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© 2017 Pearson Education, Inc. Slide 19-1
Phase Changes
Suppose you were to remove an ice cube from the freezer, initially at –20ºC, and then warm it by transferring heat at a constant rate.
Figure (b) shows the temperature as a function of time.
During the phase changes of melting then boiling, energy is being added to break molecular bonds, but the temperature remains constant.
© 2017 Pearson Education, Inc. Slide 19-2
Phase Changes
A phase diagram is used to show how the phases and phase changes of a substance vary with both temperature and pressure.
At the normal 1 atm of pressure, water crosses the solid-liquid boundary at 0ºC and the liquid-gas boundary at 100ºC.
At high altitudes, where p < 1 atm, water freezes at slightly above 0ºC and boils at a temperature below 100ºC.
In a pressure cooker, p > 1 atm and the temperature of boiling water is higher, allowing the food to cook faster.
© 2017 Pearson Education, Inc. Slide 19-3
If the pressure of liquid water is suddenly
decreased, it is possible that the water will
A. Freeze.
B. Condense.
C. Boil.
D. Either A or C.
QuickCheck
© 2017 Pearson Education, Inc. Slide 19-4
A phase change is characterized by a change in thermal energy without a change in temperature.
The amount of heat energy that causes 1 kg
of substance to undergo a phase change is the heat of transformation (or latent heat)L of that substance.
The heat required for a system of mass M to undergo a phase change is
Phase Change and Heat of Transformation
Lava—molten rock—undergoes a
phase change when it contacts the
much colder water. This is one way
in which new islands are formed.
© 2017 Pearson Education, Inc. Slide 19-5
If you heat a substance in an insulated container,
is it possible that the temperature of the substance
remains unchanged?
A. Yes
B. No
QuickCheck
© 2017 Pearson Education, Inc. Slide 19-6
The specific heat c can be
measured from the slope of
the linear increases.
The heat needed for the
phase changes is:
Suppose you start with a system in its solid phase and heat
it at a steady rate.
where Lf is the latent heat of
fusion and Lv is the latent
heat of vaporization.
Phase Change and Heat of Transformation
© 2017 Pearson Education, Inc. Slide 19-7
Melting/Boiling Temperatures and Heats of
Transformation
Example 1 - Latent Heat
A bag containing 0ºC ice is much more effective in absorbing energy than
one containing the same amount of 0ºC water. How much heat transfer is
necessary to raise the temperature of 0.800 kg of water from 0ºC to 30.0ºC?
How much heat transfer is required to first melt 0.800 kg of 0ºC ice and then
raise its temperature?
Example 2 – Latent Heat
How much heat transfer is required to raise the temperature of a 0.750-
kg aluminum pot containing 2.50 kg of water from 30.0ºC to the boiling
point and then boil away 0.750 kg of water? (b) How long does this take
if the rate of heat transfer is 500 W?
© 2017 Pearson Education, Inc. Slide 19-10
Consider two systems with different temperatures T1 and T2 that can interact thermally with each other but are isolated from everything else.
Heat will naturally flow from the hotter to the colder system until they reach a common final temperature Tf.
If Qi is the heat transferred to system i, then
Calorimetry
© 2017 Pearson Education, Inc. Slide 19-11
50 g of ice at 0ºC is added to 50 g of liquid water at
0ºC in a well-insulated container, also at 0ºC. After a
while, the container will hold
A. All ice.
B. > 50 g of ice, < 50 g of liquid water.
C. 50 g of ice, 50 g of liquid water.
D. < 50 g of ice, > 50 g of liquid water.
E. All liquid water.
QuickCheck
© 2017 Pearson Education, Inc. Slide 19-12
Problem-Solving Strategy: Calorimetry Problems
© 2017 Pearson Education, Inc. Slide 19-13
Example 3
A 200 g piece of iron at 120ºC and a 150 g piece of copper
at -50ºC are dropped into an insulated beaker containing
300 g of ethyl alcohol at 20ºC. What is the final temperature?
In-class Activity #1
In a calorimeter of negligible heat capacity, 200 g of steam at 150ºC and
100 g of ice at −40ºC are mixed. The pressure is maintained at 1 atm.
What is the final temperature, and how much steam, ice, and water are
present?
© 2017 Pearson Education, Inc. Slide 19-15
Conduction Convection
Radiation Evaporation
Heat-Transfer Mechanisms
© 2017 Pearson Education, Inc. Slide 19-16
For a material of cross-section area A and length L, spanning a temperature difference ΔT = TH – TC,the rate of heat transfer is
where k is the thermal conductivity, which characterizes whether the material is a good conductor of heat or a poor conductor.
Conduction
© 2017 Pearson Education, Inc. Slide 19-17
Conduction
© 2017 Pearson Education, Inc. Slide 19-18
Example 4
A 1.8-m-wide by 1.0-m-tall by 0.65-m-deep home freezer is
insulated with 5.0-cm-thick Styrofoam insulation. At what
rate must the compressor remove heat from the freeze to
keep the inside at -20ºC in a room where the air
temperature is 25ºC?
© 2017 Pearson Education, Inc. Slide 19-19
Thermal energy is easily transferred through air, water, and other fluids because the air and water can flow.
When the fluid is heated it generally expands and becomes less dense.
Buoyancy then causes the warmer fluid to flow upward, while the cooler fluid sinks to take its place.
This transfer of thermal energy by the motion of a fluid is called convection.
Convection
Warm water (colored) moves
by convection.
© 2017 Pearson Education, Inc. Slide 19-20
All objects emit energy in the form of radiation, electromagnetic waves generated by oscillating electric charges in the atoms that form the object.
This satellite image shows radiation emitted by the ocean waters off the east coast of the United States.
You can clearly see the warm waters of the Gulf Stream, a large-scale convection that transfers heat to northern latitudes.
Radiation
© 2017 Pearson Education, Inc. Slide 19-21
If heat energy Q is radiated in a time interval Δt by an object with surface area A and absolute temperature T,the rate of heat transfer is
The parameter e is the emissivity of the surface, a measure of how effectively it radiates.
The value of e ranges from 0 to 1.
σ = 5.67 ×10–8 W/m2K4 is the Stefan-Boltzmann constant.
Radiation
© 2017 Pearson Education, Inc. Slide 19-22
Suppose the temperature of the sun suddenly dropped
to half its present value. The sun’s power output would
decrease by a factor of
A. 1/2
B. 1/4
C. 1/8
D. 1/16
QuickCheck
© 2017 Pearson Education, Inc. Slide 19-23
Example 5
The intensity of solar radiation is 1370 W/m2. What is the
temperature of the sun’s surface? Note: The distance to
the sun is on average 1.50 x 1011 m and the radius of the
sun is 6.96 x 108 m.
In-class Activity #2
A firewalker runs across a bed of hot coals without
sustaining burns. Calculate the heat transferred by
conduction into the sole of one foot of a firewalker given
that the bottom of the foot is a 3.00-mm-thick callus with a
conductivity at the low end of the range for wood (0.08 W/m
ºC) and its density is 300 kg/m3. The area of contact is 25.0
cm2, the temperature of the coals is 700ºC, and the time in
contact is 1.00 s.