19 Second Law Thermo Heat Engines and 2

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19 Second Law Thermo • Heat Engines and 2 nd Law Thermodynamics • Hk:

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

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.

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

Heat Engine Flow Chart

Summary • Heat engines use part of heat and reject rest, efficiency = work

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

19 -2 Refrigerators and the Second Law of Thermodynamics

Refrigerators and the Second Law of Therm.

Refrigerators and the Second Law of Therm.

Omit 19 -3 Equivalence of the Heat. Engine and Refrigerator Statements

Omit 19 -3 Equivalence of the Heat. Engine and Refrigerator Statements

19 -4 The Carnot Engine

19 -4 The Carnot Engine

Carnot Theorem

Carnot Theorem

The Carnot Cycle Heat Engine

The Carnot Cycle Heat Engine

Omit 19 -5 Heat Pumps

Omit 19 -5 Heat Pumps

19 -6 Irreversibility and Disorder

19 -6 Irreversibility and Disorder

Irreversible Processes Introduce Disorder. Example: Container of gas collides inelastically with wall. Translational kinetic

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

19 -7 Entropy

Entropy • Entropy is a measure of the disorder of a system • Low

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

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 -8 Entropy and the Availability of Energy

Omit 19 -9 Entropy and the Probability

Omit 19 -9 Entropy and the Probability

Summary: • Heat engines accept heat and output work and heat. Refrigerators reverse this

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%

Heat engines typically have efficiencies in the range 1. 3% to 5% 2. 20% to 60% 3. 80% to 90%