Rev Lect Thermodynamic
Rev Lect Thermodynamic
Rev Lect Thermodynamic
REV LECTURE
Second Law of
Thermodynamics
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Introduction
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The Second Law of Thermodynamics --
Introduction
• The first law of thermodynamics states that energy is
conserved.
• Scientists in the 19th century noticed that many
processes that did not violate the law of conservation
of energy, never-the-less did not occur naturally.
• They formulated the second law of thermodynamics.
• Statement of the second law of thermodynamics by
R.J.E. Clausius (1822 – 1888)
Heat flows naturally from a hot object to a cold
object; heat will not flow spontaneously from a
cold object to a hot object. 4
20-2 Heat Engines
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Heat Engines
• By conservation of energy:
|QH| = |W| + |QL|.
• The high and low temperatures TH and TL are called
the operating temperatures of the engine.
• We will considering only engines that run in a
repeating cycle, that is, the system returns repeatedly
to its starting point, and thus can run continuously.
• Absolute value signs are used because we are worried
only about the magnitudes.
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Steam Engine
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Internal Combustion Engine
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Efficiency
|W| |
e= Q |
H
|QL| |
e =1 -
QH|
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Example 20-1
Car efficiency.
An automobile engine has an efficiency of 20 percent
and produces an average of 23,000 J of mechanical
work per second during operation. How much heat is
discharged from this engine per second?
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Efficiency
• The lower we can make |QL| the more efficient the engine will
be.
• If |QL| could be reduced to zero we would have a 100 percent
efficient engine.
• Experience has shown however, that it impossible to reduce |QL|
to zero.
• That such a perfect engine, running continuously in a cycle (a
perpetual motion machine) is not possible is another way of
expressing the second law of thermodynamics.
No device is possible whose sole effect is to transform a given
amount of heat completely into work.
• This is the Kelvin-Planck statement of the second law of
thermodynamics
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20-3 Reversible and Irreversible Process;
the Carnot Engine
• In the early nineteenth century, the French scientist
N.L. Sadi Carnot (1796 – 1832) studied in detail the
process of transforming heat into mechanical energy.
• Goal to increase inefficiency.
• In 1824 Carnot invented, on paper, the Carnot engine.
This is the ideal engine.
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Reversible and Irreversible
Processes
• The Carnot engine involves reversible processes.
• A reversible process is one carried out infinitely
slowly, so that the process can be considered as a
series of equilibrium states, and the whole process
could be done in reverse with no change in magnitude
of the work done or heat exchanged.
• Of course this cannot be done since it would take an
infinite time.
• All real processes are irreversible: they cannot be
done infinitely slowly, there can be turbulence in the
gas, friction will be present, and so on.
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Carnot's Engine
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Carnot Cycle
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Carnot Efficiency and the Second Law of
Thermodynamics
|QL| TL
eideal = 1 - =1-
|QH| TH
Carnot’s Theorem
All reversible engines operating between the same
two constant temperatures TH and TL have the same
efficiency. Any irreversible engine operating
between the same two temperatures will have an
efficiency less than this.
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Example 20-2
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Example 20-3
A phony claim?
An engine manufacturer makes the following claims:
The heat input per second of the engine is 9.0 kJ at
475 K. The heat output per second is 4.0 kJ at 325 K.
Do you believe these claims?
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20-4 Refrigerators, Air Conditioners, and
Heat Pumps
• The operating principle of refrigerators, air conditioners,
and heat pumps is just the reverse of the heat engine.
• By doing work |W|, heat is taken from a low-temperature
region, TL (inside the refrigerator), and a greater amount
of heat is exhausted at a high temperature, TH (into the
room).
• The work is usually done by a compressor motor that
compresses a fluid.
• A perfect refrigerator—one where no work is required
to take heat from the low-temperature region to the
high-temperature region—is not possible. 22
Clausius
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Coefficient of Performance (CP)
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CP Ideal
|QL| | |QL|
and CP = W| = |QH| - |QL|
TL
so CPideal = TH - TL
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Coefficient of Performance (CP)
• The coefficient of performance for a heat pump
acting as a heater can heat a house in the winter by
taking heat |QL| from the outside at low temperature
and delivering heat |QH| to the warmer inside of the
house, by doing work |W|. Thus for a heat pump:
|QH| |W|
CP = [heat pump]
Heat pump.
A heat pump has a coefficient of performance of 3.0
and is rated to do work at 1500 W. (a) How much
heat can it ass to a room per second? (b) If the heat
pump were turned around to act as an air conditioner
in the summer, what would you expect its coefficient
of performance to be assuming all else stays the
same?
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20-5 Entropy
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Definitions
Entropy is:
a measure of the disorder of a system.
a measure of the energy in a system or process
that is unavailable to do work. In a reversible
thermodynamic process, entropy is expressed as
the heat absorbed or emitted divided by the
absolute temperature.
dS = dQ/T
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Entropy
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Total Entropy
• For any reversible process:
∆ S = ∆ Ssyst + ∆ Senv = 0
• For irreversible processes:
∆ S = ∆ Ssyst + ∆ Senv > 0
• The second law: the entropy of an isolated system
never decreases. It either stays constant (reversible
process) or increases (irreversible process).
• Although the entropy in one part of the universe may
decrease in any process, the entropy of some other part
of the universe always increases by a greater amount,
so the total entropy always increases.
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20-7 Order to Disorder
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The Second Law
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