General Physics i 20

7/28/2019 General Physics i 20

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Chapter 20Entropy and

the 2

nd

Law ofThermodynamics


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Things to learn

We define the concept of entropy and

understand the 2nd law ofthermodynamics (Entropy never

decreases)

We will understand engines andrefrigerators as devices

interconverting heat and work.


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20-2 Irreversible Processes

and EntropyIrreversible process

Inverse process is improbable !

Fig 20-1 free expansion

[entropy is increasing]

Entropy : multiplicity of a system

Reversible process

Inverse process happens[entropy is invariant]


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20-3 Change in

Entropy

Irreversible

process


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Reversible

process


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Entropy

calculation


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SP 20-1


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Water is heated on a stove. Find the entropy

changes of the water as its temperature rises.

CP 1


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CP 2

Find the entropychanges of the

two processes of an

ideal gas.


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SP 20-2


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Entropy as a state function

Entropy change is independent of the path.

Once we know i and f states,

we know the Entropy change.


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SP 20-1 Once again

free expansion

Gas was the only system (closed).Entropy of the closed system has been

increased! irreversible process

Corresponding Reversible process

Gas + Reservoir was the closed system

Entropy of the closed system has been

unchanged reversible processReservoir gave the same dQ to gas at the same T


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20-4 The 2nd Law of

Thermodynamics

The entropy of a closed system never decreases

Reversible process : constant

Irreversible process : increasing


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20-5 The entropy in the real world

Heat Engine

Heat (QH) is supplied fromthe hotter reservoir.

(gasoline explosion)

Through the expansion of gas,

work W is performed.

(rotate the crank)

Partial heat (QL) is lost to the

colder reservoir.

(outside the engine)


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The Carnot Engine

= ideal engine (reversible).

= the most efficient engine.

No energy loss due to

friction and turbulence.


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The Carnot Engine

(Work)

Work = Heat in Heat out


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The Carnot Engine

(Entropy)


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The Carnot Engine (Efficiency)

Perfect engine

impossible


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SP 20-4


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20-6 Refrigerator

Refrigerator is cooling

a cold reservoir cooler.

Input : Work (compressor)

Output : QL(heat lost by the cold reservoir)


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20-7 The efficiencies of real

engines No real engine has

efficiency greater than

C. If X > C, couple X to

Carnot refrigerator:

Violate the 2nd law of

thermodynamics


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20-8 A statistical view of

entropy

Macroscopic definition

dS=dQ/T

Microscopic definition

S=k ln W

k=Boltzmann constant W=multiplicity

W=1 for each microscopic configuration


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Multiplicity calculation

S=k ln W

Count the number of ways todistribute balls in the two

partitions


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Multiplicity calculation

As n ,the peak value of the W becomes

sharper and sharper at

n1=n2=n/2Entropy (log multiplicity)

becomes maximum at this point

For a system with a large number ofparticles (order of mol), system tends to

make a transition into a state with more

multiplicity, which has a larger entropy.

Second law of thermodynamics

General Physics i 20

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