Download Introduction to Modern Physics PHYX 2710

Physics of Technology
PHYS 1800
Lecture 27
Introduction
Review for Test 3
Section 0
Lecture 1
Slide 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 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
Review for Test 3
Lecture 27 Slide 2
Notes on Test
1. Covers Chapters 9-11
2. ~8 short answer problems or questions (5 point
each)
3. 3 Numerical problems based heavily on the
material from the homework and Lab/Demo
sessions (20 points each). One problem each from
Chapters 9, 10 and 11.
4. You will have a formula sheet just like the one in
Introduction
Section 0 Lecture 1 Slide 3
the handout.
5. Test is Thursday March 26 1:30-2:45 in ESLC 46.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 3
Physics of Technology
PHYS 1800
Lecture 27
Review for Test 3
Introduction
Section 0
Lecture 1
Slide 4
Introduction and Review
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 4
What Do We Need To Measure?
• What is the minimum about things we need to know?
• Where things are—a length, L
• When things are there—a time, t
• How thing interact with gravity—a mass, M
• How things interact with E&M—a charge, Q
• How thing inter act with weak nuclear force
Introduction
Section with
0 Lecture
1 Slide
5
• How things
interact
strong
nuclear
force
• Random collections of objects—a temperature, T
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 5
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 6
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)
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 6
Newton’s Laws in Review
•
1st Law —a special case of the 2nd Law for statics,
with a=0 or Fnet=0
• An objects velocity remains unchanged, unless
a force acts on the object.
•
2nd Law (and 1st Law)—How motion of a object is
effected by a force.
– The acceleration of an object is directly
proportional to the magnitude of the imposed
force and inversely proportional to the mass of
the object. The acceleration is the same
direction as that of the imposed force.
F  ma
units : 1 newton = 1 N = 1 kg  m s2
•
Introduction
Section 0
Lecture 1
Slide 7
3rd Law —Forces come from interactions with
other objects.
• 
For every action (force), there is an equal but
opposite reaction (force).
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 7
Conservation of Energy
Energy 
Energy: The potential to do work.
Conservation of Energy: The total
energy of a closed system remains
constant.
– Energy can be converted from one
form to another.
– Not all forms of energy can be fully
recovered.
Introduction
Section 0
Lecture 1
Slide 8
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Time 
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 8
Momentum and Impulse
• Multiply both sides of Newton’s second law
by the time interval over which the force acts:
• The left side of the equation is impulse, the
(average) force acting on an object multiplied
by the time interval over which the force acts.
 v 
Fnet  ma  m

 t 
Fnet t  mv
• How a force changes the motion of an object
depends on both the size of the force and how
long the force acts.
• The right side of the equation is the change
in the momentum of the object.
Introduction
Section 0
Lecture 1
Slide 9
• The momentum of the object is the mass of
the object times its velocity.
p  mv
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009

Review for Test 3
Lecture 27 Slide 9
Impulse-Momentum Principle
The impulse acting on an object produces a change
in momentum of the object that is equal in both
magnitude and direction to the impulse.
impulse = change in momentum
= p
In analogy,
Introduction
Section 0

Lecture 1
work
INTRODUCTION TO Modern Physics PHYX 2710
Slide 10
= change in energy
= ΔE
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 10
Formulas We Know and Love
Formulas as They Will Appear on the Test Sheet
d  vo t  a t
v
a
d
1
2
f
  o t   t
2
 do 

