Leon Balents (UCSB) Lorenz Bartosch (Yale/Frankfurt) Anton Burkov (UCSB) Matthew Fisher (UCSB)
Temperature and Heat - UCSB · 2020. 1. 1. · Copyright © 2008 Pearson Education Inc., publishing...
Transcript of Temperature and Heat - UCSB · 2020. 1. 1. · Copyright © 2008 Pearson Education Inc., publishing...
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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
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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
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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.
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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.
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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.”
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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.
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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.
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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.
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Thermal expansion—linear
• A change in
length will
accompany a
change in
temperature.
The size of the
change will
depend on the
material.
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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.
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Coefficients of expansion
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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).
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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.
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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.
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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.
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Specific heat values
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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.
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Heats of Fusion and Heats of Vaporization
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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.
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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).
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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.
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Thermal conductivity
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Thermal Conductivity – Lattice Waves – Longitudinal and Transverse
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Thermal Conductivity – k (watts/m-Kelvin)
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Conduction of heat I
• Consider Example 17.12.
• What makes a picnic
cooler effective?
• Figure 17.25 at right
illustrates the problem.
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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.
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Conduction of heat III
• Consider Example 17.14.
• There are variations of the metal bar problem.
• Figure 17.27 below illustrates the problem.
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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.
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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.