Physics 201,Lecture 22,Pg 1Reeder - 05-06
Physics 201 Lecture 22
Thermal Physics
? Temperature
? Thermal Expansion
? Ideal Gas Law
? Atoms,Molecules and Moles
? Kinetic Theory
Physics 201,Lecture 22,Pg 2Reeder - 05-06
Thermodynamics
? A practical,blue-collar” science initially concerned with economics,
industry,real life problems,
? DYNAMICS -- Concerned with the concepts of energy transfers
between a system and its environment and the resulting
temperature variations
? Concerns itself with the physical and chemical transformations of
matter in all of its forms,solid,liquid,and gas
? Concerns the processes the,violate” conservation of mechanical
energy -- friction ……
Physics 201,Lecture 22,Pg 4Reeder - 05-06
Temperature,
Zeroth Law of Thermodynamics
? Two objects in thermal equilibrium with each other are at
the same temperature
? Temperature is the property that determines whether or not
an object is in thermal equilibrium with other objects
Celsius Scale
? Temperature of an ice-water mixture is
defined as 0o C
?This is the freezing point of water
? Temperature of a water-steam mixture is
defined as 100o C
?This is the boiling point of water
? Distance between these points is divided
into 100 segments
Physics 201,Lecture 22,Pg 6Reeder - 05-06
Pressure-Temperature Graph
? All gases extrapolate to the same temperature at 0 pressure
? This temperature is absolute zero
Kelvin Scale
? When the pressure of a gas
goes to zero,its temperature is
–273.15o C
? This temperature is called
absolute zero
? This is the zero point of the
Kelvin scale
?–273.15o C = 0 K
? To convert,TC = TK – 273.15
? Temperature readings are nearly
independent of the gas
? Pressure varies with temperature
when maintaining a constant volume
Physics 201,Lecture 22,Pg 10Reeder - 05-06
Question
You measure your body temperature with a thermometer calibrated in
degrees Kelvin,What do you hope the reading is (assuming you are not
trying to fake some sort of illness)?
1,307 K
2,310 K
3,313 K
4,317 K
98oF ~ 37oC ~ 310oK
Physics 201,Lecture 22,Pg 11Reeder - 05-06
Thermal Expansion
? The thermal expansion of an object is a consequence of
the change in the average separation between its
constituent atoms or molecules
? At ordinary temperatures,molecules vibrate with a small
amplitude
? As temperature increases,the amplitude increases
?This causes the overall object as a whole to expand
Physics 201,Lecture 22,Pg 12Reeder - 05-06
Linear Expansion
? For small changes in temperature
? The coefficient of linear expansion,??,depends on the
material
?See table in text
?These are average coefficients,they can vary
somewhat with temperature
? L ? ? L o ? t
Physics 201,Lecture 22,Pg 13Reeder - 05-06
Area Expansion
? Two dimensions expand
? A ? ? A o ? t,? ? 2 ?
Physics 201,Lecture 22,Pg 14Reeder - 05-06
Question
You are given a 4.000-cm diameter steel ball bearing and a brass
plate with a 3.994-cm diameter hole at room temperature (20oC),
You are asked to squeeze the steel ball through the brass plate,
You are allowed to increase or decrease the temperature of the
two objects,However,they should both be at equal temperature
at all times,What can you do to succeed?
(?brass = 2 x 10-5 /oC,?steel = 1 x 10-5 /oC)
A,Cool the ball and the plate.
B,Heat the ball and the plate.
C,You can never succeed.
Physics 201,Lecture 22,Pg 15Reeder - 05-06
Volume Expansion
? Three dimensions expand
?For liquids,only coefficient of volume expansion exists
? V ? ? V o ? t
f o r s o l i d s,? ? 3 ?
Physics 201,Lecture 22,Pg 16Reeder - 05-06
Question,Expansion
? Two spheres are made of the same metal
and have the same radius,but one is hollow
and the other is solid at room temperature,
The temperature of both spheres is
increased by the same amount,Which
sphere expands more?
?Solid sphere
?Hollow sphere
?They expand by the same amount
All dimensions increase
The increase is proportional to
original size of that dimension
For the sphere the relevant
dimension is its outer radius
Physics 201,Lecture 22,Pg 17Reeder - 05-06
Question
Liquids always expand when heated.
A,yes.
B,no.
Unusual behavior of water,
? At the temperature of water increases from 0oC to 4
oC,it contracts and its density increases
? Above 4 oC,water exhibits the expected expansion
with increasing temperature
? Maximum density of water is 1000 kg/m3 at 4 oC
Physics 201,Lecture 22,Pg 18Reeder - 05-06
Applications of Thermal Expansion –
Bimetallic Strip
? Thermostats
?Use a bimetallic strip
?Two metals expand
differently
Physics 201,Lecture 22,Pg 19Reeder - 05-06
Aside,The Periodic Table
Physics 201,Lecture 22,Pg 20Reeder - 05-06
The Periodic Table Explained?
proton
neutron
electron
Look carefully
Physics 201,Lecture 22,Pg 21Reeder - 05-06
Energy vs Mass
He (m=4.0026 u) O (M=15.9995 u)
4 x He = 16.01 u
Mass difference = 0.01 u
= binding energy
So energy is the same as mass somehow
E = mc2sure
Physics 201,Lecture 22,Pg 22Reeder - 05-06
Atoms,Molecules,and Moles
? 1 mole = 6.022 x 1023 molecule (NA = Avogadro’s Number)
? 1 ? = 1 atomic mass unit = (mass of 12C atom)/12
? approximately # neutrons + # protons
? atomic weight W
? Mass of 1 mole of,stuff” in grams = molecular mass in ?
? e.g.,1 mole of N2 has mass of 2x14=28 grams
Physics 201,Lecture 22,Pg 23Reeder - 05-06
The Ideal Gas Law
? PV = NkBT
?P = pressure in N/m2 (or Pascals)
?V = volume in m3
?N = number of molecules
?T = absolute temperature (K)
?kB = Boltzmann’s constant
? kB = 1.28 x 10-23 J/K
? note,PV has units of N-m or J (energy!)
Physics 201,Lecture 22,Pg 24Reeder - 05-06
The Ideal Gas Law
? PV = NkBT
? Alternate way to write this
?N = number of moles (n) x NA molecules/mole
?PV= NkBT
? nNAkB T
? n(NAkB)T
?nRT
? PV = nRT
?R = ideal gas constant = NAkB = 8.31 J/(mol-K)