HNRS 227 Lecture #2 Chapters 2 and 3
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Transcript HNRS 227 Lecture #2 Chapters 2 and 3
HNRT 227 Fall 2015
Chapter 4
Heat and Temperature
presented by Prof. Geller
10 September 2015
1
Do You Remember These?
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
2
iClicker Question
What is the result if you add 1015 to
1015?
A 1015
B 1030
C 10150
D 1025
E 2 x 1015
3
iClicker Question
Which unit of the following is the
largest?
A Kilometer
B Astronomical Unit (AU)
C Light Year (ly)
D Gigameter
4
iClicker Question
What is acceleration?
A The change in distance per
change in time.
B The change in position per
unit of time.
C The ratio of the change in
velocity per change in time.
D The change of time per unit
of length.
5
iClicker Question
What happens to the velocity and
acceleration of an object in free fall?
A The velocity decreases as the acceleration
remains the same.
B The velocity increases as the acceleration
remains the same.
C The velocity increases and the
acceleration decreases.
D The velocity increases and the
acceleration increases.
E Both velocity and acceleration decrease.
6
iClicker Question
Neglecting air resistance, what are the
forces acting on a bullet after it has
left the barrel of a rifle?
A
B
C
D
The force of air acting up and gravity
acting down.
Only the force of gravity acting
straight down.
There are no forces acting at this
point.
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All of the above are true.
Question for Thought
A kWhr is
A
B
C
D
a unit of work
a unit of energy
a unit of power
More than one of the above is true.
A kWhr is a unit of work, and since energy is the
ability to do work, it is also a unit of energy. In
terms of units, a watt is a joule per second, and an
hour, as a second, is a unit of time. The time units
cancel, leaving a unit of a joule, which can be used to
measure either work or energy.
8
Question for Thought
What happens to the kinetic energy of
a falling book when the book hits the
floor?
A
B
C
The kinetic energy is destroyed.
The kinetic energy is converted to
heat only.
The kinetic energy is converted to
heat and sound.
9
The magnitude of the force that a baseball
player exerts with a 5 kilogram baseball bat
on a 0.2 kilogram ball is measured to be 50
Newtons. What is the magnitude of the
force that the ball exerts on the bat?
A. 6 Newtons
B. 10 Newtons
C. 50 Newtons
D. 60 Newtons
E. 1.0 Newtons
10
F2
Box
F1
In the diagram above, a box is on a frictionless
horizontal surface with forces F1 and F2 acting as
shown. If the magnitude of F1 is greater than the
magnitude of F2, then the box is
A. moving at constant speed in the direction of F1
B. moving at constant speed in the direction of F2
C. accelerating in the direction of F1
D. accelerating in the direction of F2
E. not moving at all.
11
When a satellite is a distance R from the
center of Earth, the force due to gravity on
the satellite is F. What is the force due to
gravity on the satellite when its distance
from the center of the Earth is 2R?
A. 9 F
B. 4 F
C. F / 4
D. F / 9
E. F / 16
12
Which of the following best describes the law of
conservation of energy?
A. Energy must not be used up faster than it is
created or the supply will run out.
B. Energy can be neither created nor destroyed.
C. Energy is conserved because it is easily
destroyed.
D. Conservation is a law describing how to destroy
matter.
E. Energy conservation is a law recently passed by
Congress.
13
What is the power consumed by a 100
Joules energy source in 4 seconds?
A. 400 Watts
B. 100 Watts
C. 200 Watts
D. 50 Watts
E. 25 Watts
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Main Concepts for Chapter 4
Kinetic Molecular Theory
Temperature
Heat
Phases of matter
Thermodynamics
15
1st Law of Thermodynamics
In an isolated system, the
total amount of energy,
including heat energy, is
conserved.
ENERGY IS CONSERVED
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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
17
Temperature
Units
Fahrenheit
Celsius
Kelvin
Where is absolute zero?
At what temperature does water
freeze? Boil?
18
19
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
20
Specific Heat
Ability of a material to absorb heat
energy
proportional to mass
proportional to change in temperature
Q = m*c*DT (note: not in textbook)
c is the SPECIFIC HEAT of the substance
amount of energy needed to increase
temperature of 1 gram of substance 1 degree
Celsius
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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
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Phases and Phase Diagram
(NOT IN TEXT but IMPORTANT)
23
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
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Question for Thought
Which is true about temperature and
heat.
A
Temperature is a measure of the
average kinetic energy of the molecules of a
substance.
B
Heat is the total internal energy of
the molecules involved in an energy
transfer.
C
Both A and B above are true.
D
Neither A nor B above are true.
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Question for Thought
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.
A
This explains why most materials become less
dense as their temperature is increased.
B
This explains why most materials become more
dense as their temperature is increased.
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Question for Thought
Would the tight packing of more insulation,
such as glass wool, in an enclosed space
increase or decrease the insulation value?
A
B
Increase the insulation value.
Decrease the insulation value.
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.
27
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.
28
Question for Thought
Cooler air is found in low valleys on
calm nights.
A True
B False
True. 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.
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Question for Thought
Is air a good insulator?
A
B
Yes
No
Air is a good insulator because it 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.
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Question for Thought
Can you 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.
31
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
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measure of the chemical energy released.
Question for Thought
A piece of metal feels cooler than a
piece of wood at the same temperature.
A
B
True
False
True. 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.
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Question for Thought
Can you 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
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“hidden.”
Question for Thought
The condensation of water vapor on a
bathroom mirror warms the bathroom.
A
B
True
False
True. 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.
35
Question for Thought
Which provides more cooling for a
styrofoam cooler?
A
B
One with 10 pounds of ice at 0
degrees C
One with 10 pounds of ice water at 0
degrees C?
The 10 pounds of ice (A) 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
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therefore absorbs less heat.
Question for Thought
Consider that 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?
A
Summer
B
Winter
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.
37
Question for Thought
A burn from steam at 100 degrees C is
more severe than a burn from 100
degrees C water.
A
B
True
False
True. One hundred degree Celsius steam
contains more energy (540 cal/g) than
100°C water, so the steam burn would be
more severe.
38
Question for Thought
Relative humidity typically increases after
sunset.
A
B
True
False
True. 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.
39
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)
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Answer
4.
Q DKE and DKE
Q mcDT mcDT
1 2
1
mv Q mv 2
2
2
1 2
mv
2
1.00 cal
1 2
mv
2
4.184 J
DT
mc
2
T
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
kgC
0.03 C
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Question
Lead is a soft, dense metal with a
specific heat of 0.028 kcal/kgC, a
melting point of 328.0o 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?
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Answer
13.
Q1 mcDT
kcal
250.0 kg0.028
308.0C
kgC
2,156 kcal
Q2 mLf
kcal
250.0 kg5.5
kg
1,375 kcal
QTotal Q1 Q2
2,156 kcal 1,375 kcal
3,531 kcal
3,500 kcal
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