Transcript HNRS 227 Lecture #2 Chapters 2 and 3

HNRS 227 Lecture 5 and 6
Chapter 4 and Chapter 5
Heat and Temperature
Wave Motion and Sound
presented by Prof. Geller
Recall from Chapters 1, 2, 3
 Units of length, mass and time, and metric Prefixes
 Density and its units
 The Scientific Method
 Speed, velocity, acceleration
 Force
 Falling objects
 Newton’s Laws of Motion
 Newton’s Law of Universal Gravity
 Work
 Potential Energy and Kinetic Energy
 Conversion of Energy
 Types/Sources of Energy
Main Concepts for
Chapters 4 and 5
Kinetic Molecular Theory
Temperature
Heat
Phases of matter
Thermodynamics
Forces, Vibrations and Wave Motion
Sound Waves and their characteristics
Reflection, Refraction, Resonance
1st Law of Thermodynamics
In an isolated system, the
total amount of energy,
including heat energy, is
conserved.
ENERGY IS CONSERVED
Temperature
A relative term reflecting how
vigorously atoms in a substance are
moving and colliding
Alternative definition
the average kinetic energy of the
molecules in a region
Temperature
Units
Fahrenheit
Celsius
Kelvin
Where is absolute zero?
At what temperature does water
freeze? Boil?
Heat
Heat is a form of energy
the energy flows from a warmer object to a
cooler object
Units of heat
calorie
amount of temperature needed to raise
temperature of 1 gram of water 1 degree Celsius
kilocalorie (kcal or Calorie)
amount of temperature needed to raise
temperature of 1 kg of water 1 degree Celsius
Specific Heat
Ability of a material to absorb heat energy
proportional to mass
proportional to change in temperature
Q = m*c*DT
c is the SPECIFIC HEAT of the substance
amount of energy needed to increase
temperature of 1 gram of substance 1 degree
Celsius
Transfer of Heat
Conduction
“movement of heat by collisions between
vibrating atoms or molecules”
Convection
“transfer of heat by the physical motion of
masses”
cooler liquids or gasses descend while warmer
liquids or gasses rise
Phases and Phase Diagram
(NOT IN TEXT but IMPORTANT)
2nd Law of Thermodynamics
Two key components
heat flows from a warmer body to a cooler
body
entropy increases remains constant or
increases in time
Question for Thought
Define temperature and heat.
Temperature is a measure of the average kinetic
energy of the molecules of a substance. Heat is
the total internal energy of the molecules
involved in an energy transfer.
Question for Thought
Why do most materials become less dense as
their temperature is increased?
 As the temperature of a solid increases, the vibrations of the
individual molecules become larger. When these vibrations become
larger, the average distance between the molecules increases to
accommodate these larger oscillations, and the solid expands. In a
liquid or a gas, the individual molecules move faster as the
temperature increases, and the collisions between individual
molecules become more violent. Since the molecules are moving
faster, they move farther apart as they travel a larger distance in the
time between collisions.
Question for Thought
Would the tight packing of more insulation, such
as glass wool, in an enclosed space increase or
decrease the insulation value? Explain.
 Tight packing would tend to decrease the insulation value of glass
wool because it would squeeze the wool together and give the heat
more paths to travel. It is the presence of many small pockets of air,
with unattached molecules, that gives glass wool and other similar
insulation materials their insulating properties.
Question for Thought
 A true vacuum bottle has a double-walled, silvered bottle
with the air removed from the space between the walls.
Describe how this design keeps food hot or cold by
dealing with conduction, convection and radiation.
 The vacuum between the walls prevents heat transfer by
means of convection or conduction, while the silvered
walls reflect radiated energy back into (or away from) the
food, preventing energy transfer by radiation.
Question for Thought
Why is cooler air found in low valleys
on calm nights?
 Cooler air is denser than warmer air. This denser air
weighs more per volume than the warmer air and
