ch20-2nd law of thermo

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Physics Lecture ResourcesPhysics Lecture Resources

Prof. Mineesh GulatiHead-Physics Wing

Happy Model Hr. Sec. School, Udhampur, J&K

Website: happyphysics.com


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Ch 20 The Second LawC

h 20 The Second Lawof Thermodynamicsof Thermodynamics

2005 Pearson Education


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20.1 Directions of Thermodynamic Processes20.1 Directions of Thermodynamic Processes



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20.2 Heat Engines20.2 Heat Engines

Efficiency of an engine

H

C

H

C

H

11Q

Q

Q

Q

Q

We !!!



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20.3 Internal20.3 Internal--Combustion EnginesCombustion Engines

Cycle of a four-stroke internal-combustion engine



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1

11

!

r

e

Thermal efficiency in Otto

cycleOtto cycle



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Diesel Cycle



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20.4 Refrigerators20.4 Refrigerators

CH

CC

QQ

Q

W

QK

!!

Coefficient of performance

of a refrigerator



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20.5 The Second Law of thermodynamics20.5 The Second Law of thermodynamics


It is impossible for any system to undergo a

process in which it absorbs heat from a

reservoir at a single temperature and

converts that heat completely into

mechanical work, with the system ending inthe same state in which it begin


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20.6 The Carnot Cycle20.6 The Carnot Cycle



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H

C

H

C

T

T

Q

Q!

H

C

H

C

T

T

Q

Q!

Heat transfer in a Carnot engine

or

H

CH

H

C

Carnot1

T

TT

T

Te !!

Efficiency of a Carnot engine



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CH

C

Carnot

TT

TK

!

coefficient of performance of a Carnot refrigerator

H

C

H

C

H

C

Q

Q

Q

Q

T

T!!

H

C

H

C

H

C

Q

Q

Q

Q

T

T!!

definition of Kelvin temperature


No engine can be more

efficient than a Carnot engine

operating between the same

two temperatures


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20.7 Entropy20.7 Entropy

!(2

1 T

dQS

Entropy change in a reversible

process


Entropy is used to measured ofdisorder


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Example 20.6Example 20.6

One kilogram of water at 0One kilogram of water at 0 is heated to is heated to100. Compute its change in entropy.100. Compute its change in entropy.

ANS:ANS:

2

1

22

1 21

1

3

ln

373(1 )(4190 / )(ln )

273

1.31 10 /

T

T

TdQ dTS S S mc mc

T T T

Kkg J kg K K

J K

( ! ! ! !

! y

! v



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0dQT

!


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20.8 Microscopic Interpretation of Entropy20.8 Microscopic Interpretation of Entropy


For any system, the most probable

macroscopic state is the one with thegreatest number of corresponding

microscopic state, which is also the

macroscopic state and the greatest

entropy


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wkS ln!

Microscopic expression for entropy



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The Kelvin temperature scale is based onthe efficiency of the Carnot cycle and is

independent of the properties of any

specific material. The zero point on the

Kelvin scale is called absolute zero.


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