram of steel. In this chapter, we will
define the quantity of heat to raise the temperature
and to change the phase of a substance.
•
•
•
•
Sadi Carnot
1796 – 1832
French Engineer
Founder of the science
of thermodynamics
• First to recognize the
relationship between
work and heat
Thermodynamics is the study of processes in which energy is
transferred as heat and as work.
IThe internal energy is the sum of all the energy of all the
molecules in an object:
 random translational kinetic energy
 rotational kinetic energy
 vibrational energy
 intermolecular energy associated with their bonding.
ZEROTH LAW
The Zeroth law states that "If bodies A and B are each
separately in thermal equilibrium with body C, then A and B
are in thermal equilibrium with each other."
The common property between A and B is called temperature.
Topics
1) The First Law of Thermodynamics
2) Work Done on a Gas
3) Pressure - Volume Graph
4) Thermodynamic Processes
5) The Second Law of Thermodynamics
6) Heat Engines
7) Carnot cycle
8) Entropy
The Laws of Thermodynamics (02 of 38)
First Law of Thermodynamics
System
U
Q
W
Environment
DU = Q - W
The Laws of Thermodynamics (03 of 38)
The Ideal Gas Law
Volume
(m3)
Pressure
(Pa)
PV = NkT
Number of
Molecules
Absolute
Temperature
(K)
Boltzmann’s
Constant
(1.38 x 10-23 J/K)
Temperature and kinetic Theory13
Thermodynamic Processes
A. Isobaric
Constant Pressure
B. Iso-volumetric Constant Volume
C. Isothermal
Constant Temp.
D. Adiabatic
No Heat Transfer
between systems
The Laws of Thermodynamics (06 of 38)
Pressure - Volume Graph
Isotherms
(lines of constant
temperature)
P
Pressure
T4
T3
T2
T1
Area under curve
represents work
Internal energy
is proportional
to temperature
V
Volume
The Laws of Thermodynamics (05 of 38)
HEAT ENGINES
Hot Res. TH
Qhot
Engine
Qcold
Cold Res. TC
Wout
A heat engine is any
device which through
a cyclic process:
• Absorbs heat Qhot
• Performs work Wout
• Rejects heat Qcold
THE SECOND LAW OF
THERMODYNAMICS
Hot Res. TH
Qhot
Wout
Engine
Qcold
Cold Res. TC
It is impossible to construct an
engine that, operating in a
cycle, produces no effect other
than the extraction of heat
from a reservoir and the
performance of an equivalent
amount of work.
Not only can you not win (1st law);
you can’t even break even (2nd law)!
THE SECOND LAW OF
THERMODYNAMICS
Hot Res. TH
Hot Res. TH
400 J
100 J
Engine
400 J
400 J
Engine
300 J
Cold Res. TC
• A possible engine.
Cold Res. TC
• An IMPOSSIBLE
engine.
EFFICIENCY OF AN
ENGINE
Hot Res. TH
QH
W
Engine
QC
The efficiency of a heat engine
is the ratio of the net work
done W to the heat input QH.
e=
W
=
QH
Cold Res. TC
e=1-
Q H - QC
QH
QC
QH
EFFICIENCY EXAMPLE
Hot Res. TH
800 J
W
An engine absorbs 800 J and
wastes 600 J every cycle. What
is the efficiency?
QC
Engine
e=1-
600 J
Cold Res. TC
600 J
e=1-
800 J
-----
QH
e = 25%
Question: How many joules of work is done?
EFFICIENCY OF AN IDEAL
ENGINE (Carnot Engine)
Hot Res. TH
QH
Engine
QC
W
maember perfect engine, the
quantities Q of heat gained
and lost are proportional to
TH -temperatures
TC
the absolute
T.
e=
TH
Cold Res. TC
e=1-
TC
TH
The Carnot Cycle
Th= 550 K
For the engine
= 890 J
Qh
Work done by engine each cycle
Engine
W
W  Qh - Qc
Qc
W  890 J - 470 J  420 J
= 470 J
Tc
The efficiency of the engine
W
420 J
e
 0.472

Qh
890 J
 47.2 %
The Laws of Thermodynamics (26 of 38)
The Carnot Cycle
Th= 550 K
Temperature of the cool reservoir
Qc Tc

Qh Th
Qc
 Tc  Th
Qh
 470 J 
Tc  550 K 

 550 J 
= 890 J
Qh
Engine
Qc
 290 K
W = 420 J
= 470 J
Tc
The engine undergoes 22 cycles per second,
its mechanical power output
J  cycles 

W
 22

 Wf   420
P
cycle
s



t
 9.24 kW
The Laws of Thermodynamics (27 of 38)
The Carnot Cycle
Th
A carnot engine absorbs 900 J of
heat each cycle and provides 350 J
of work
Qh
The efficiency of the engine
Qc
W 350 J

e
 0.389
Qh 900 J
= 900 J
Engine
 38.9 %
W = 350 J
Tc
The heat ejected each cycle
W  Qh - Qc
Qc  Qh - W  900 J - 350 J
 550 J
The Laws of Thermodynamics (28 of 38)
The Carnot Cycle
Th
A carnot engine absorbs 900 J of
heat each cycle and provides 350 J
of work
Qh
The engine ejects heat at 10 oC
The temperature of the hot
reservoir
Qc
Qc Tc

Qh Th
= 900 J
Engine
W = 350 J
=550 J
Tc = 283 K
Qh
 Th  Tc
Qc
 900 J 
Tc  283 K 

 550 J 
 463 K  190 oC
The Laws of Thermodynamics (29 of 38)
The Carnot Cycle
Th= 650 K
A carnot engine operates
between a hot reservoir at 650 K
and a cold reservoir at 300 K. If
it absorbs 400 J of heat at the
hot reservoir, how much work
does it deliver?
Qh
= 400 J
Engine
W=?
Qc
Th - Tc
W
e

Qh
Th
Tc= 300 K
 Th - Tc 
W  Qh 

 Th 
 650 K - 300 K 
W  400 J

650
K


 215 J
The Laws of Thermodynamics (30 of 38)
Entropy
Entropy is a measure of the disorder of a system. This gives us
yet another statement of the second law:
Natural processes tend to move toward
a state of greater disorder.
Example: If you put milk and sugar in your coffee and stir it,
you wind up with coffee that is uniformly milky and sweet.
No amount of stirring will get the milk and sugar to come
back out of solution.
The Laws of Thermodynamics (33 of 38)
Entropy
Another example: when a tornado hits a building, there is
major damage.
You never see a tornado approach a pile of rubble and leave
a building behind when it passes.
Thermal equilibrium is a similar process –
the uniform final state has more disorder than
the separate temperatures in the initial state.
The Laws of Thermodynamics (34 of 38)
Entropy
Another consequence of the second law:
In any natural process, some energy
becomes unavailable to do useful work.
If we look at the universe as a whole, it seems inevitable that,
as more and more energy is converted to unavailable forms,
the ability to do work anywhere will gradually vanish. This is
called the heat death of the universe.
The Laws of Thermodynamics (35 of 38)
Summary
First law of thermodynamics:
ΔU  Q - W
Isothermal process: temperature is constant.
Adiabatic process: no heat is exchanged.
Work done by gas at constant pressure:
W  PΔU
Heat engine changes heat into useful work
(requires temperature difference).
Efficiency of a heat engine:
Carnot efficiency:
QL
W
e
 1QH
QH
TL
ec  1 TH
The Laws of Thermodynamics (36 of 38)