1. WilliamThompson (Lord Kelvin)
Thermodynamics
Understanding the words
Temperature
Heat
Heat capacity
The 0, 1, 2 laws of thermodynamics
(one of) Kelvin’s legacy’s
WilliamThompson (Lord Kelvin)

2. What is Heat?
I feel hot
Perception as to hot and cold defined relative to our own body temperature, i.e. object is hotter or colder than oneself
Objective measurement of temperature
Macroscopic, display of temperature gauge
Microscopic behaviour of atoms and molecules

3. Measuring temperature
Properties of materials change with temperature
Length
Volume
Resistance

4. Hotter things become longer
Solids get bigger when they get hot
A 1 metre long bar heated by 1 degree gets bigger by
Steel ≈0.01 mm
Glass ≈ mm
Rails expand and may buckle on a hot summer day

5. Hotter things take up more volume -1
Most materials get bigger when they get hot (but not water 0°C -> 4°C gets smaller!)
Thermometer relies on a thermal expansion of a liquid (e.g.mercury)
Thin tube
(Gives big length change for small increase in volume)
Large volume of reservoir

6. Hotter things change their resistance
All hotter metals have a higher electrical resistance or conductivity
Digital thermometer
All hotter semiconductors have a lower electrical resistance
key definition between to distinguish metals and insulators!

7. Example You have a (glass) jar and you can’t get the metal lid off.
What should you do:
a) ask your girlfriend
b) run the jar & lid under cold water
c) run the jar & lid under hot water

8. Solution:
a) ask your girlfriend
b) run the jar & lid under cold water
c) run the jar & lid under hot water
Because the metal has a substantially higher coefficient of thermal expansion than the glass, heating them will make both of them bigger, but the metal will be more bigger.

9. Insulation Example of good (thermal) insulators
A vacuum, polystyrene, fibreglass, plastic, wood, brick
(low density/foam structure, poor electrical conductors)
Examples of poor insulators, i.e. good conductors
Most metals (but stainless steel better than copper) e.g. gold contact used within IC chips to prevent heating
Gases, liquids
(high density, “mobile”, good electrical conductors)

10. Temperature and scales
Temperature scales (melting & boiling of water)
Degrees Celsius (MP 0°C 100°C)
Degrees Kelvin (MP K BP K)
Degree Fahrenheit (MP 32° F BP 212°F)
Explain how a temperature scale is constructed.

11. The Common Temperature Scales
Fahrenheit & Celsius
Celsius & Fahrenheit
The Celsius & Fahrenheit scales are based on the phase transition temperatures for pure water

12. Converting between scales
State the relation between Kelvin & Celsius Scales.
Kelvin to Celsius
K = C
C = K
Fahrenheit to Celsius (Not IB)
F = C x (9/5) + 32
C = (F - 32) x (5/9)

13. Example Convert the following temperatures into °F and K
Boiling water, 100°C
Freezing water, 0°C
Absolute zero, °C
212°F, K
32°F, K
-460°F, 0K

14. Stop here

15. Type of thermometer
Change in electrical resistance (convenient but not very linear)
Change in length of a bar (bimetallic strip)
Change in volume of a liquid
Change in volume of gas (very accurate but slow and bulky)

16. Heat and internal energy
Can you describe the difference between the terms.
Temperature
Heat
Internal Energy

17. Temperature & Absolute Temperature
Temperature is a property that determines the direction of thermal energy transfer between two bodies in thermal contact.
Absolute temperature is a measure of the average kinetic energy of the molecules of a substance.
Average kinetic energy is proportional to absolute temperature in Kelvin.

18. Example
What is the kinetic energy of an oxygen molecule at room temperature ( 21˚C)?
(k = 1.38x10-23J/K)
Since we know the kinetic energy, how is it travelling?
This is called the root mean squared speed or rms speed.
KE = 3/2 kT
= 3/2(1.38x10-23 x 294)
= 6.09 x Joules
We could equate
KE = 1/2 mv2 = KE = 3/2 kT
and get v2 = 3kT/m
mass must be in kg!!!! Not u.

19. Heat (Energy) Is the flow of energy in or out of a system.
Heat (energy) flows because of temperature difference
Bigger temperature difference bigger heat flow
Less insulation give more heat flow for the same temperature difference
Heat will not flow between two bodies of the same temperature

20. Equilibrium
Two objects of different temperature when placed in contact will reach the same temperature + =
Cold milk
Light brown coffee
Hot black coffee

21. Heat transfer = energy transfer
Energy measured in Joules but heat often measured in Calories
One cal raises one gram of water from 14.5°C to 15.5°C
1 cal = 4.186J
Doing work on something usually makes it hot
Joules Experiment!
1st law of thermodynamics heat and work are both forms of energy

22. Sir James Joule James Prescott Joule 1818-1889
Stirring water made it warm
Change in temperature proportional to work done
Showing equivalence of heat and mechanical energy
Also that electrical current flow through a resistor causes heating

23. Joules Experiment

24. Internal Energy
Is the total potential and kinetic energy of the molecules in a substance.
Potential energy is associated with intermolecular forces.
Kinetic energy includes both translational and rotational motion.