t
v f  vo 

t

1
2
f
2
 o 
ma mb
r2
Ra3 Rb3 Gma


Ta2 Tb2 4 2
G=6.67·10-11 N-m2/kg2
g=9.8 m/s2
PE gravity  mgh
Fgravity  G
t
 f  o 
t
F  ma
W  F d
 net  I 
  Fl
KEtrans  12 mv 2
KErot  12 I 2
p  F t
L   t
Introduction Section 0 Lecture 1 Slide
p  mv
L  I
W
I  mr 2
Power 
INTRODUCTION
TO Modern Physics PHYX 2710
t
v2
ac 
r
11
Felastic  k x
PEelastic  12 k x 2
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 11
Physics of Technology
PHYS 1800
Lecture 27
Introduction
Review for Test 3
Section 0
Lecture 1
Slide 12
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 12
Test 3 Review Concepts
Concepts and Terms to Be Familiar With
Know what pressure and density are and how this relates to fluids.
Know Pascal’s Principle and how to apply it to hydraulics problems.
Know how buoyant force is related to pressure and Archimedes’ Principle.
Know what an ideal gas is and what the ideal gas law says about pressure volume
and temperature of an ideal gas.
Understand how conservation of mass is related to flow rate.
Understand the difference between laminar and turbulent flow.
Understand Bernoulli’s Principle as a fluid form of the conservation of energy.
Be able to state the four laws of thermodynamics.
Be able to define heat and temperature and explain how they are different.
Understand heat capacity, heat of fusion (melting), and heat of vaporization (boiling).
Be able to do simple calorimitry problems.
Be able to qualitatively explain the difference between the three forms of heat
Introduction Section 0 Lecture 1 Slide 13
transfer: conduction, convection and radiation.
Be able to explain what a heat engine is and what the components of work, high
INTRODUCTION TO Modern Physics PHYX 2710
temperature reservoir
and low temperature reservoir.
Fall 2004
What is efficiency of a heat engine? Of a Carnot engine?
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 13
Formulas We Know and Love
New Formulas as They Will Appear on the Test Sheet
PF/A
kB=1.38 10-23 J/K
flow rate  v A
  m /V
PV  Nk BT
P  12  v 2   g h  constant
W  P V
Q  m c T
e  W / QH 
Introduction
Section 0
Lecture 1
QH  QC
QH
eCarnot 
TH  TC
TH
Slide 14
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 14
Physics of Technology
PHYS 1800
Lecture 27
Review for Test 3
Introduction
Section 0
Lecture 1
Slide 15
Fluids and Pressure
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 15
Test 3 Review Concepts
Fluids and Pressure
Concepts and Terms to Be Familiar With
Know what pressure and density are and how this relates to fluids.
Know Pascal’s Principle and how to apply it to hydraulics problems.
Know how buoyant force is related to pressure and Archimedes’ Principle.
Know what an ideal gas is and what the ideal gas law says about pressure
volume and temperature of an ideal gas.
Understand how conservation of mass is related to flow rate.
Introduction
Section 0 Lecture
1 Slide
16
Understand
the difference
between
laminar
and turbulent flow.
Understand Bernoulli’s Principle as a fluid form of the conservation of
energy.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 16
States of Matter
Introduction
Section 0
Lecture 1
Slide 17
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 17
Pressure
• The man weighs more, so he exerts
a larger force on the ground.
• The woman weighs less, but the
force she exerts on the ground is
spread over a much smaller area.
• Pressure takes into account both
force and the area over which the
force is applied.
– Pressure is the ratio of the force to
the area over which it is applied:
– Units: 1 N/m2 = 1 Pa (pascal)
– Pressure is the quantity that
determines whether the soil will
yield.
Introduction Section 0 Lecture 1 Slide
INTRODUCTION TO Modern Physics PHYX 2710
F
P
A
18
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 18
Dennison’s Laws of Fluids
• When push comes to shove, fluids are just like
