quickcheck - avconline.avc.edu
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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
© 2017 Pearson Education, Inc. Slide 20-2
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.
© 2016 Pearson Education Inc.
© 2017 Pearson Education, Inc. Slide 20-6
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?
3
© 2017 Pearson Education, Inc. Slide 20-7
Molecular Collisions Are Reversible
© 2017 Pearson Education, Inc. Slide 20-8
A Car Crash Is Irreversible
© 2016 Pearson Education, Inc.
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?
4
© 2017 Pearson Education, Inc. Slide 20-10
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.
© 2016 Pearson Education Inc.
© 2017 Pearson Education, Inc. Slide 20-12
Energy-Transfer Diagrams
5
© 2017 Pearson Education, Inc. Slide 20-13
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
© 2017 Pearson Education, Inc. Slide 20-14
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.)
© 2016 Pearson Education Inc.
6
© 2017 Pearson Education, Inc. Slide 20-16
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.
© 2016 Pearson Education Inc.
© 2017 Pearson Education, Inc. Slide 20-18
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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© 2017 Pearson Education, Inc. Slide 20-19
The efficiency of this heat
engine is
A. 0.60
B. 0.50
C. 0.40
D. 0.20
QuickCheck
© 2016 Pearson Education, Inc.
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.
© 2017 Pearson Education, Inc. Slide 20-21
A Heat-Engine Example: Slide 1 of 3
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© 2017 Pearson Education, Inc. Slide 20-22
A Heat-Engine Example: Slide 2 of 3
© 2017 Pearson Education, Inc. Slide 20-23
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
A Heat-Engine Example: Slide 3 of 3
© 2017 Pearson Education, Inc. Slide 20-24
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
9
© 2017 Pearson Education, Inc. Slide 20-25
How much work is done in
one cycle?
A. 6000 J
B. 3000 J
C. 2000 J
D. 1000 J
QuickCheck
© 2017 Pearson Education, Inc. Slide 20-26
How much heat is
exhausted to the
cold reservoir?
A. 7000 J
B. 5000 J
C. 3000 J
D. 2000 J
QuickCheck
© 2017 Pearson Education, Inc. Slide 20-27
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.
10
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?
© 2016 Pearson Education, Inc.
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
© 2016 Pearson Education Inc.
11
Internal-combustion engines
© 2016 Pearson Education Inc.
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.
© 2016 Pearson Education Inc.
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:
© 2016 Pearson Education Inc.
12
Principle of the mechanical refrigeration cycle
© 2016 Pearson Education Inc.
© 2017 Pearson Education, Inc. Slide 20-35
The coefficient of performance
of this refrigerator is
A. 0.40
B. 1.50
C. 1.67
D. 2.00
QuickCheck
© 2016 Pearson Education, Inc.
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?
13
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.
© 2016 Pearson Education Inc.
The second law of thermodynamics
• 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.
© 2016 Pearson Education Inc.
The second law of thermodynamics
• 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.
© 2016 Pearson Education Inc.
14
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?