Thermal Physics

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Chapter 10 Thermal Physics
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• Temperature
• Thermodynamics branch of physics studying
  thermal energy of systems
• Temperature (T), a scalar measure of the
  thermal (internal) energy of a system
• SI unit K (Kelvin)
• Kelvin scale has a lower limit (absolute
• zero) and has no upper limit

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• Kelvin scale
• Kelvin scale is defined by the temperature of
  the triple point of pure water
• Triple point set of pressure and temperature
  values at which solid, liquid, and gas phases can
  coexist
• International convention
• T of the triple point of water is

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• The zeroth law of thermodynamics
• If two (or more) bodies in contact dont change
  their internal energy with time, they are in
  thermal equilibrium
• 0th law of thermodynamics if bodies are in
  thermal equilibrium, their temperatures are equal

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• Measuring temperature
• Temperature measurement principle if bodies A
  and B are each in thermal equilibrium with a
  third body C, then A and B are in thermal
  equilibrium with each other (and their
  temperatures are equal)
• The standard temperature for the Kelvin scale is
  measured by the constant-volume gas thermometer

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Constant-volume gas thermometer
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• Celsius and Fahrenheit scales
• Celsius scale
• Fahrenheit scale

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Chapter 10 Problem 3
Convert the following temperatures to their
values on the Fahrenheit and Kelvin scales (a)
the boiling point of liquid hydrogen, 252.87C
(b) the temperature of a room at 20C.
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• Thermal expansion
• Thermal expansion increase in size with an
  increase of a temperature
• Linear expansion
• Volume expansion

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Thermal expansion
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Chapter 10 Problem 14
A cube of solid aluminum has a volume of 1.00 m3
at 20C. What temperature change is required to
produce a 100-cm3 increase in the volume of the
cube?
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• Temperature and heat
• Heat (Q) energy transferred between a system
  and its environment because of a temperature
  difference that exists between them
• SI Unit Joule
• Alternative unit calorie (cal)

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• Avogadros number
• Mole amount of substance containing a number
  of atoms (molecules) equal to the number of atoms
  in a 12 g sample of 12C
• This number is known as Avogadros number (NA)
• NA 6.02 x 1023 mol -1
• The number of moles in a sample
• N total number of atoms (molecules)
• m total mass of a sample, m0 mass of a single
  atom (molecule) M molar mass

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• Ideal gases
• Ideal gas a gas obeying the ideal gas law
• R gas constant
• R 8.31 J/mol K
• kB Boltzmann constant
• kB 1.38 x 1023 J/K

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• Ideal gases
• The gas under consideration is a pure substance
• All molecules are identical
• Macroscopic properties of a gas P, V, T
• The number of molecules in the gas is large, and
  the average separation between the molecules is
  large compared with their dimensions the
  molecules occupy a negligible volume within the
  container
• The molecules obey Newtons laws of motion, but
  as a whole they move randomly (any molecule can
  move in any direction with any speed)

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• Ideal gases
• The molecules interact only by short-range
  forces during elastic collisions
• The molecules make elastic collisions with the
  walls and these collisions lead to the
  macroscopic pressure on the walls of the
  container
• At low pressures the behavior of molecular gases
  approximate that of ideal gases quite well

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Ideal gases
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• Ideal gases
• Root-mean-square (RMS) speed

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• Translational kinetic energy
• Average translational kinetic energy
• At a given temperature, ideal gas molecules have
  the same average translational kinetic energy
• Temperature is proportional to the average
  translational kinetic energy of a gas

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• Internal energy
• For the sample of n moles, the internal energy
• Internal energy of an ideal gas is a function of
  gas temperature only

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Chapter 10 Problem 30
A tank having a volume of 0.100 m3 contains
helium gas at 150 atm. How many balloons can the
tank blow up if each filled balloon is a sphere
0.300 m in diameter at an absolute pressure of
1.20 atm?
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• Distribution of molecular speeds
• Not all the molecules have the same speed
• Maxwells speed distribution law
• Nv?v fraction of molecules with speeds in the
  range from v to v ?v

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• Distribution of molecular speeds
• Average speed
• RMS speed
• Most probable speed

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Questions?
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• Answers to the even-numbered problems
• Chapter 10
• Problem 28
• 3.0 mol
• (b) 1.80 1024 molecules

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Answers to the even-numbered problems Chapter 10
Problem 42 3.34 105 Pa