other stuff.
• Pascal’s Principle: Pressure extends uniformly in all
directions in a fluid.
• Boyle’s Law: Work on a fluid equals PΔV
• Bernoulli’s Principle: Conservation of energy for fluids
Introduction
Section 0
Lecture 1
Slide 19
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 19
Pascal’s Principle
• Fluid pushes outward uniformly in all
directions when compressed.
• Any increase in pressure is transmitted
uniformly throughout the fluid.
• Pressure exerted on a piston extends
uniformly throughout the fluid, causing
it to push outward with equal force per
unit area on the walls and the bottom of
the cylinder.
• This is the basis of Pascal’s Principle:
– Any change in the pressure of a
fluid is transmitted uniformly in all
directions
throughout
fluid.
Introduction Section
0 Lecture 1the
Slide
20
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 20
•
•
•
•
•
•
•
Gas molecules lack strong interactions.
Pressure is understood as resulting from
momentum transfer to the container walls
through unbalanced collisions
Pressing on one surface adds force and
hence imparts impulse to the gas
That impulse is taken up as added collisons
(pressure) on other surfaces
The random nature of the motion of gas
particles assures that the force is distributed
evenly to all surfaces
For fixed walls, a decrease in V results in an
increase in P
For expandable walls (like a balloon) the
volume “appears elsewhere to make up for
the lost volume
Introduction
Section 0
Lecture 1
Pascal’s Principle for
Gases
Slide 21
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 21
•
•
•
•
+
Section 0 + Lecture 1
+
+
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
+
Physics of Technology—PHYS 1800
Spring 2009
Pascal’s Principle
for Liquids
+ + + + + +
+ + + + + +
Introduction
+
+ + + + + +
•
Liquid molecules have strong interactions.
Liquids do not compress much
Pressure is understood as resulting from
momentum transfer to the container walls
through unbalanced spring forces
Pressing on one surface adds force that is
transferred to other springs
The network nature of the forces on the particles
assures that the force is distributed evenly to all
surfaces
For expandable walls (like a balloon) the volume
“appears elsewhere to make up for the lost
volume
For fixed walls, a small decrease in V (a
compression) results in a large increase in P
For solids, you can think of the strong forces
holding the atoms in there equilibrium positions,
equivalent to fixed walls
+ + + + + +
•
•
•
+
+
Slide
+ 22
+
+
+
Review for Test 3
Lecture 27 Slide 22
Archimedes’ Principle
• The average density of an object compared to a fluid determines
whether the object will sink or float in that liquid.
• The upward force that pushes objects back toward the surface in
liquids is called the buoyant force.
• Archimedes’ Principle: The buoyant force acting on an object
fully or partially submerged in a fluid is equal to the weight of the
fluid displaced by the object.
Introduction
Section 0
Lecture 1
Slide 23
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 23
Archimedes’ Principle
•
For example, consider a block submerged in water, suspended from a string.
– The pressure of the water pushes on the block from all sides.
– Because the pressure increases with depth, the pressure at the bottom of the block
is greater than at the top.
– There is a larger force (F = PA) pushing up at the bottom than there is pushing
down at the top.
– The difference between these two forces is the buoyant force.
The buoyant force is proportional to both
the height and the cross-sectional area of
the block, and thus to its volume.
Introduction
Section 0
The volume of the fluid displaced is
directly related to the weight of the fluid
displaced.
Lecture
1 Slide 24
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Weight  mg  Vdg
Volume  Ah
Excess Pressure P 
W dgAh

