Thermal Physics the big ideas

1
Thermal Physics - the big ideas

• We perform a world wind tour of the big ideas of
  thermal physics.
• The physics of heat.

2
Temperature

• Its what we measure with a thermometer.
• This is an operational definition.
• Its proportional to the average kinetic energy
  of the molecules of a substance.
• More on this later.

3
Temperature Scale

• There are 2 commonly used metric scales
• Celsius (C)
• Centigrade
• Kelvin (K)
• This is the official SI scale.
• We need two bits of info to fix a temperature
  scale.
• Two fixed points (freezing and boiling)
• One fixed point and the unit size.

4
Temperature Scales II

• Examplethe Maverick temperature scale (M).
• Water freezes at 50M
• Water boils at 100M
• This fixes the unit size.
• There are 50 M between freezing boiling.

5
Converting Scales

• The Fahrenheit temperature scale (Chap 22)
• Water freezes at 32F
• Water boils at 212F
• There are 180 F between freezing boiling.
• What is 70M on the Fahrenheit scale?
• 70M is 20 M above freezing.
• Thats 2/5 of the way between freezing boiling!
• Whats 2/5 of the way between freezing boiling
  on the Fahrenheit scale?
• (180 F)2/5 72 F
• This is 72 F above freezing!
• The temperature on the Fahrenheit scale is 32 F
  72 F 104 F!

6
Converting Scales II

• To convert a temperature TA on scale A (A) to TB
  on scale B (B)
• 0) Determine conversion relationship
• TAbp - TAfp TBbp - TBfp
• We will use this to determine a conversion
  factor.
• 1) How far is TA from the freezing point on scale
  A?
• (TA - TAfp)
• 2) Multiply by the conversion factor
• (TBbp - TBfp)/(TAbp - TAfp)
• 3) Add the freezing point temp from scale B
• TBfp
• TB (TA - TAfp) (TBbp - TBfp)/(TAbp - TAfp)
  TBfp

7
Problem 1

• Convert normal body temperature (98.6 F) to C.
• 98.6 F 37 C

8
The Kelvin Scale

• We use a physical property that changes with
  temperature for making thermometers.
• Pressure is one example.
• Color, electrical resistances, size

• Notice the lines converge at a single
  temperature.
• This temperature is called absolute zero (0 K).
• The unit size is the same as for the Celsius
  scale 1 K 1 C

9
The Kelvin Scale II

• 0 K - 273 C
• 1 K 1 C
• You must use the Kelvin Scale whenever you are
  dividing by temperature.
• Examples to follow.

10
Temperature and Kinetic Energy

• Temperature is a macroscopic quantity.
• How does that compare to the microscopic world of
  molecules?
• Molecules are in constant motion.
• They have nonzero kinetic energy.
• The average kinetic energy of the molecules in an
  ideal gas can be computed.

11
Temperature and Kinetic energy

• T must be in Kelvins
• kB is known as Boltzmanns constant
• 1.38 X 10-23 J/K
• rms is the root-mean-square value of something.

12
Problem 2

• What is the rms speed of a nitrogen molecule at a
  temperature of 300 K? Nitrogen is a diatomic
  molecule.
• There is Avogadros number of molecules in one
  mole of nitrogen.
• 6.02 X 1023 molecules/mole
• One mole of N2 has a mass of 214 g 28 g.
• 517 m/s

13
Calorimetry

• Heat is not the same as temperature.
• Heat is energy that is transferred between a
  system and its environment because of a
  temperature between them.

14
Units of Heat

• 1 calorie (cal) is the heat needed to raise the
  temperature of 1 g of water from 14.5 C to 15.5
  C.
• 1 kcal 1 Cal (the dietary calorie).
• The SI unit for heat is the joule!
• 1 cal 4.186 J
• This is known as the mechanical equivalent of
  heat.
• James Prescott Joule and his honeymoon.

15
Specific Heat

• Use Q to represent an amount of heat (energy).
• To change the temperature of a given mass (m) of
  a substance by DT requires the following
• c is the specific heat.
• J/(kgC)
• Water has a high specific heat.

16
Problem

• A 223-g lead ball at a temperature of 83.2 C is
  placed in a light calorimeter containing 178 g of
  water at 24.5C. Find the equilibrium temperature
  of the system.
• 26.7 C

17
Weather is driven by water
18
Change of Phase

• Changing the temperature of an object changes its
  kinetic energy.
• Changing the phase of an object changes its
  internal potential energy.
• Both require heat.

19
Latent Heat

• The heat necessary to change the phase of a
  substance depends only on the mass m.
• The coefficient of proportionality is called the
  Latent Heat.
• L Q/m
• J/kg
• Lf - latent heat of fusion
• Solid to liquid (vice versa)
• Lv - latent heat of vaporization
• Liquid to gas (vice versa)
• Q is positive is the substance is releasing heat
  and negative if it is absorbing heat.

20
Calorimetry - Conservation of energy

• A 105-g copper calorimeter contains 307 g of
  water at room temperature (23 C). If 52 g of ice
  at 0 C is added to the calorimeter, what is the
  final temperature of the system?
• Assume calorimeter is at same initial temp as
  water.
• Heat transfers until everything is at the same
  temperature.
• 8.5 C