Transcript Physics 114 – Lecture 36

Physics 114 – Lecture 36
• Chapter 13 Temperature and Kinetic Theory
• §13.1 Atomic Theory of Matter
• Democritus speculated about progressive divisions of
matter. Would a stage be reached when the matter could
not be divided further without changing the nature of
that matter? He thought that this would occur.
• He defined the smallest individual piece of matter to be
an atom of that substance
• Mass of an atom – define mass of 12C atom to be
12.0000 unified atomic mass units
• → 1 u ≡ 1.6605 X 10-27 kg
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• Evidence for atomic theory of matter, Brown, 1827,
observed random motion of very small particles
(pollen grain) suspended in liquid – known as
Brownian motion
• Einstein, 1905, showed from a theoretical analysis
that a typical atomic diameter ~ 10-10 m
Brownian Motion
Solid
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Liquid
Gas
Physics 114 – Lecture 36
• Study Example 13.1 – Distance between atoms
• §13.2 Temperature and Thermometers
• Temperature is a measure of how hot or how cold a
substance may be
• Most substances expand when heated – e.g.,
expansion joints or compressible spacers are needed
to accommodate this effect, bridges, concrete
surfaces, …
• This expansion may be used to design a thermometer.
Other attributes of a substance which changes with
temperature may also be used
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• Examples
• Temperature Scales
• Need two easily
reproducible temperatures
• Freezing of water –
00 C ≡ 320 F
• Boiling point of water –
1000 C ≡ 2120 F
• → Δ T (boiling point of water – freezing point of water)
= 1000 C = 1800 F - Celsius and Fahrenheit Temp Scales
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• Conversion of Fahrenheit temperatures
to Celsius temperatures and
vice versa
5 0
T C   T F - 32
9
0
and
9 0
T F  T C   32
5
0
• Study Example 13.2
• For very accurate temperature
measurement one must use a
constant volume gas
thermometer – see text
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• §13.3 Thermal Equilibrium and the Zeroth Law of
Thermodynamics
• If two bodies at different temperatures are placed in
thermal contact they reach a common temperature
and are said to be in thermal equilibrium
• If bodies A and B are separately in thermal
equilibrium with a third body, C, then bodies A and B
will be in thermal equilibrium with each other
• This statement is known as the zeroth law of
therodynamics
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§13.4 Thermal Expansion
Linear Expansion
ΔL = α L0 ΔT
where α = coefficient of linear expansion
With L = L0 + ΔL → L = L0 (1 + α ΔT)
Units of α are (0C)-1
For most materials α may only be considered constant
over a limited range in temperature
• Study Examples 13.3, 13.4 and 13.5
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Volume Expansion
ΔV = βV0 ΔT
where β = coefficient of linear expansion
With V = V0 + ΔV → V = V0 (1 + β ΔT)
Again, units of β are (0C)-1
If the material is isotropic then it is easily shown that
β ≈ 3α
Notice that the coefficient of linear expansion has no
meaning for fluids – liquids and gases – since they
have no definite shape
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Anomalous Behaviour of Water Below 40C
With the density of water is given by, ρ = m/V
For a given mass of water, at temperature, T,
ρ = m/V = m/[V0(1 + βΔT)] = m/V0 X [1/(1 + βΔT)]
= ρ0 X [1/(1 + βΔT)]
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