Download Ch.19 (section 1 only)

19 Second Law Thermo
• Heat Engines and 2nd Law
Thermodynamics
• Hk: 27, 35.
Second Law of Thermodynamics
• Heat flows spontaneously from a
substance at a higher temperature to a
substance at a lower temperature.
• The reverse situation does not occur
spontaneously.
• Ex. Hot drink cools but does not reheat
Heat Engines
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Device that uses heat to perform work
Hot Reservoir (e.g. steam)
Cool Reservoir (e.g. pool of water)
Efficiency is work done per unit of input
heat (e = W/QH)
• Ex. A heat engine does 100J of work
when given 300J from the hot reservoir.
The efficiency is 100J/300J = 0.33 =
33%.
Heat Engine Flow Chart
Summary
• Heat engines use part of heat and reject
rest, efficiency = work out/heat in
• 2nd Law of Thermodynamics prescribes
that efficiency cannot be 100%
• /
19-2
Refrigerators and the Second
Law of Thermodynamics
Refrigerators and the Second Law of Therm.
Omit
19-3
Equivalence of the HeatEngine
and Refrigerator Statements
19-4
The Carnot Engine
Carnot Theorem
The Carnot Cycle Heat Engine
Omit
19-5
Heat Pumps
19-6
Irreversibility and Disorder
Irreversible Processes Introduce Disorder.
Example: Container of gas collides inelastically with
wall. Translational kinetic energy is added to general
thermal energy of gas
19-7
Entropy
Entropy
• Entropy is a measure of the disorder of a
system
• Low entropy (ordered)
• High entropy (disordered)
• Reversible Process: Entropy is constant
• Irreversible Process: Entropy increases
Second Law in terms of Entropy
• the total entropy of the universe does not
change when a reversible process occurs.
• the total entropy does increase when an
irreversible process occurs.
Omit
19-8
Entropy and the Availability
of Energy
Omit
19-9
Entropy and the Probability
Summary:
• Heat engines accept heat and output
work and heat. Refrigerators reverse
this process. Carnot engine is most
efficient engine.
• 2nd Law of Therm. dictates the direction
of natural processes.
• Irreversible processes increase
disorder.
• Entropy is a measure of disorder and
increases for any irreversible process.
Heat engines typically have
efficiencies in the range
1. 3% to 5%
2. 20% to 60%
3. 80% to 90%
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0%
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0%
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