Transcript Lecture 25 - USU Department of Physics

Physics of Technology
PHYS 1800
Lecture 25
Heat Engines and the
2nd Law of Thermodynamics
Introduction
Section 0
Lecture 1
Slide 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 1
PHYSICS OF TOF
ECHNOLOGY
- PHYS 1800
PHYSICS
TECHNOLOGY
ASSIGNMENT SHEET
Spring 2009Spring
Assignment
Sheet
2009
Date
Day
Lecture
Chapter
Feb 16
M
Presidents Day
17
Tu
Angular Momentum (Virtual Monday)
18
W
Review
19
H
Test 2
20
F*
Static Fluids, Pressure
Feb 23
M
Flotation
25
W
Fluids in Motion
27
F*
Temperature and Heat
Mar 2
M
First Law of Thermodynamics
4
W
Heat flow and Greenhouse Effect
6
F*
Climate Change
Mar 9-13
M-F
Spring Break
Mar 16
M
Heat Engines
18
W
Power and Refrigeration
20
F*
Electric Charge
Mar 23
M
Electric Fields and Electric Potential
25
W
Review
26
H
Test 3
27
F*
Electric Circuits
Mar 30
M
Magnetic Force Review
Apr 1
W
Electromagnets
3
F
Motors and Generators
Apr 6
M
Making Waves
8
W
Sound Waves
10
F*
E-M Waves, Light and Color
Apr 13
M
Mirrors and Reflections
Introduction
Section
0 Lecture 1 Slide 2
15
W
Refraction and Lenses
17
F*
Telescopes and Microscopes
Apr 20
M
Review
22
W
Seeing Atoms
24
F
The really BIG & the really small
INTRODUCTION TO Modern Physics PHYX 2710
May
1
F
Final Exam: 09:30-11:20am
No Class
8
5-8
5-8
9
9
9
10
10
10
No Classes
11
11
12
12
13
9-12
13
14
9-12
14
15
15
16
17
17
17
1-17
18 (not on test)
21 (not on test)
Homework Due
-
6
7
8
-
9
10
11
No test week
12
Fall 2004
* = Homework Handout
*Homework Handout
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 2
Physics of Technology
PHYS 1800
Lecture 25
Heat Engines and the
2nd Law of Thermodynamics
Introduction
Section 0
Lecture 1
Slide 3
Review of Thermodynamics
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 3
Describing Motion and Interactions
Position—where you are in space (L or meter)
Velocity—how fast position is changing with time (LT-1 or m/s)
Acceleration—how fast velocity is changing with time (LT-2 or m/s2)
Force— what is required to change to motion of a body (MLT-2 or kg-m/s2 or N)
Inertia (mass)— a measure of the force needed to change the motion of a body (M)
Energy—the potential for an object to do work. (ML2T-2 or kg m2/s2 or N-m or J)
Work is equal to the force applied times the distance moved. W = F d
Kinetic Energy is the energy associated with an object’s motion. KE=½ mv2
Potential Energy is the energy associated with an objects position.
Gravitational potential energy PEgravity=mgh
Spring potential energy PEapring= -kx
Momentum— the potential of an object to induce motion in another object (MLT-1 or kg-m/s)
Introduction
Section 0
Lecture 1
Slide 4
Angular Momentum and Rotational Energy— the equivalent constants of motion for rotation (MT-1 or
kg/s) and (MLT-2 or kg m/s2 or N)
Modern Physics PHYX 2710
Pressure— forceINTRODUCTION
dividedTOby
the area over which the force is applied (ML-1T-1 or kg/m-s or N/m2 or Pa)
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 4
Dennison’s Laws Thermal Poker
(or How to Get a Hot Hand in Physics)
• 0th Law: Full House beats Two Pairs
• 1st Law: We’re playing the same game (but with a wild card)
• 2nd Law: You can’t win in Vegas.
• 3rd Law: In fact, you always loose.
• 0th Law: Defines Temperature
• 1st Law: Conservation of Energy (with heat)
•
2nd
Introduction
Section 0
Lecture 1
Slide 5
Law: You can’t recover all heat losses
(or defining entropy)
INTRODUCTION TO Modern Physics PHYX 2710
•
Fall 2004
3rd
Law: You can never get to absolute 0.
