materi termo-Chapter7

8/11/2019 materi termo-Chapter7

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The Second Law ofThermodynamics

Chapter 7


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The first law of thermodynamics states that during any

cycle that a system undergoes, the cyclic integral of

the heat is equal to the cyclic integral of the work. However, we know from our experience that because

a proposed cycle does not violate the first lawdoes not

ensure that the cycle will actually occur.

It is this kind of experimental evidence that led to theformulation of the second law of thermodynamics.


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In its broader significance the second law

acknowledges that processes proceed in a certain

direction but not in the opposite direction. A hot cup of coffeecools by virtue of heat transfer to

the surroundings, but heat will not flow from the cooler

surroundings to the hotter cup of coffee.

Gasolineis used as a car drives up a hill, but the fuellevel in the gasoline tank cannot be restored to its

original level when the car coasts down the hill.


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7.1 HEAT ENGINES AND REFRIGERATORS


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The first law of thermodynamics states that during any cyclethat a system undergoes, the cyclic integral of the heat is equalto the cyclic integral of the work.


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7.2 THE SECOND LAW OF THERMODYNAMICS

Two classical statements of the second

law

KelvinPlanck statement

Clausius statement.


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The KelvinPlanck statement:

It is impossible to construct a device that will operate in

a cycle and produce no effect other thanthe raising of

a weight and the exchange of heat with a singlereservoir.

See Fig. 7.6.


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The Clausiu s s tatement:

It is impossible to construct a device that operates in a

cycle and produces no effect other than the transfer of

heat from a cooler body to a hotter body.See Fig. 7.7.


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Equivalence of the two statements

(source)

(sink)

=0


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(Impossible device)


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7.3 THE REVERSIBLE PROCESS

A reversible process for a system

-- is defined as a process that once having taken place

can be reversed and in so doing leave no change ineither systemor surroundings.


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7.4 FACTORS THAT RENDERPROCESSES IRREVERSIBLE

Friction


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Unrestrained Expansion


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Heat Transfer through a Finite Temperature

Difference

Mixing of Two Different Substances


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Other Factors

hysteresis effects

i 2R loss encountered in electrical circuits

ordinary combustion


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7.5 THE CARNOT CYCLE


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7.6 TWO PROPOSITIONS REGARDING THEEFFICIENCY OF A CARNOT CYCLE

First Proposition

It is impossible to construct an engine that

operates between two given reservoirs and is moreefficient than a reversible engineoperating

between the same two reservoirs.


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Second PropositionAll engines that operate on the Carnot cycle between two

given constant temperaturereservoirs have the same

efficiency.


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The thermodynamic scale


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7.7 THE IDEAL-GAS TEMPERATURE SCALE

Ttp.


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How is the ideal-gas temperature determined ?


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7.8 Ideal versus Real Machines


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