pushes the warmer air out of the way as it sinks down to
its lowest level. The warmer, less dense air sits on top
of the cooler air because it weighs less per volume.
Question for Thought
Why is air a good insulator?
Air is not very dense and conduction is not
very efficient at transferring energy
because the molecules are much farther
apart than they are in solids or liquids.
Question for Thought
 Explain the meaning of the mechanical equivalent of
heat.
 Mechanical energy can be converted to heat as it
changes from external mechanical energy to the internal
kinetic energy of the molecules. A given quantity of
mechanical energy always yields a known amount of
heat.
Question for Thought
What do people really mean when they
say that a certain food “has a lot of
Calories?
 When people refer to the “Calorie content of food,” they are referring
to the amount of chemical energy available from the food. One way
to measure the chemical energy of foodstuff is to find out how much
heat is released by complete oxidation. A Calorie (kcal) is a
measure of the heat release and thus is a measure of the chemical
energy released.
Question for Thought
A piece of metal feels cooler than a piece
of wood at the same temperature. Explain
why.
 The metal is more efficient at conducting heat away from
your hand than wood, so it feels cooler because your
hand senses heat leaving your body.
Question for Thought
Explain how latent heat of fusion and
latent heat of vaporization are
“hidden.”
 The latent heats of fusion and vaporization go into or are
released from internal energy during a phase transition.
There is no temperature change associated with these
heat transfers, so they are “hidden.”
Question for Thought
What is condensation? Explain on a
molecular level, how the condensation of
water vapor on a bathroom mirror warms
the bathroom.
 Condensation occurs when more vapor molecules are returning to
the liquid state than are leaving the liquid state. When a water
vapor molecule joins a group of liquid water molecules, it has to
give up its latent heat of vaporization. This heat is transferred to the
surrounding air molecules such as the air in the bathroom.
Question for Thought
Which provides more cooling for a styrofoam
cooler, one with 10 pounds of ice at 0 degrees C
or one with 10 pounds of ice water at 0 degrees
C?
 The 10 pounds of ice provide more cooling because as the ice
undergoes the phase change into water, it absorbs heat. Ten
pounds of ice water simply absorbs heat according to the value of
its specific heat until it reaches room temperature and therefore
absorbs less heat.
Question for Thought
 Explain why a glass filled with a cold beverage seems to
“sweat.” Would you expect more sweating inside a
house during the summer or during the winter? Explain.
 Water condenses out of the air onto the cooler surface
of a glass because the air near the glass is cooled,
lowering its temperature to the dew point. Since the
warmer air can hold more water vapor in the summer, it
would have more water vapor to condense. Therefore,
you would expect more condensation in the summer.
Question for Thought
Why is a burn from steam at 100 degrees
C more severe than a burn from 100
degrees C water?
One hundred degree Celsius steam
contains more energy (540 cal/g) than
100°C water, so the steam burn would be
more severe.
Question for Thought
Relative humidity typically increases after
sunset. Explain how this is possible when no
additional water vapor is added or removed from
the air.
 Cooling of air reduces the capacity of air to hold water vapor.
Relative humidity is a ratio of water in the air to how much water it
can hold. Thus a decrease of capacity increases the relative
humidity, even when the amount of water vapor in the air is
constant.
Question
A 1.0 kg metal head of a geology hammer
strikes a solid rock with a velocity of 5.0
meters per second. Assuming that all of
the energy is retained by the hammer
head, how much will the temperature
increase? (specific heat of hammer head
is 0.11 kcal/kgC)
Answer
4.
Q  DKE and DKE 
Q  mcDT  mcDT
T