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 5
Heat
• What is heat?
• What is the relationship between quantity of heat
and temperature?
• What happens to a body (solid, liquid, gas) when
thermal energy is added or removed?
Thermal Energy
Solid: Atoms vibrating in all directions about their
fixed equilibrium (lattice) positions. Atoms
constantly colliding with each other.
Liquid: Atoms still oscillating and colliding with
each other but they are free to move so that the long
range order (shape) of body is lost.
Introduction Section 0 Lecture 1 Slide 6
Gas: No equilibrium position, no oscillations, atoms
are free and move in perpetual high-speed “zig-zag”
dance punctuated by collisions.
solid
liquid
INTRODUCTION TO Modern Physics PHYX 2710
gas
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 6
Heat
k BT  12 mv 2
kB is Boltzmann’s
constant
+
+
+
+
Lecture 1
+
+
Section 0
+
Introduction
+
+
=1.38 10-23 J/K
Solid
Slide 7
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 7
Temperature and Heat
• When two objects at different temperatures are placed in
contact, heat will flow from the object with the higher
temperature to the object with the lower temperature.
• Heat added increases temperature, and heat removed
decreases temperature.
• Heat and temperature
are not the same.
• Temperature is a
quantity that tells us
which direction the
heat will flow.
Introduction
Section 0 Lecture
Heat is a form
of energy.
(Here comes
conservation of energy!!!)
1
Slide 8
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 8
Joule’s Experiment and the First Law of Thermodynamics
• Joule’s experiments led to Kelvin’s statement of the first law of
thermodynamics.
– Both work and heat represent transfers of energy into or out of a
system.
– If energy is added to a system either as work or heat, the internal
energy of the system increases accordingly.
• The increase in the internal
energy of a system is equal
to the amount of heat added
to a system minus the
amount of work done by the
system.
U = Q - W
Introduction
Section 0
Lecture 1
Slide 9
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 9
Gas Behavior and The First Law
Consider a gas in a cylinder with a movable piston.
If the piston is pushed inward by an external force, work is done on
the gas, adding energy to the system.
• The force exerted on the
piston by the gas equals the
pressure of the gas times the
area of the piston:
F = PA
• The work done equals the
force exerted by the piston
times theIntroduction
distance
the
Section
0 piston
Lecture 1
moves:
Slide 10
W = Fd = (PA)d = PV
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 10
Physics of Technology
PHYS 1800
Lecture 25
Heat Engines and the
2nd Law of Thermodynamics
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Slide 11
Heat Engines
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 11
Heat Engines
What is a heat engine?
It is a device that uses input heat
to generate useful work.
e
From the 1st Law (Conservation
of Energy)
W
QH

W  Qnet  U
In cyclic engines we return to the
original state every cycle so

U 0
Introduction Section 0
and
W  QH  QC
Lecture 1
Slide 12
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 12
Heat Engines
What is a heat engine?
All heat engines share these
main features of
operation:
– Thermal energy (heat) is
introduced into the engine.
– Some of this energy is
converted to mechanical
work.
– Some heat (waste heat) is
released into the
environment
Introduction Sectionat
0 aLecture 1 Slide 13
temperature lower than the
input temperature.
QH  W  QC
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 13
Efficiency
Efficiency is the ratio of the
net work done by the
engine to the amount of
heat that must be
supplied to accomplish
this work.
W
e
QH
Or from the 1st Law
QH Section
 Q0C
Introduction
e
Lecture 1
Slide 14
QH
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 14
A heat engine takes in 1200 J of heat from the hightemperature heat source in each cycle, and does 400 J
of work in each cycle. What is the efficiency of this
engine?
a)
b)
c)
33%
40%
66%
QH = 1200 J
W = 400 J
e = W /Introduction
QH
Section 0 Lecture 1
= (400 J) / (1200 J)
= 1/3 = 0.33
= 33%
Slide 15
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 15
How much heat is released into the environment
in each cycle?
a)
b)
c)
d)
33 J
400 J
800 J
1200 J
QC = QHIntroduction
-W
Section 0 Lecture
= 1200 J - 400 J
= 800 J
1
Slide 16
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 16
Carnot Engine and Carnot Cycle
• Carnot considered the ideal (most efficient
possible) engine for a give TH and TC.
