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© 2017 Pearson Education, Inc. Slide 20-1

A large –20ºC ice cube is dropped into a

super-insulated container holding a small amount

of 5ºC water, then the container is sealed. Ten

minutes later, is it possible that the temperature of

the ice cube will be colder than –20ºC?

A. Yes

B. No

C. Maybe. It would depend on other factors.

QuickCheck


When two gases are brought into thermal contact, heat

energy is transferred from the warm gas to the cold gas

until they reach a common final temperature.

Energy could still be conserved if heat was transferred in

the opposite direction, but this never happens.

The transfer of heat energy from hot to cold is an example

of an irreversible process, a process that can happen

only in one direction.

Irreversible Processes and the Second

Law of Thermodynamics

© 2016 Pearson Education, Inc.

Which statement about

these two thermodynamic

processes is correct?

A. Both are reversible.

B. Both are irreversible.

C. The upper one is

reversible and the lower

one is irreversible.

D. The upper one is

irreversible and the lower

one is reversible.

Q20.1Metal box

at 0°C

Ice at 0°CLiquid water at

0°C

Metal box

at 0°C

Metal box

at 70°C

Ice at 0°CLiquid water at

40°C

Metal box

at 40°C

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Directions of thermodynamic processes

• The direction of a reversible process can be reversed by an

infinitesimal change in its conditions.

• The system is always in or very close to thermal equilibrium.

• A block of ice melts irreversibly when we place it in a hot

metal box.

© 2016 Pearson Education Inc.

Directions of thermodynamic processes

• A block of ice at 0°C can be melted reversibly if we put it in

a 0°C metal box.



The figure shows two boxes

containing identical balls.

Once every second, one ball

is chosen at random and

moved to the other box.

What do you expect to see

if you return several hours

later?

Although each transfer is reversible, it is more likely

that the system will evolve toward a state in which

N1 ≈ N2 than toward a state in which N1 >> N2.

The macroscopic drift toward equilibrium is irreversible.

Which Way to Equilibrium?

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Molecular Collisions Are Reversible


A Car Crash Is Irreversible


Q20.2

A. a b

B. b c

C. c a

D. two or more of A, B, and C

An ideal gas is taken around the

cycle shown in this p-V diagram,

from a to b to c and back to a.

Process b c is isothermal. Which

of the processes in this cycle could

be reversible?

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An energy reservoir is an object or a part of the

environment so large that its temperature does not

change when heat is transferred between the system

and the reservoir.

A reservoir at a

higher temperature

than the system is

called a hot reservoir.

A reservoir at a lower

temperature than the

system is called a

cold reservoir.

Energy Reservoirs

The second law of thermodynamics

• The second law of thermodynamics can be stated in several

ways:

It is impossible for any system to undergo a process in

which it absorbs heat from a reservoir at a single

temperature and converts the heat completely into

mechanical work, with the system ending in the same

state in which it began.

• We will call this the “engine” statement of the second law.

It is impossible for any process to have as its sole result

the transfer of heat from a cooler to a hotter body.

• We’ll call this the “refrigerator” statement of the second law.



Energy-Transfer Diagrams

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Turning work into heat is

easy — just rub two

objects together!

Shown is the energy

transfer diagram for this

process.

The conversion of work into heat is 100% efficient, in

that all the energy supplied to the system as work is

ultimately transferred to the environment as heat.

Work into Heat


It is impossible to invent a

“perfect engine” that transforms

heat into work with 100%

efficiency and returns to its initial

state so that it can continue to

do work as long as there is fuel.

The second law of

thermodynamics forbids a

“perfect engine.”

Transforming heat into work is not easy.

To be practical, a device that transforms heat into work

must return to its initial state at the end of the process

and be ready for continued use.

Heat into Work

Heat engines

• A heat engine is any device that

partly transforms heat into work

or mechanical energy.

• All motorized vehicles other than

purely electric vehicles use heat

engines for propulsion.

• (Hybrid vehicles use their

internal-combustion engine to

help charge the batteries for the

electric motor.)


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In a steam turbine of a

modern power plant,

expanding steam does work

by spinning the turbine.

The steam is then

condensed to liquid water

and pumped back to the

boiler to start the process

again.

First heat is transferred to the water in the boiler to

create steam, and later heat is transferred out of the

water to an external cold reservoir, in the condenser.

Heat Engines

Heat engines

• Simple heat engines operate

on a cyclic process during

which they absorb heat QH

from a hot reservoir and

discard some heat QC to a

cold reservoir.

• Shown is a schematic

energy-flow diagram for a

heat engine.



We can measure the

performance of a

heat engine in terms

of its thermal

efficiency e defined

as

Actual car engines and steam generators have

e ≈ 0.1 – 0.5.

