PHY 102 S08

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• Activity A5-CI due by 4 pm Friday 02/29
• Chapter 4 Mallard HW quiz new due date end of
  the day next Monday 03/03
• Chapter 4 quiz in class next Tuesday 03/04
• Exam 2 postponed to Thursday 03/06

Tuesday, February 26 Spring 2008
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Chapter 4
Heat and the Second Law of Thermodynamics
Great Idea Heat is a form of energy that flows
from warmer to cooler objects
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2nd Law of Thermodynamics
Heat will not flow spontaneously from a cold to a
hot body.
Like the flow of air from high pressure regions
to low pressure regions, heat tends to flow from
hot objects (regions of higher energy) to cold
objects (regions of lower energy).
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Heat Engines
An engine converts energy to mechanical work
TH high temperature reservoir
working body
schematic representation of a heat engine
TC low temperature reservoir
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Heat Engines
The efficiency of an engine is the work done by
the engine divided by the energy put into it.
W Qnet QH - QC
Work done by the engine is equal to the net heat.
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Thermal Power Plants
A thermal power plant uses a heat engine to
generate electricity from heat obtained from
natural resources.
TH 600 C
efficiency
e 57
TC 100 C
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2nd Law of Thermodynamics
An engine that does nothing but convert heat to
useful work cannot be constructed.
http//auto.howstuffworks.com/engine1.htm
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Sample Exercise
A typical nuclear power plant delivers heat from
the reactor to the turbines at a temperature of
540 C. If the turbines release heat at a
temperature of 200 C, what is the maximum
possible efficiency of these turbines?
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Sample Exercises
In one cycle, a heat engine does 500 J of work
and releases 700 J of heat to a lower-temperature
reservoir. a. How much heat does it take in
from the higher-temperature reservoir? b. What
is the efficiency of the engine?
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Refrigerators and Heat Pumps
A heat pump, or refrigerator, is a device that
moves heat from a cooler
reservoir to a warmer reservoir by means of
work supplied from an external source.
The heat released, QH , equals the energy put
into the engine from both work, W , and heat,
QC. QH W QC
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Refrigerators and Heat Pumps
A refrigerator moves heat from the cooler air
from the interior to the warmer room.
A heat pump moves heat from the cooler air
outdoors into the warmer interior of a house or
building.
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2nd Law of Thermodynamics
Equivalency of first two statements of the 2nd
law
The spontaneously flow of heat from cold to hot
would make the engine 100 efficient.
A 100 efficient engine would require the
spontaneously flow of heat from cold to hot.
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Violations of Laws of Thermodynamics
W gt QH - QC
e 100
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2nd Law of Thermodynamics
Every isolated system becomes more disordered
with time.
ORDER VS. DISORDER
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2nd Law of Thermodynamics
ORDER VS. DISORDER
ICE
LIQUID WATER
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2nd Law of Thermodynamics
Probability Number of ways to arrange three
orange and three green numbered balls 720 total
36 ordered (3 orange then 3 green)
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2nd Law of Thermodynamics
Entropy is a quantity that describes the extent
to which a system loses the ability to do useful
work. It is a measure of the disorder or
randomness of a system
The entropy of the universe or of an isolated
system can only increase or remain constant. Its
entropy cannot decrease.