Download HNRS 227 Lecture #2 Chapters 2 and 3

HNRS 227 Fall 2008
Chapter 4
Heat and Temperature
presented by Prof. Geller
iClicker Question
What is the result if you add 1015 to 1015?
A 1015
B 1030
C 10150
D 1025
E 2 x 1015
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.
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
iClicker Question
Which unit of the following is the largest?
A Kilometer
B Astronomical Unit (AU)
C Light Year (ly)
D Parsec (pc)
E Gigameter
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.
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iClicker Question
Question for Thought
Neglecting air resistance, what are the forces
acting on a bullet after it has left the barrel of a
rifle?
A
The force of air acting up and gravity
acting down.
B
Only the force of gravity acting straight
down.
C
There are no forces acting at this point.
D
All of the above are true.
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.
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.
F2
Box
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
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.
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
2
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.
Main Concepts for Chapter 4
Kinetic Molecular Theory
Temperature
Heat
Phases of matter
Thermodynamics
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
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
1st Law of Thermodynamics
In an isolated system, the
total amount of energy,
including heat energy, is
conserved.
ENERGY IS CONSERVED
Temperature
Units
Fahrenheit
Celsius
Kelvin
Where is absolute zero?
At what temperature does water
freeze? Boil?
3
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*ΔT (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
Phases and Phase Diagram
(NOT IN TEXT but IMPORTANT)
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
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
4
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.
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.
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.
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.
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
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.
5
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.
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.
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.
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
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 “hidden.”
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 therefore
absorbs less heat.
6
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.
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.
Answer
Q = mcΔT ∴ mcΔT
T
=
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.
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)
Question
4.
Q = ΔKE and ΔKE =
Question for Thought
1 2
1
mv ∴ Q = mv 2
2
2
=
⎛ 1 2 ⎞ ⎛⎜1.00 cal ⎞
mv ×
⎝2
⎠ ⎝ 4.184 J ⎠
1 2
mv ∴ ΔT =
mc
2
2
1
m
(1.0kg )⎛⎝ 5.0 ⎞⎠ (1.00 cal) 1.00 kcal
2
s
×
⎛
1, 000.0 cal
kcal ⎞
(1.0 kg )⎜ 0.11
(4.184 J)
⎝
kg°C ⎠
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?
= 0.03° C
7
Answer
13.
Q1 = mcΔT
⎛
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
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