19 Second Law Thermo Heat Engines and 2
- Slides: 25
19 Second Law Thermo • Heat Engines and 2 nd 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 • • 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 100 J of work when given 300 J from the hot reservoir. The efficiency is 100 J/300 J = 0. 33 = 33%.
Heat Engine Flow Chart
Summary • Heat engines use part of heat and reject rest, efficiency = work out/heat in • 2 nd 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 Heat. Engine 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. • 2 nd 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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