```CHAPTER
5
The Second Law of
Thermodynamics
The process
One kg of air, initially at 5 bar, 350 K and 3 kg of
carbon dioxide, initially at 2 bar and 450 K, are
confined to opposite sides of a rigid, well
insulated container. The partition is free to
move and allows heat conduction from one gas
to the other without energy storage in the
partition. The air and carbon dioxide each
behave as ideal gases. Determine the final
equilibrium temperatures and final pressures,
assuming constant specific heats.
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 2
Introduction to the Second Law - The Direction of Natural Processes
Insulation (Q = 0)
The system
Air
1 kg
5 bar
350 K
Partition
Movement
with Heat
Transfer
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Carbon Dioxide
3 kg
2 bar
450 K
Chapter 5 Module 1 Slide 3
Introduction to the Second Law - The Direction of Natural Processes
• Known
– Initial states of the
two gases
– Closed system
– Free motion of
partition
– Heat flow between
the two gases.
• Assume
– Both gases are ideal with
constant specific heats.
– WS = 0.
– No heat stored in
partition.
– KE and PE effects
negligible.
– No friction in partition.
• Find
– Final pressures
– Final temperatures
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 4
Introduction to the Second Law - The Direction of Natural Processes
Solution
• Apply First Law
–Total energy of the combined
system is constant.
• Apply Zeroth Law
–Thermal equilibrium in the final
equilibrium state (TA2 = TB2).
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 5
Introduction to the Second Law - The Direction of Natural Processes
• Once equilibrium has been
process (overall), can the
system of A+B go back to
the initial state
spontaneously?
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 6
Introduction to the Second Law - The Direction of Natural Processes
“Everyone realizes that the reverse process
cannot happen. But why not? The total energy of
[the isolated] system would remain constant in
the reversed process as it did in the original, and
there would be no violation of the first law.
There must therefore be some other natural
principle, in addition to the first law and not
derivable from it, which determines the direction
in which a process can take place in an isolated
system.”
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 7
Introduction to the Second Law - The Direction of Natural Processes
Other natural processes...
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 8
Introduction to the Second Law - The Direction of Natural Processes
Natural processes
• Spontaneous processes
–How do they use energy?
• Inefficiencies
–How do they arise?
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 9
Introduction to the Second Law - The Direction of Natural Processes
Natural processes
• Water flows down hill.
• Objects fall to the earth
and never rebound to their
original elevation.
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 10
Introduction to the Second Law - The Direction of Natural Processes
Natural processes
• Power plants eject hot fluid into
streams that cannot be recovered
to do useful work.
• Refrigerators eject heat into the
surroundings that cannot be
recovered to do useful work.
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 11
Introduction to the Second Law - The Direction of Natural Processes
Common Experiences
• Automobiles never extract the heating
value of the gasoline fully and convert it
completely into kinetic energy. Hot
exhaust gases are emitted to the
environment and that energy cannot be
recovered to do work.
• Heat exchangers cannot be run backwards
to reheat and re-cool the fluid streams.
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 12
Introduction to the Second Law - The Direction of Natural Processes
A conjecture...
“There must therefore be some
to the first law and not derivable
from it, which determines the
direction in which a process can
take place in an isolated system.”
(W.
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 13
Introduction to the Second Law - The Direction of Natural Processes
Thermal Energy Reservoirs
• Large bodies of water or atmosphere
– Can release or absorb large amounts of
energy without a temperature change
•
•
•
•
Two phase systems
Industrial furnaces
Sources and sinks
Thermal pollution
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 14
Introduction to the Second Law - The Direction of Natural Processes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
FIGURE 5-9
Part of the heat received by
a heat engine is converted
to work, while the rest is
rejected to a sink.
Heat Engine:
A special device that
allows the conversion
of heat into work.
5-1
Heat engines:
• Receive heat from a high-temp source
– Solar, oil furnace, nuclear reactor, etc
• Convert part of heat to work (rotating shaft)
• Reject waste heat to low-temp sink
– Atmosphere, rivers, lakes, etc.
• Operate on a cycle.
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 16
Introduction to the Second Law - The Direction of Natural Processes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
FIGURE 5-10
Schematic of a steam power
plant.
and cycle so
Wout – Win will
equal Qin - Qout
External Combustion
Engine – Thermodynamic
Cycle
Internal combustion
engine – mechanical
cycle – not a heat
engine
5-2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
FIGURE 5-14
Schematic of a heat
engine.
Efficiency?
Work out compared
to heat in.
Thermal pollution can be
a problem.
5-3
st
1
•
•
•
•
Law and Efficiency
Wnet,out = Qin – Qout
Thermal efficiency = net work out/heat in
ηth = Wnet,out/Qin = 1 - Qout/Qin
ηth = 1 – QL/QH
Instructor’s Visual Aids
Heat Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 1 Slide 19
Introduction to the Second Law - The Direction of Natural Processes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
FIGURE 5-16
A heat-engine cycle cannot be
completed without rejecting some
heat to a low-temperature sink, not a
high-temperature reservoir to use
again.
One cannot save Qout!
Waste heat is that used
to heat up the gas.
5-4
Over arching fundamental
ideas...
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 21
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Central ideas & observations
• There is a natural direction
of all processes towards a
state of lower potential to do
work.
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 22
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Central ideas & observations
• There is tendency in all
processes and cyclic devices
to produce unrecoverable
energy in either doing or
consuming work.
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 23
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Central ideas & observations
•Thermal inefficiency is a
part of all processes and
cycles - there is always
some wasted heat or
work.
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 24
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
What can produce work?
• Temperature differences.
• Pressure differences.
• Electric charge differences.
• Electrical potential differences
• Difference of chemical
composition (chemical potential).
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 25
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
The expenditure of energy
• Once the work is done, we
cannot recreate these
differences (i.e., the ability
to do work) without
energy (work or heat).
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 26
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Clausius statement of the
Second Law...
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 27
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Clausius statement
It is impossible for any
system to operate in such a
way that the sole result
would be an energy transfer
by heat from a cooler to a
hotter body.
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 28
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Kelvin-Planck statement
of the Second Law...
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 29
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Kelvin-Planck statement
It is impossible for any system to
operate in a thermodynamic
cycle and deliver a net amount of
work to its surroundings while
receiving energy by heat transfer
from a single thermal reservoir.
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 30
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Other efficiencies
• desired output/required input
• Water heater – energy of hot water out
over energy in (watch cost of energy in)
• Motor – mechanical energy output over
electrical energy input
• Lighting – amount of light output over
electrical energy input
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 31
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Other efficiencies
• For a power plant:
– ηoverall = ηcombustion ηthermal ηgenerator=
Wnet,electric/HHV ∙mnet
– HHV = higher heating value – water leaves as
a liquid
– LHV = lower heating value – water leaves as
a vapor
Instructor’s Visual Aids
Heat, Work and Energy. A First Course in Thermodynamics
Chapter 5 Module 2 Slide 32
The Second Law of Thermodynamics - Additional Observations
on the Nature of Processes and Cycles
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
FIGURE 5-23
The efficiency of a
cooking appliance
represents the fraction
of the energy supplied
to the appliance that is
transferred to the food.
5-5
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