1 2
1
mv  Q  mv 2
2
2

1 2
mv
2
1 2  
1.00 cal 
mv

2
 4.184 J 
 DT 
mc
2
1
 m 
1.0kg 5.0  1.00 cal 1.00 kcal
2
s


kcal 
1, 000.0 cal
1.0 kg 0.11
4.184 J

kgC 
 0.03 C
Question
Lead is a soft, dense metal with a specific
heat of 0.028 kcal/kgC, a melting point of
o
328.0 C and a heat of fusion of 5.5
kcal/kg. How much heat must be
provided to melt a 250.0 kg sample of
lead with a temperature of 20.0o C?
Answer
13.
Q1  mcDT

kcal 
 250.0 kg0.028
308.0C 


kgC
 2,156 kcal
Q2  mLf
 kcal 
 250.0 kg5.5

kg 
 1,375 kcal
QTotal  Q1  Q2
 2,156 kcal  1,375 kcal
 3,531 kcal
 3,500 kcal
Forces and Vibrations
Vibration
back and forth motion
Amplitude
extent of displacement from the equilibrium position
Cycle
one complete vibration
Period
time required to complete one cycle
Frequency
number of cycles per second
Relationship between period and frequency
T = 1 / f
f=1/T
Waves
Longitudinal
disturbance that causes particles to move closer
together or farther apart IN THE SAME DIRECTION
the wave is moving
Transverse
disturbance that causes motion PERPENDICULAR to
the direction that the wave is moving
In general, liquids carry longitudinal waves but
not transverse waves
Same terms as vibrations
v = l* f
Sound Waves
Sound does not travel in a vacuum
Sound moves through solids faster than
any gas
Velocity of sound is effected by
composition and temperature of gas
Reflection, Refraction and
Interference
Reflection
waves bouncing back off of a boundary
Refraction
change in direction of wave crossing a
boundary
Interference
interaction of waves
destructive interference
constructive interference
Resonance
Natural frequency
frequency of vibration determined by the
object’s composition and shape
Resonance
when frequency of external force matches
natural frequency
Doppler Effect and Sonic Boom
Doppler Effect
Apparent change in frequency of a
wave caused by the relative motion of
the source or observer
pitch of train approaching, departing
Sonic boom
shock wave caused by object moving
at speed of sound or faster
Question for Thought
What is a wave?
A wave is a disturbance that moves
through a medium such as a solid or the
air.
Question for Thought
Is it possible for a transverse wave to
move through air? Explain.
The book view is “No,” because there is
no force acting on the air to return it to its
original position after the wave has
dislocated it. However, what about EM.
Question for Thought
A piano tuner hears three beats per
second when a tuning fork and a note are
sounded together and six beats per
second after the string is tightened. What
should the tuner do next, tighten or loosen
the string? Explain.
 Loosen. Since the beat frequency depends upon the
difference between the two frequencies, you wish to go
in the direction of fewer beats per second.
Question for Thought
Why do astronauts on the moon have to
communicate by radio even when close to
one another?
There is no medium such as air to
transmit sound on the moon.
Question for Thought
What is resonance?
The condition where the frequency of an
external force matches the frequency of
an object is resonance.
Question for Thought
Explain why sound travels faster in warm
air than in cool air.
Gas molecules have a greater kinetic energy
and move faster in warm air than in cold air.
These molecules are able to transfer an impulse
from one molecule to the next faster.
Question for Thought
Do all frequencies of sound travel with the
same velocity? Explain using the wave
equation.
Longer wavelengths have lower frequencies.
Since the velocity of sound is equal to the
product of the frequency times the wavelength,
the velocity is a constant.
Question for Thought
What eventually happens to a sound wave
traveling through the air?
The energy of the sound wave is
eventually dissipated into heat.
Question for Thought
What gives a musical note its
characteristic quality?
The presence and strength of various
overtones determine the characteristic
sound of a musical note.
Question for Thought
Does a supersonic aircraft make a sonic
boom only when it cracks the sound
barrier? Explain.
 The sonic boom is from the building up of a pressure
wave in front of the moving aircraft. Since this pressure
wave is present as long as the plane is moving faster
than the speed of sound, the aircraft continually makes a
sonic boom.
Question for Thought
What is an echo?
An echo is the return of a sound wave to
its source after the wave has been
reflected.
Question for Thought
Why are fundamental frequencies and
overtones also called resonant
frequencies?
They all produce standing waves or
resonance in whatever is oscillating.
Question
The distance between the center of a
condensation and the center of an
adjacent rarefaction is 65.23 cm. If the
frequency is 256.0 Hz, how fast are these
waves moving?
Answer
2.
v  fl
f  256.0 Hz
l  2  65.23 cm 
1
v  256.0 2  65.23 cm 

s 
 3.340  104 cm / s (or 343.0 m / s)
Question
A warning buoy is observed to rise every
5.0 seconds as crests of waves pass by it.
What is the period of these waves? What
is the frequency?
Answer
3.
a  T  5.0s
1
T
1

5.0s
b f 
 0.2 Hz
Question
Sound from the siren of an emergency
vehicle has a frequency of 750 Hz and
moves with a velocity of 343 meters per
second. What is the distance from one
condensation to the next?
Answer
4.
v  fl  l 
v
f
m
f  750.0 Hz
s
m
343.0
s
l 
1
750.0
s
343.0 m s


750.0
s 1
 0.457 m
v  343.0