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Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
PowerPoint® Lectures for
University Physics, Twelfth Edition
– Hugh D. Young and Roger A. Freedman
Lectures by James Pazun
Chapter 17
Temperature and Heat
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Goals for Chapter 17
• To delineate the three different temperature scales
• To describe thermal expansion and thermal stress
• To consider heat, phase changes, and calorimetry
• To study how heat flows with convection,
conduction, and radiation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Introduction
• Growing up in Pittsburgh, molten steel was a common sight. Still it is imposing at 1500oC.
• The worst common burns you can imagine are steam burns. You have not only water heated to its boiling point but gaseous steam carrying the heat of vaporization. It’s a great deal of energy in a small space.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Measuring temperature
• There are many ways to measure
temperature, but the two devices
mentioned below take advantage of
a gas or liquid sample which
expands if heat is added and
contracts if heat is removed.
• A cylinder of gas will show pressure
rise if volume is kept constant.
• A small container of liquid will see
the liquid increase in volume as
temperatures rise. Mercury was
chosen “early on” because it’s so
dense, a small volume can record
large temperature ranges.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
The zeroth law of thermodynamics
• Simply stated? “Heat will always travel from a hot reservoir to a cold one without outside energy forcing an unnatural transfer.”
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thermometers? Just our way of trying to see where the heat is
• The measure of temperature is a way of expressing how much heat one object is holding relative to another.
• There are several examples shown at right. You can base a thermometer on thermal expansion of a gas, differential expansion of bimetal strips, even on something as wild as laser-doppler shift.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
The coldest we can ever get?
• Early experiments observed changes in pressure or volume as
temperature changed.
• It was noticed that the linear trends lead to a consistent lowest
temperature that we call “absolute zero”—labeled 0K after Lord Kelvin.
• Refer to Example 17.1.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Conversions are expected
• Values on the temperatures scales (Fahrenheit, Centigrade/Celsius,
and Kelvin) may be readily interconverted. Physics professors will
want values to eventually be in Kelvins because that’s the form in
SI units.
• See Figure 17.7 below.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thermal expansion—linear
• A change in
length will
accompany a
change in
temperature.
The size of the
change will
depend on the
material.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Changing temperature changes atomic spacing
• Molecules can be visualized as bedsprings and spheres. More
heat (higher temperatures) is reflected by the motion of the
atoms relative to each other.
• See Figure 17.9 below.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Coefficients of expansion
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thermal changes in material length and volume
• Refer to Problem-Solving Strategy 17.1.
• Consult Example 17.2 (change in length).
• Consult Example 17.3 (change in length II).
• Consult Example 17.4 (change in volume).
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thermal expansion we see constantly
• Water is interesting. There are no
other liquids that expand to become
less dense as a solid than they are
as a liquid. This is fortunate, if
lakes were to freeze and dense ice
sink to the bottom, everything in
the water would die as the liquid
became solid from the bottom up.
• Thermal expansion joints allow
roads to expand and contract
without any stress to the material
used to build.
• Refer to Example 17.5.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
James Joule and the mechanical equivalent of heat
• Joule knew a mass
above the ground had
potential energy. He
dropped an object on a
cord, turning a paddle
in water monitored by
a very accurate
thermometer.
• His conclusion was to
connect energy
conservation (potential
and kinetic) to heat as
a third form observed.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Specific heat
• A specific heat value reveals
how much temperature will
change when a given amount of
a substance absorbs a given
amount of heat.
• Water is a “benchmark” as one
ml of water will absorb 1 cal of
heat to raise its temperature by
1oC.
• Refer to Example 17.6 and
Example 17.7.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Specific heat values
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Phase changes and temperature behavior
• A solid will absorb heat according to its heat
capacity, becoming a hotter solid.
• At the melting point, a solid will absorb its
heat of fusion and become a liquid. An
equilibrium mixture of a substance in both its
liquid and solid phases will have a constant
temperature.
• A cold liquid will absorb heat according to its
heat capacity to become a hotter liquid.
• At the boiling point, a liquid will absorb its
heat of vaporization and become a gas. An
equilibrium mixture of liquid and gas will have
a constant temperature.
• A cold gas can absorb heat according to its heat
capacity and become a hotter gas.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Heats of Fusion and Heats of Vaporization
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Using well-behaved water to measure other systems
• Because water is a good thermal sink, is readily available, and reproducibly absorbs 4.184 J for every gram to rise in temperature by 1oC, it is often used to measure another object’s change in heat energy by comparison.
• For example, an unknown metal might be massed, raised to a known temperature (say to 100oC in a boiling water bath), then added to a known amount of cold water. The resulting change in the temperature of the water will allow heat absorbed to be calculated and then the heat capacity of the unknown metal.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Heat calculations
• Follow Problem-Solving Strategy 17.2.
• Refer to Example 17.8 (no phase change).
• Refer to Example 17.9 (changes in both temperature and
phase).
• Refer to Example 17.10 (an example that could be done in a
kitchen).
• Refer to Example 17.11 (combustion, temperature change,
and phase change).
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Why, and how well, do materials transfer heat?
• Figure 17.23 illustrates
the model.
• Table 17.5 lists thermal
conductivities. They are
dramatically different,
from very large values
for conductors like
metals to very small
values for insulators
like styrofoam or wood.
• Consider Problem-
Solving Strategy 17.3.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thermal conductivity
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Thermal Conductivity – Lattice Waves – Longitudinal and Transverse
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thermal Conductivity – k (watts/m-Kelvin)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Conduction of heat I
• Consider Example 17.12.
• What makes a picnic
cooler effective?
• Figure 17.25 at right
illustrates the problem.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Conduction of heat II
• Consider Example 17.13.
• This is a good reason not to pick up a metal frying pan by
its bare handle.
• Figure 17.26 below illustrates the problem.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Conduction of heat III
• Consider Example 17.14.
• There are variations of the metal bar problem.
• Figure 17.27 below illustrates the problem.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Convection of heat
• Heating by moving large
amounts of hot fluid,
usually water or air.
• Figure 17.28 at right
illustrates heat moving by
convection.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Radiation of heat
• Infrared lights, hot metal
objects, a fireplace,
standing near a running
furnace … these are all
objects heating others by
broadcast of EM radiation
just lower in energy than
visible red.
• Consider Example 17.15.
• Consider Example 17.16.