 dgh
A
a
Review for Test 3
Lecture 27 Slide 24
Flow Rate
• The volume of a portion of water of length L flowing past
some point in a pipe is the product of the length times
the cross-sectional area A, or LA.
• The rate at which water moves through the pipe is this
volume divided by time: LA / t.
• Since L / t = v, the rate of flow = vA.
Introduction
Section 0
Lecture 1
Slide 25
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 25
Laminar vs Turbulent Flow
• Laminar flow is smooth flow, with no eddies or other disturbances.
– The streamlines are roughly parallel.
– The speeds of different layers may vary, but one layer moves smoothly past
another.
• Turbulent flow does have eddies and whorls; the streamlines are no
longer parallel.
Introduction
Section 0
Lecture 1
Slide 26
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 26
Bernoulli’s Principle
• How does a large passenger jet
manage to get off the ground?
• What forces keep it in the air?
• How is a ball suspended in mid-air by
a leaf blower?
• What happens if we do work on a
fluid?
Work  KE  PE  E
 constant
Total
• Bernoulli’s principle applies
1 2
conservation of energy to the flow of
PV

mv  mgh  E  constant
fluids:
2
• The sum of the pressure plus the
•
kinetic energy per unit volume of
1 2
0 Lecture
1 Slide 27
•
aIntroduction
flowing Section
fluid must
remain
P  v  gh  E/V  constant
2
constant.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 27
Pressure Changes
with Area
How does pressure
vary in pipes and
hoses?
• Will the pressure be greatest in the narrow section or the
wide section?
• The speed will be greater in the narrow section.
• To keep the sum P + 1/2 dv2 constant, the pressure must be
larger where the fluid speed is smaller (h is fixed).
• If the speed increases, the pressure decreases. (This goes
Introduction
0 Lecture 1 Slide 28
against
ourSection
intuition.)
• This can be shown using vertical open pipes as pressure
gauges.
• The height of the column of water is proportional to the
pressure.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 28
Physics of Technology
PHYS 1800
Lecture 27
Introduction
Review for Test 3
Section 0
Lecture 1
Slide 29
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 29
Physics of Technology
PHYS 1800
Lecture 27
Review for Test 3
Introduction
Section 0
Lecture 1
Slide 30
Temperature and Heat
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 30
Test 3 Review Concepts
Temperature and Heat
Concepts and Terms to Be Familiar With
Be able to state the four laws of thermodynamics.
Be able to define heat and temperature and explain how they are different.
Understand heat capacity, heat of fusion (melting), and heat of vaporization
(boiling).
Be able to do simple calorimitry problems.
Introduction
Section 0
Lecture 1
Slide 31
Be able to qualitatively explain the difference between the three forms of
heat transfer: conduction, convection and radiation.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 31
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 32
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
Review for Test 3
Lecture 27 Slide 32
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 33
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
Review for Test 3
Lecture 27 Slide 33
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 34
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 34
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 35
W = Fd = (PA)d = PV
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 35
Ideal Gas Behavior
•
In an isothermal process, the temperature does not change.
– The internal energy must be constant.
– The change in internal energy, U, is zero.
– If an amount of heat Q is added to the gas, an equal amount of work
W will be done by the gas on its surroundings, from U = Q - W.
•
In an isobaric process, the pressure of the gas remains constant.
– The internal energy increases as the gas is heated, and so does the
temperature.
– The gas also expands, removing some of the internal energy.
– Experiments determined that the pressure, volume, and absolute
temperature of an ideal gas are related by the equation of state:
PV = NkT
Introduction
Section 0
where N is the number of molecules
k is
Lecture 1 and
Slide
36 Boltzmann’s
constant.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 36
Heat and Specific Heat Capacity
• The specific heat capacity of a material is the quantity
of heat needed to change a unit mass of the material by
a unit amount in temperature.
– For example, to change 1 gram by 1 Celsius degree.
– It is a property of the material, determined by experiment.
– The specific heat capacity of water is 1 cal/gC: it takes 1
calorie of heat to raise the temperature of 1 gram of water
by 1C.
• We can then calculate how much heat must be
absorbed by a material to change its temperature by a
given amount:
Q =Introduction
mcT
Section 0
where Lecture
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
1
Slide 37
Q = quantity of heat
m = mass
c = specific heat capacity
T = change in temperature
Review for Test 3
Lecture 27 Slide 37
If the specific heat capacity of ice is 0.5 cal/gC°, how much heat
would have to be added to 200 g of ice, initially at a temperature
of -10°C, to raise the ice to the melting point?
a)
b)
c)
d)
1,000 cal
2,000 cal
4,000 cal
0 cal
Introduction
m = 200 g
c = 0.5 cal/gC°
T = -10°C
Q = mcT
(200
g)(0.5 cal/gC°)(10°C)
Section 0 Lecture 1 =Slide
38
= 1,000 cal
(heat required to raise the temperature)
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 38
Phase Changes and Latent Heat
• When an object goes through a change of phase or state, heat is
added or removed without changing the temperature. Instead, the
