kuliah 9 fisika
TRANSCRIPT
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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(
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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(
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2345;!7
2345;c7
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A heat um can heat a house in the winter(
2345>7
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