Lecture 15
Fluids in Motion
(Chapter 14)
Temperature & Heat
(Chapter 15)
Midterm Exam 1 on Mon. October 4
(Chapters 2-8,10-14; Lectures 1-15)
28-Sep-10
Midterm #1 - Monday Oct. 4
Chap. 2: NEWTON'S FIRST LAW OF MOTION: All sections
except “The Moving Earth”
Chap. 3: LINEAR MOTION: All sections
Chap. 4: NEWTON'S SECOND LAW OF MOTION: All
Chap. 5: NEWTON'S THIRD LAW OF MOTION: All sections
Chap. 6: MOMENTUM: All but “More Complicated Collisions”
Chap. 7: ENERGY: All except “Sources of Energy”
Chap. 8: ROTATIONAL MOTION: All but “Simulated Gravity”
Chap.10: PROJECTILE AND SATELLITE MOTION: only
“Projectile Motion”, “Fast-Moving Projectiles—Satellites”
Chap.11: ATOMIC STRUCTURE: All but “Antimatter” and
“Dark Matter”
Chap.12: SOLIDS: “Density” only
Chap.13: LIQUIDS: All but “Surface Tension” & “Capillarity”
Chap.14: GASES: All except Plasma
Flow Rate and the Equation of Continuity
If the density doesn’t change – typical for
liquids – this simplifies to
.
Where the pipe is wider, the flow is slower.
Equation of Continuity
for Liquids.
Bernoulli’s Principle
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Still Air
A
Wind
Where the speed of a fluid
increases the pressure in the
fluid decreases.
This phenomenon is due to
energy conservation; when
fluid’s kinetic energy increases
(velocity increases) its internal
potential energy (pressure)
decreases.
L
Water Pipe
Water is flowing continuously in the pipe shown below. The velocity
of the water is greatest at B.
Where is the pressure in the water greatest?
A)
B)
C)
A
D) equal everywhere
C
B
Demo: Blow It Up
Hold a sheet of paper in front of your mouth
and blow; the paper will rise.
L
A
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Check Yourself
Wind blowing over the ocean causes waves to
build due to Bernoulli’s principle.
Where is the pressure lowered?
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Blow the Roof
If wind blows hard
enough the low
pressure above
can create a large
enough force to lift
the roof off.
L
A
New Orleans Superdome after hurricane Katrina
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Demo: Blow It Off
Bend cardboard into a U-shape. Place on
table, legs down, and try to blow it off.
Side view
Fast moving air in the
channel between the
card and the table
creates a low pressure
region, pressing the
card downward.
A
Front
view
L
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Demo: Blow the Funnel
Blow hard through a
funnel with a ping
pong ball in the
funnel’s bowl.
BLOW
Ping
Pong
Ball
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Airplane Wing
Pressure difference created by Bernoulli
effect creates upward lift.
LIFT FORCE
L
Wing
A
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Applications of Bernoulli’s Principle:
Baseball
A ball’s path will curve due
to its spin, which results in
the air speeds on the two
sides of the ball not being
equal.
Demo: Throwing You a Curve
Force on a spinning ball is perpendicular to the
forward motion and towards the sided spinning
with the flow of air around the ball.
Spin
No Spin
L
L
Bernoulli
Forward Motion
H
L
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Demo: Keep It Up
Objects in a moving steam of fluid are pulled to the
center of the stream because pressure is lower
inside the stream than outside.
L
L
A
A
L
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A
A
A
A
L
Temperature (T)
• Temperature (T) is a measure of how “hot” or
“cold” something is
• Temperature is a measure of the random kinetic
energy of each particle in an object.
– The greater the motion/vibration the greater the T
– The smaller the motion/vibration the lower the T
• SI Unit: kelvin (K)
– E.g., room temperature is about 295K
• Kelvin is the natural temperature scale
– 0 K is lowest possible temperature
– No negative temperatures
– Random internal KE is zero at T = 0 K
Other Temperature Scales
The Celsius scale:
Water freezes at 0° Celsius.
Water boils at 100° Celsius.
