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Chapter 16
Temperature
and the
Kinetic Theory of Gases
Overview of Thermodynamics
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Thermodynamics extend the ideas of
temperature and internal energy to systems
of many particles.
Concerning with energy transfers between a
many-particle system and its environment
and the resulting variations in temperature of
the system or changes in thermodynamic
state.
Kinetic theory explains the bulk properties of
matter and the correlation between them and
the mechanics of atoms and molecules.
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16.1 Temperature
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We associate the concept of temperature with
how hot or cold an objects feels to the touch.
Our sense of touch provides us with a
qualitative indication of temperature.
Our senses are unreliable for this purpose.
We need a reliable and reproducible way for
establishing the relative “hotness” or
“coldness” of objects that is related solely to
the temperature of the object
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Thermometers are developed for these
measurements.
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Thermal Contact
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Two objects are in thermal contact with
each other if energy can be exchanged
between them.
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The exchanges can be in the form of heat
or electromagnetic radiation.
The exchanges in energy are due to a
temperature difference.
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Thermal Equilibrium
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Thermal equilibrium between two
objects is a situation such that the two
objects would not exchange energy by
heat or electromagnetic radiation if they
were placed in thermal contact.
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Zeroth Law of
Thermodynamics
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Two objects A and B are separately in
thermal equilibrium with a third object C.
A and B are in thermal equilibrium with
each other, if they are placed in thermal
contact.
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Let object C be a thermometer.
Since any two of them are in thermal
equilibrium with each other, they must be
the same in some physical property.
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Zeroth Law of
Thermodynamics, Application
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Object C (thermometer) is placed in contact with A until
it they achieve thermal equilibrium
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Object C is then placed in contact with object B until
they achieve thermal equilibrium
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The reading on C is recorded
The reading on C is recorded again
If the two readings are the same, A and B are also in
thermal equilibrium
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16.2 Temperature
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Temperature can be thought of as the
property that determines whether an
object is in thermal equilibrium with
other objects.
Two objects in thermal equilibrium
with each other are at the same
temperature.
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If two objects have different temperatures, they
are not in thermal equilibrium with each other
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Thermometers
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A thermometer is a device that is used
to measure the temperature of a system
with which the thermometer is in
thermal equilibrium.
All thermometers make use of some
physical property that exhibits a change
with temperature and can be calibrated
for a measurement.
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Thermometers, cont
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The properties include
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The volume of a liquid
The length of a solid
The pressure of a gas at a constant volume
The volume of a gas at a constant pressure
The electric resistance of a conductor
The color of a hot object
The temperature scale of a thermometer can
be established on the basis of one of these
physical properties.
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Thermometer, Liquid in Glass
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A common type
of thermometer is
a liquid in a glass
capillary tube.
The material in
the capillary tube
expands as it is
heated.
The liquid is
usually mercury
or alcohol.
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Calibrating a Thermometer
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A thermometer can be calibrated by
placing it in contact with some
environments that remain at constant
temperature.
Common systems involve water
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A mixture of ice and water at atmospheric
pressure
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Called the ice point or freezing point of water
A mixture of water and steam in equilibrium at
atmospheric pressure
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Called the steam point or boiling point of water
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Celsius Scale
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The ice point of water is defined to be
0oC
The steam point of water is defined to
be 100oC
The length of the column between these
two points is divided into 100 equal
segments, called degrees.
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Problems with
Liquid-in-Glass Thermometers
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An alcohol thermometer and a mercury
thermometer may agree only at the
calibration points
The discrepancies between the two
thermometers are especially large when
the temperatures being measured are
far from the calibration points, because
mercury and alcohol have different
thermal expansion properties.
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Gas Thermometer
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The gas thermometer offers a way to
define temperature by directly relating
temperature to internal energy.
Temperature readings are nearly
independent of the substance used in
the thermometer.
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Constant-Volume
Gas Thermometer
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The physical property
used in this device is
the pressure variation
with temperature of a
fixed-volume gas.
The volume of the gas
in the flask is kept
constant by raising or
lowering the reservoir B
to keep the mercury
level at A constant
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Constant Volume
Gas Thermometer, cont
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The thermometer is calibrated by using
a ice water bath and a steam water bath
The pressures of the mercury under
each situation are recorded
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The volume is kept constant by adjusting A
The information is plotted
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Constant Volume
Gas Thermometer, final
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To find the
temperature of a
substance, the gas
flask is placed in
thermal contact with
the substance
The pressure is
found on the graph
The temperature is
read from the graph
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Absolute Zero
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The thermometer
readings are virtually
independent of the gas
in the flask.
If the lines for various
gases are extended, the
pressure is always zero
when the temperature
is –273.15o C
This temperature is
called absolute zero
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Kelvin Temperature Scale
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Absolute zero is used as the basis of
the Kelvin temperature scale.
The size of the degree on the Kelvin
scale is the same as the size of the
degree on the Celsius scale
To convert: TC = T – 273.15
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TC is the temperature in Celsius
T is the Kelvin (absolute) temperature
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Kelvin Temperature Scale, 2
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The Kelvin temperature scale is now
based on two new fixed points
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Adopted in 1954 by the International
Committee on Weights and Measures
One point is absolute zero
The other point is the triple point of water
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This is the single temperature and pressure at
which ice, water, and water vapor can coexist in
thermal equilibrium.
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Absolute
Temperature Scale, 3
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The triple point of water occurs at 0.01o
C and 4.58 mm of mercury
This temperature was set to be 273.16K
on the Kelvin temperature scale.
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The unit of the absolute scale is the kelvin
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Absolute
Temperature Scale, 4
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The absolute scale is also called the
Kelvin scale
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The triple point temperature is 273.16 K
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Named for William Thomson, Lord Kelvin
No degree symbol is used with kelvins
The kelvin is defined as 1/273.16 of the
temperature of the triple point of water
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Some Examples of
Absolute Temperatures
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This figure gives some
absolute temperatures at
which various physical
processes occur
The scale is logarithmic
The temperature of
absolute zero has never
been achieved.
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Experiments only have come
close
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Energy at Absolute Zero
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According to classical physics, the kinetic
energy of the gas molecules would become
zero at absolute zero.
The molecular motion would cease
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Therefore, the molecules would settle out on the
bottom of the container
Quantum theory modifies this statement and
indicates that some residual energy would
remain at this low temperature.
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This energy is called the zero-point energy
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Fahrenheit Scale
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A common scale in everyday use in
the US
Named for Daniel Fahrenheit
Temperature of the ice point is 32oF
Temperature of the steam point is 212oF
There are 180 divisions (degrees)
between the two reference points
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Comparison of Scales
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Celsius and Kelvin have the same size
degrees, but different starting points
TC = T – 273.15
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Celsius and Fahrenheit have difference
sized degrees and different starting
points
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TF  TC  32 F
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Comparison of Scales, cont
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To compare changes in temperature
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TC  T  TF
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Ice point temperatures
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0oC = 273.15 K = 32oF
steam point temperatures
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100oC = 373.15 K = 212oF
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