25. Three Phases – Atomic Model
Three States of Ordinary Matter
Solid ® liquid ® gas

26. Atomic Model of Matter Comparing Molecular Forces
Solid – Largest molecular forces
Liquid
Gas – Weakest molecular forces
When the kinetic energy of the molecules become comparable to the energy required for separation the molecules change there position and separate (PE increase). This is a phase transition
Melting or vaporizing

27. Avogadro constant
One mole a any substance is that quantity of the substance whose mass in grams is numerically equal to the substances molar mass, μ.
EX: The moloar mass of O2 is 32 g mol-1
NA = 6.02 x 1023 molecules mol-1

28. Example
How many grams are there in a quantity of oxygen gas containing 1.2 x 1025 molecules?
The number of moles is
(1.20 x 1025)/6.02 x 1023
= mol
Since the molar mass is 32 g mol-1
The mass is x 32
= 638 g or kg

29. Example So, how fast is that O2 molecule traveling? O2 = 32 g/mole
v2 = 3kT/m (rms speed of a molecule)
m = 0.032/(6.023 x 1023) = 5.3x10-26 kg v2 = 3(1.38 x 10-23J/K)(294)/(5.3x10-26 kg)
v = 479 m/sec

30. Transferring heat energy
3 mechanisms
Conduction
Heat transfer through material
Convection
Heat transfer by movement of hot material
Radiation
Heat transfer by light

31. Conduction of heat Conduction in solids Conduction in metal
Heat energy causes atoms to vibrate, a vibrating atom passes this vibration to the next
Conduction in metal
Heat energy causes electrons to gain energy, electrons travel through metal (conduction) and carry heat energy with them
Metals are good conductors of both heat and electricity

32. Conduction of heat
The atoms at the bottom are at a higher temperature and will oscillating more strongly than those at the top.

33. Rate of heat flow
Heat flow (H) is energy transfer per unit time, depends on
Temperature difference
Thermal conductivity (k)
k (copper) = 385 W/(m K)
k (glass) = 0.8 W/(m K)
k (air) =0.02 W/(m K) A TH TC L

34. Example
Two rods of the same cross-sectional area are joined together. The right rod is a better conductor of heat than the rod at left. The ends are kept at fixed temperatures.
In which rod is the rate of heat transfer the largest?
Is the temperature at the joining point lower are higher than 54 ˚C?
Heat entering the joint must equal the heat leaving the joint. (Conservation of Energy). Hence, the rate of heat transfer is the same.
Since the second conductor is poor a much larger temperature gradient can be maintained. Thus, the temperature at the junction will be larger.

35. Thermal conduction vs thermal resistance
Also can use thermal resistance, cf
Can make equation of heat flow more general

36. Convection of heat “Hot air rises” (and takes its heat with it!)
Radiators

37. Convection of heat “Hot air rises” (and takes its heat with it!)
Cumulus clouds

38. Figure 16-11 Alternating Land and Sea Breezes

39. Convection of heat
Hurricanes
Plate tectonics

40. Radiation of heat Don’t confuse with radioactivity
Instead realize that light carries heat (e.g. the sun heats the earth)
Anything above absolute zero radiates heat
P a AT4 Stefan-Boltzmann law.

41. Radiation of heat
involves the generation and absorption of photons. Unlike conduction or convection, however, radiation requires no intervening medium to transport the heat.
All objects radiate energy continuously in the form of electromagnetic waves
The hotter an object the more power it radiate sand the shorter the wavelength of the peak emission wavelength

42. Not all things emit heat the same
Heat emission from an object area A
P = AesT4
s = Stafan’s constant = 5.6x10-8 W/(m2 K4)
e = emissivity of a body, 0 -1
ecopper = 0.3
ecarcoal ≈ 1

43. Example Estimate the upper limit to the heat emission of the sun
Suns temperature 7000k
Sun’s radius 7x108m
Emission, P = AesT4
Area = 4pr2 = 6.2 x 1018 m2
Emissivity ≈ 1
H = 6.2 x 1018 x 5.6x10-8 x 70004
Sun’s output = 8.3 x 1026 W

44. Are heat emitter also good absorbers?
Black and dull on the surface
Best emitter/absorber
Charcoal
Blackbody radiators
perfect absorber & emitter
White and polished/shiny
Good Reflectors
Stay cool in the summer

45. Figure 16-12 The Thermos Bottle
Discuss the operation of a thermos making reference to methods of heat exchange.

46. Assignment
Questions from Packet. 1, 2, 3, 5, 7, 10, 11, 12, 14, 18, 19.

47. The “colour” of heat
Peak wavelength of light emitted depends on temperature
Spectrum includes all wavelength longer than the peak but not many above
20°C - peak in infrared (need thermal imaging camera to see body heat)
800°C - peak in red (electric coil, fire glows reds)
3000° - peak in blue (but includes green and red light hence appears white)
2.7K peak in micro-wave (background emission in the universe left over from the Big Bang)