Heat Engines

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The efficiency of this heat

engine is

A. 0.60

B. 0.50

C. 0.40

D. 0.20

QuickCheck


Q-RT20.1

A. An engine that in one cycle absorbs 2500 J of heat and

rejects 2250 J of heat

B. An engine that in one cycle absorbs 50,000 J of heat and

does 4000 J of work

C. An engine that in one cycle does 800 J of work and

rejects 5600 J of heat

Rank the following heat engines in order from highest to

lowest thermal efficiency.


A Heat-Engine Example: Slide 1 of 3

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Shown is the heat-

engine process on

a pV diagram.

No work is done

during the isochoric

step 2 → 3.

The net work per

cycle is

Wnet = Wlift – Wext = (Ws)1→2 + (Ws)3→1



Some heat engines use an ideal gas as the working

substance.

A gas heat engine can be represented by a closed-cycle

trajectory on a pV diagram.

The net work done during a full cycle is

Ideal-Gas Heat Engines

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How much work is done in

one cycle?

A. 6000 J

B. 3000 J

C. 2000 J

D. 1000 J

QuickCheck


How much heat is

exhausted to the

cold reservoir?

A. 7000 J

B. 5000 J

C. 3000 J

D. 2000 J

QuickCheck


Which heat engine has the larger efficiency?

QuickCheck

A. Engine 1

B. Engine 2

C. They have the same efficiency.

D. Can’t tell without knowing the number of moles of gas.

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Example 1 - The pV diagram in the figure shows a cycle of a heat engine

that uses 0.250 mole of an ideal gas having γ=1.40. The curved part ab

of the cycle is adiabatic.

(a) Find the pressure of the gas at point a.

(b) How much heat enters this gas per cycle? When?

(c) How much heat leaves this gas in a cycle? When?

(d) How much work does this engine do in a cycle?

(e) What is the thermal efficiency of the engine?


In-class Activity #1 – A gasoline truck engine takes in 10,000 J and

delivers 2000 J of mechanical work per cycle. The heat is obtained by

burning gasoline with heat of combustion Lc = 5.0 x 104 J/g.

(a) What is the thermal efficiency of this engine?

(b) How much heat is discarded in each cycle?

(c) If the engine goes through 25 cycles per second,

what is its power output in watts?

(d) How much gasoline is burned in each cycle?

Internal-combustion engines


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Internal-combustion engines


Refrigerators

• A refrigerator takes heat

from a cold place (inside

the refrigerator) and gives it

off to a warmer place (the

room). An input of

mechanical work is

required to do this.

• A refrigerator is essentially

a heat engine operating in

reverse.

• Shown is an energy-flow

diagram of a refrigerator.


QH = QC +Win

Refrigerators: Coefficient of performance

• From an economic point of view, the best refrigeration cycle

is one that removes the greatest amount of heat from the

inside of the refrigerator for the least expenditure of

mechanical work.

• The relevant ratio is therefore |QC|/|W|; the larger this ratio,

the better the refrigerator.

• We call this ratio the coefficient of performance, K:


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Principle of the mechanical refrigeration cycle



The coefficient of performance

of this refrigerator is

A. 0.40

B. 1.50

C. 1.67

D. 2.00

QuickCheck


Example 2 - A freezer has a coefficient of performance of 2.40. The

freezer is to convert 1.80 kg of water at 25.0ºC to 1.80 kg of ice at -5.0ºC

in one hour.

(a) What amount of heat must be removed from the water at

25.0ºC to convert it to ice at - 5.0ºC?

(b) How much electrical energy is consumed by the freezer

during this hour?

(c) How much wasted heat is rejected to the room in which the

freezer sits?

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• The second law of thermodynamics can be stated in several

ways:

It is impossible for any system to undergo a process in

which it absorbs heat from a reservoir at a single

temperature and converts the heat completely into

mechanical work, with the system ending in the same

state in which it began.

• We will call this the “engine” statement of the second law.

It is impossible for any process to have as its sole result

the transfer of heat from a cooler to a hotter body.

• We’ll call this the “refrigerator” statement of the second law.



• If a workless refrigerator were possible, it could be used in

conjunction with an ordinary heat engine to form a

100%-efficient engine, converting heat QH − |QC| completely

to work.



• If a 100%-efficient engine were possible, it could be used in

conjunction with an ordinary refrigerator to form a workless

refrigerator, transferring heat QC from the cold to the hot

reservoir with no input of work.


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In-class Activity #2 - A refrigerator has a coefficient of

performance of 2.10. Each cycle it absorbs 3.42×104 J of

heat from the cold reservoir.

(a) How much mechanical energy is required each

cycle to operate the refrigerator?

(b) During each cycle, how much heat is discarded

to the high-temperature reservoir?

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