Kuliah 9 fisika

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• The Second Law of Thermodynamics—Introduction

• Heat Engines

•

Refrigerators, Air Conditioners, and Heat Pums

© 2014 Pearson

Education, Inc.


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The a!sence of the rocess i""ustrated a!o#e indicatesthat conser#ation of energy is not the who"e story$ If itwere, mo#ies run !ac%wards wou"d "oo% erfect"ynorma" to us&


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The second "aw of thermodynamics is a statement a!outwhich rocesses occur and which do not$ There are manyways to state the second "aw' here is one(

Heat can f"ow sontaneous"y from a hot o!)ect to a co"do!)ect' it wi"" not f"ow sontaneous"y from a co"d o!)ect

to a hot o!)ect$


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It is easy to roduce therma" energy using wor%, !uthow does one roduce wor% using therma" energy*

This is a heat engine'

mechanica" energy can !e o!tained from therma"energy on"y when heatcan f"ow from a higher

temerature to a "ower temerature$


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+e wi"" discuss on"y engines that run in a reeatingcyc"e' the change in interna" energy o#er a cyc"e is ero,as the system returns to its initia" state$

The high temerature reser#oir transfers an amount ofheat QH to the engine, where art of it is transformed

into wor% W and the rest, QL, is e-hausted to the "ower

temerature reser#oir$ .ote that a"" three of these

/uantities are ositi#e$


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A steam engine is onetye of heat engine$


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The interna" com!ustion engine is a tye of heat engineas we""$


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+hy does a heat engine need a temerature difference*

0therwise the wor% done on the system in one art ofthe cyc"e wi"" !e e/ua" to the wor% done !y the system

in another art, and the net wor% wi"" !e ero$


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The efficiency of the heat engine is the ratio of the wor%done to the heat inut(

1sing conser#ation of energy to e"iminate W , we find(

23456a7

23456!7


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The Carnot engine was created to e-amine the efficiency ofa heat engine$ It is idea"ied, as it has no friction$ Each "egof its cyc"e is re#ersi!"e$

The Carnot cyc"e consists of(•Isotherma" e-ansion

•Adia!atic e-ansion

•

Isotherma" comression•Adia!atic comression

An e-am"e is on the ne-t s"ide$


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8or an idea" re#ersi!"e engine, the efficiency can !ewritten in terms of the temerature(

8rom this we see that 399: efficiency can !e achie#edon"y if the co"d reser#oir is at a!so"ute ero, which is

imossi!"e$

Rea" engines ha#e some frictiona" "osses' the !estachie#e ;95


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These a"iances can !e thought of as heat enginesoerating in re#erse$

=y doing wor%, heat is

e-tracted from the co"dreser#oir and e-haustedto the hot reser#oir$


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Refrigerator erformance is measured !y the coefficientof erformance 2C0P7(

Su!stituting(

2345;a7

2345;!7

2345;c7


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A heat um can heat a house in the winter(

2345>7


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