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Chapter 10
Thermal Energy
Chapter Objectives
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Define Temperature
Converting between the 3 temperature scales
Identify Linear Expansion
Utilize the Coefficient of Linear Expansion
Compare Linear, Area, and Volumetric Expansion
Apply the Kinetic Theory of Gases
Temperature
• Heat is defined as the energy exchanged
between objects.
• Temperature is the measurement of the
average kinetic energy of an object.
Thermal Equilibrium
• Two objects in thermal equilibrium with
each other are at the same temperature.
Temperature Scales
• Fahrenheit is the scale used in the United States
– Freezing point of water
• 32o
– Boiling point of water
• 212o
• Celsius is the most common used scale around
the world
– Often thought of as a member of the metric system
• Kelvin scale is the most common used scale in the
world of science
– Along with the Celsius scale, these are the SI accepted
measurements
Celsius Scale
• The Celsius scale is set up based on the
critical points of water
– Freezing point
• 0o
– Boiling point
• 100o
• Positive and negative values exist on the
Celsius scale
Kelvin Scale
• The Kelvin scale is defined as 1/273.16 of the
temperature of the triple point of water.
– There is no negative values on the Kelvin scale.
– The lowest point is often called absolute zero.
• Freezing point of water
– 273.15
• Boiling point of water
– 373.15
• For most calculations in this class, temperature
should be in Kelvin.
Triple Point
• The triple point of water is the point at
which conditions allow for all three phases
of water to exist in equilibrium.
• Those conditions describe a single, specific
pressure and temperature.
– 0.01 oC
– 4.58 mm Hg
• 0.006 atm
Finding Freezing and Boiling Points
• Boiling points and Freezing points can change as
environmental conditions change
– Standard critical points are measured at accepted
atmospheric pressure
• As pressure goes up,
– Boiling point goes up
– Freezing point goes up
• As pressure goes down,
– Boiling point goes down
– Freezing point goes down
Converting Between Scales
• To convert between
the Fahrenheit and
Celsius scales,
TF = 9/5TC + 32
• To convert between
Celsius and Kelvin
scales,
T = TC + 273.15
Linear Expansion
• Much like the behavior of gases, solids and liquids
expand.
• Each material expands at different rates based on
their chemical make-up.
• Due to this, a constant must be used for each
material called the average coefficient of linear
expansion.
L =
L0 T
Notice this calculates
the change in length,
not the final length.
Area and Volumetric Expansion
• If the linear
dimension expands,
then the area
dimension must
expand as well.
• For this, there is an
• And also an average of
volumetric expansion.
average coefficient
of area expansion.
A =
A0T
V
= V0T
Kinetic Theory of Gases
1) The number of molecules are large, but the
space between them is large compared with the
volume.
2) The molecules move randomly in any direction.
3) All collisions of the molecules are perfectly
elastic.
4) The forces between molecules are negligible.
5) The gas is a pure substance. Meaning all
molecules are identical.
Ideal Gas Law
• The ideal gas law varies
slightly for physics
versus chemistry.
• That is due to
Boltzmann’s Constant
(kB).
• kB = 1.38 x 10-23 J/K
• Chemistry’s version uses the
ideal gas constant (R).
• R = 8.31 J/(mol *K)
PV = nRT
n is number of moles
PV = NkBT
N is number of particles
P is pressure (N/m2)
V is volume (m3)
T is temperature (K)
rd
3
Version of Ideal Gas Law
• Assuming the amount of gas (N1= N2) remains constant in
a closed container, we can derive a 3rd version of the ideal
gas law.
• This version will also help us to see the basis for the 3 gas
laws from chemistry (Boyle’s Law, Charles’ Law, and GayLussac’s Law).
Leaving us with
P1V1 P2V2
=
T1
T2