1. Temperature, Heat, and the First Law of Thermodynamics
Chapters 19, 20
Temperature, Heat, and the First Law of Thermodynamics

2. 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
William Thomson
(Lord Kelvin) ( )

3. 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

4. The zeroth law of thermodynamics
If two (or more) bodies in contact don’t 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

5. 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

6. Constant-volume gas thermometer

7. Celsius and Fahrenheit scales
Celsius scale:
Fahrenheit scale:
Anders Cornelius
Celsius
( )
Gabriel Daniel
Fahrenheit
( )

8. Thermal expansion
Thermal expansion: increase in size with an increase of a temperature
Linear expansion:
Volume expansion:

9. Thermal expansion

10. Chapter 19
Problem 5
A copper telephone wire has essentially no sag between poles 35.0 m apart on a winter day when the temperature is – 20.0°C. How much longer is the wire on a summer day when T = 35.0°C?

11. 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):

12. Q Q Absorption of heat Specific heat (c): heat capacity per unit mass
Common states (phases) of matter: solid, liquid, gas
Latenet heat (L): the amount of energy per unit mass transferred during a phase change (boiling, condensation, melting, freezing, etc.) Q Q

13. Absorption of heat Q Q

14. Absorption of heat

15. Absorption of heat

16. Chapter 20
Problem 17
A 1.00-kg block of copper at 20.0°C is dropped into a large vessel of liquid nitrogen at 77.3 K. How many kilograms of nitrogen boil away by the time the copper reaches 77.3 K? (The specific heat of copper is cal/g °C. The
latent heat of vaporization of nitrogen is 48.0 cal/g.)

17. Avogadro’s 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 Avogadro’s 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
Amedeo Avogadro
( )

18. 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
Ludwig Eduard
Boltzmann
( )

19. Heat and work
Thermodynamic cycle

20. Heat and work
Work is done by the system:
Work is done on the system :

21. The first law of thermodynamics
Work and heat are path-dependent quantities
Quantity Q + W = ΔEint (change of internal energy) is path-independent
1st law of thermodynamics: the internal energy of a system increases if heat is added to the system or work is done on the system

22. The first law of thermodynamics
Adiabatic process: no heat transfer between the system and the environment
Isochoric (constant volume) process
Free expansion:
Cyclical process:

23. Chapter 20
Problem 29
Consider the cyclic process depicted in the figure. If Q is negative for the process BC and ΔEint is negative for the process CA, what are the signs of Q, W, and that are associated with each process?

24. Heat transfer mechanisms
Thermal conduction
Conduction rate:
Thermal resistance:
Conduction through a composite rod:
Thermal conductivity

25. Heat transfer mechanisms
Thermal radiation
Radiation rate:
Stefan-Boltzmann constant:
Absorption rate:
Emissivity
Josef Stefan
( )

26. Chapter 20
Problem 46
At high noon, the Sun delivers 1000 W to each square meter of a blacktop road. If the hot asphalt loses energy only by radiation, what is its steady-state temperature?

27. Heat transfer mechanisms
Convection

28. Heat transfer mechanisms

29. Questions?

30. Answers to the even-numbered problems
Chapter 19
Problem 2
810°F
(b) 450 K

31. Answers to the even-numbered problems
Chapter 19
Problem 6
1.20 cm

32. Answers to the even-numbered problems
Chapter 19
Problem 18
(a) 2.99 mol
(b) 1.80 × 1024 molecules

33. Answers to the even-numbered problems
Chapter 20
Problem 26
(a) 12.0 kJ
(b) –12.0 kJ