The Fahrenheit scale:
Water freezes at 32° Fahrenheit .
Water boils at 212° Fahrenheit .
Kelvin Temperature Scale
The Kelvin scale has the same step size (size of
one degree) as the Celsius scale, but the Kelvin
scale has its zero at absolute zero.
Conversion between a Celsius temperature and
a Kelvin temperature:
Temperature Scales Compared
Random kinetic energies of atoms at different
temperatures.
Low T liquid.
High T liquid.
Thermometers
Thermometers are instruments designed to
measure temperature. In order to do this,
they take advantage of some property of
matter that changes with temperature.
• Length of a solid or liquid column
• Volume of a solid, liquid, or gas
• Electromagnetic waves (infrared light) given off
by hot objects
Common thermometers used today include the
liquid-in-glass type and the bimetallic strip.
“Thermoscan” Ear
Thermometer - Reads
infrared light emission
from body.
Internal Energy
Internal energy of an
object depends on:
• Temperature
• Mass
• Material
Water
1 kg
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Temperature
Internal Kin.
Energy
300 K
120,000 J
1 kg
200 K
80,000 J
1000
grams
100 K
40,000 J
0K
0 Joules
Temperature
Internal Kin.
Energy
300 K
120 J
200 K
80 J
Iron
Temperature
Internal Kin.
Energy
300 K
1,200,000 J
200 K (ice)
800,000 J
100 K (ice)
400,000 J
100 K
40 J
0K
0 Joules
0K
0 Joules
(ice)
Iron
1 gram
Increasing Internal Energy
Can increase internal energy (and temperature) of
object by tapping energy sources.
Chemical energy
released in fire
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Electric energy
heats burner
Heat (Q)
Definition of heat:
Heat is the energy transferred between objects
because of a temperature difference.
Can think of heat as “disordered work.”
Random KE in one object transfers to random
KE in another by collisions and other methods.
Objects are in thermal contact if heat can flow
between them.
When the transfer of heat between objects in
thermal contact ceases, they are in thermal
equilibrium. (The objects will then be at the
same temperature.)
Units for Heat
• Since heat is just a flow of energy, the SI unit is
the energy unit, the joule (J).
• Other heat units
– calorie (cal): Heat needed to raise temperature of 1
gram of water by 1°C (or 1 K)
– Calorie (Cal or kcal): Heat needed to raise temperature
of 1 kg of water by 1°C (or 1 K)
– Calorie also used to measure energy content of food
• Conversions: 1 cal = 4.186 J
1 kcal = 1 Cal (food Cal.) = 4.186 kJ
• These factors called “mechanical equivalent of
heat”
Work and Heat
May also increase object’s internal energy by
exerting force to do mechanical work on object .
Rub hands together for warmth
Strike an iron surface with
great force and red-hot sparks
are created
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Demo: Work and Heat
Increase internal energy (and thus temperature) by doing
mechanical work on an object.
Bottle of Mercury
Temperature
increases by a
few degrees
SHAKE
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Specific Heat Capacity
Specific heat capacity is
the amount of heat
energy required to raise
the temperature of one
unit mass of a material
by one degree.
Filling and crust at same
temperature yet mouth
burned only by the filling.
SI Unit: J/(kg•K)
J/(kg•°C)
Other Units:
cal/(g •°C)
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Some specific
heat capacity
values.
Specific heats
of gases are
complicated.
Filling has high specific heat capacity
Crust has low specific heat capacity
Check Yourself
Why does a piece of watermelon stay cool for a
longer time than sandwiches do when both are
removed from a cooler on a hot day?
Why is it that the climate in the desert is so hot
during the day yet so cold at night?
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Key Points of Lecture 15
• Fluid in Motion - Continuity
• Bernoulli’s Principle
•-------------------------------------------------------• Temperature
• Internal Energy
• Specific Heat Capacity
z Before Monday, read Hewitt Chap. 14, last part.
z Before next Wed., read Hewitt Chap. 15.
z Homework Assignment #11 is due before 11:00 PM on
Friday, Oct. 1.