• Carnot engine has negligible work lost to friction,
turbulence, heat loss, etc.
• Carnot also reasoned that the processes should
occur without undue turbulence.
– The engine is completely reversible: it can be turned
around and run the other way at any point in the cycle,
because it is always near equilibrium.
– This is Carnot’s ideal engine.
• The cycle devised by Carnot that an ideal engine
would have to follow is called a Carnot cycle.
Lecture 1 Slide 17
• AnIntroduction
(ideal,Section
not0 real)
engine following this cycle is
called a Carnot engine.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 17
Carnot Efficiency
• The efficiency of Carnot’s ideal engine (one using an
ideal gas with PV=NkBT) is called the Carnot
efficiency and is given by:
QH  QC
TH  TC
eC 

QH
TH
• This is the maximum efficiency possible for any
engine taking in heat from a reservoir at absolute
temperature TH and releasing heat to a reservoir at
temperature TC.
Introduction Section 0 Lecture 1 Slide 18
• This provides
a useful limiting case.
• Even Carnot’s ideal engine is less than 100% efficient.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 18
Carnot Cycle
1.
2.
3.
4.
Heat flows into cylinder at temperature TH. The fluid
expands isothermally and does work on the piston.
The fluid continues to expand adiabatically (without heat
loss).
Work is done by the piston on the fluid, which undergoes
an isothermal compression.
The fluid returns to its initial condition by an adiabatic
compression.
Introduction
Section 0
Lecture 1
Slide 19
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 19
A steam turbine takes in steam at a temperature of 400C and
releases steam to the condenser at a temperature of 120C.
What is the Carnot efficiency for this engine?
a)
b)
c)
d)
30%
41.6%
58.4%
70%
TH = 400C = 673 K
TC = 120C = 393 K
eC = (TH -Introduction
TC ) / THSection 0 Lecture 1 Slide
= (673 K - 393 K) / (673 K)
= 280 K / 673 K
= 0.416 = 41.6%
20
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 20
If the turbine takes in 500 kJ of heat in each cycle,
what is the maximum amount of work that could be
generated by the turbine in each cycle?
a)
b)
c)
d)
0.83 J
16.64 kJ
28 kJ
208 kJ
QH = 500 kJ
e = W / QH
,
Introduction Section 0 Lecture
so W = e QH
= (0.416)(500 kJ)
= 208 kJ
1
Slide 21
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 21
Physics of Technology
PHYS 1800
Lecture 25
Heat Engines and the
2nd Law of Thermodynamics
Introduction
Section 0
Lecture 1
Slide 22
Second Law of Thermodynamics
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 22
Second Law of Thermodynamics
Heat (random motion) is a special form of energy that cannot be fully
(with complete efficiency) transformed to other forms of energy.
This leads to various forms of the Second Law of Thermodynamics.
•
You can’t recover all heat losses .
•
You can’t win in Vegas.
•
No engine, working in a continuous cycle, can take heat from a
reservoir at a single temperature and convert that heat
completely to work.
•
Therefore, no engine can have a greater efficiency than a Carnot
Introduction
Section 0between
Lecture 1 the
Slide
23
engine
operating
same
two temperatures.
•
Define entropy (something that measures randomness or
disorder in an object) to take account of this.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 23
Second Law of Thermodynamics
• An engine with an efficiency
greater than the Carnot
engine would produce a
greater amount of work than
the Carnot engine, for the
same amount of heat input
QH.
• Some of this work could be
used to run the Carnot
engine in reverse, returning
the heat released by the first
engine to the highertemperature
reservoir.
Introduction Section 0 Lecture 1 Slide
24
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 24
Second Law of Thermodynamics
• The remaining work Wexcess
would be available for external
use, and no heat would end up
in the lower-temperature
reservoir.
• The two engines would take a
small quantity of heat from the
higher-temperature reservoir
and convert it completely to
work.
• This would violate the second
law of thermodynamics.
Introduction
Section 0
Lecture 1
Slide 25
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 25
Physics of Technology
Next Lab/Demo:
Fluid Dynamics
Temperature
Thursday 1:30-2:45
ESLC 46
Ch 9 and 10
Next Class:
Wednesday 10:30-11:20
BUS
Slide 26318 room
Review Ch 10
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Heat Engines
Lecture 25 Slide 26