state of matter changes: solid to liquid, for example.
• The amount of heat needed per unit mass to produce a phase
change is called the latent heat.
– The latent heat of fusion of water corresponds to the amount of heat
needed to melt one gram of ice.
– The latent heat of vaporization of water corresponds to the amount of
heat needed to turn one gram of water into steam.
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
+
INTRODUCTION TO Modern Physics PHYX 2710
+
Slide 39
+
+
+
Lecture 1
+
Section 0
+
Introduction
+
+
Solid
Lecture 27 Slide 39
If the specific heat capacity of ice is 0.5 cal/gC°, how much heat
would have to be added to 200 g of ice, initially at a temperature
of -10°C, to completely melt the ice?
Lf = 80 cal/g
a)
b)
c)
d)
1,000 cal
14,000 cal
16,000 cal
17,000
cal
Introduction
Section 0
Lecture 1
Q = mLf
= (200 g)(80 cal/g)
= 16,000 cal
(heat required to melt the ice)
Slide 40
Total heat required to raise the ice to 0 °C
and then to melt the ice is:
1,000 cal + 16,000 cal = 17,000 cal = 17 kcal
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 40
A hot plate is used to transfer 400
cal of heat to a beaker containing ice
and water; 500 J of work are also
done on the contents of the beaker
by stirring. What is the increase in
internal energy of the ice-water
mixture?
a)
b)
c)
d)
900 J
1180 J
1680 J
2180
J
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
W = -500 J
Q = 400 cal
= (400 cal)(4.19 J/cal)
= 1680 J
Slide 41
U = Q - W
= 1680 J - (-500 J)
= 2180 J
Review for Test 3
Lecture 27 Slide 41
A hot plate is used to transfer
400 cal of heat to a beaker
containing ice and water; 500 J
of work are also done on the
contents of the beaker by
stirring. How much ice melts in
this process?
a)
b)
c)
d)
0.037 g
0.154 g
6.5 g
27.25
g
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Lf = 80 cal/g
= (80 cal/g)(4.19 J/cal)
= 335 J/g
U
Slide
42 = mLf
m = U / Lf
= (2180 J) / (335 J/g)
= 6.5 g
Review for Test 3
Lecture 27 Slide 42
The Flow of Heat
• There are three basic processes for heat flow:
– Conduction
– Convection
– Radiation
Introduction
Section 0
Lecture 1
Slide 43
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 43
The Flow of Heat
Q
T
kA
t
L
– In conduction, heat flows
through a material when
objects at different
temperatures are placed in
contact with one another.
Introduction
Section 0
Lecture 1
Slide 44
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 44
The Flow of Heat
– In convection, heat is transferred by the motion of a
fluid containing thermal energy.
• Convection is the main method of heating a house.
• It is also the main method heat is lost from buildings.
Introduction
Section 0
Lecture 1
Slide 45
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 45
The Flow of Heat
– In radiation, heat energy is
transferred by electromagnetic
waves.
• The electromagnetic waves involved
in the transfer of heat lie primarily in
the infrared portion of the spectrum.
• Unlike conduction and convection,
which both require a medium to travel
through, radiation can take place
across a vacuum.
• For example, the evacuated space in
a thermos bottle.
• The radiation is reduced to a
minimum by silvering the facing walls
of the evacuated space.
Q
t
  A B T
Introduction
Section 0
4Lecture
1
Slide 46
 B  5.7 10 8 W / m 2 K 4
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 46
Physics of Technology
PHYS 1800
Lecture 27
Review for Test 3
Introduction
Section 0
Lecture 1
Slide 47
Heat Engines and the Second Law
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 47
Test 3 Review Concepts
Heat Engines and the Second Law
Concepts and Terms to Be Familiar With
Be able to explain what a heat engine is and what the components of
work, high temperature reservoir and low temperature reservoir.
What is efficiency of a heat engine? Of a Carnot engine?
Introduction
Section 0
Lecture 1
Slide 48
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 48
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 49
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 49
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 50
temperature lower than the
input temperature.
QH  W  QC
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 50
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 51
QH
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 51
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 52
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 52
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 53
• 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
Review for Test 3
Lecture 27 Slide 53
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
54
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
Review for Test 3
Lecture 27 Slide 54
A Third Statement of The Second Law of
Thermodynamics
• Entropy remains constant in reversible
processes but increases in irreversible
processes.
• The entropy of a system decreases only if it
interacts with some other system whose entropy
is increased in the process.
– This happens, for example, in the growth and
development of biological organisms.
• The
entropy of the universe or of an isolated
Introduction Section 0 Lecture 1 Slide 55
system can only increase or remain constant.
Its entropy can never decrease.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 55
A heat pump uses 200 J of work to remove 300 J of
heat from the lower-temperature reservoir. How
much heat would be delivered to the highertemperature reservoir?
a)
b)
c)
d)
100 J
200 J
300 J
500 J
W = 200 J
QC = 300
J Section 0 Lecture 1
Introduction
QH = W + QC
= 200 J + 300 J
= 500 J
Slide 56
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Review for Test 3
Lecture 27 Slide 56