general physics

1. Chapter 18
Temperature, Heat, and the First Law
of Thermodynamics

2. Things to learn
ƒ We will learn concepts useful in
thermodynamics (temperature, heat, work,
and heat capacity) and relations among them.
ƒ We will learn laws of thermodynamics and
their applications:
- Zeroth law: T equal at thermal equilibrium
- First law: energy conservation
- Adiabatic, constant-volume, cyclical, free-expansion
processes.
ƒ We will learn effects of heat:
- Change in T
- Change of phase (heat of transformation)
- Thermal expansion
ƒ We will learn modes of heat transfer.

3. 18-2 Temperature
Thermodynamics
=Thermal physics
Temperature
Hot? Cold?
Scale?
Kelvin (K) in SI unit
[273 + Celsius]
Universe was very hot right
after Big Bang(1039 K)
Now about (3K)

4. 18-3 The zeroth Law of
Thermodynamics
If bodies A and B are each in
thermal equilibrium with a
third body T, then they are in
thermal equilibrium with each
other.

5. 18-4 Measuring temperature
Practical standard
0 degrees Celsius : freezing point of water
100 degrees Celsius : boiling point of water
Easily reproducible; these may vary!
Scientific reference scale
T of Triple-point of water is defined by
273.16 K
International agreement
reproducible

6. Calibrating thermometers
Triple-point of water
= vapor, water, and
ice are in equilibrium
(273.16 K)
Make a gas
thermometer in
equilibrium with a
triple-point water
cell which has a
definite temperature

7. Constant-volume gas
thermometer
Use the fact: T= C p
T: temperature
C: constant if the
volume of a gas is fixed
P: pressure

8. Constant-volume gas thermometer
gives correct value as p3 Æ 0

9. 18-5 Celsius & Fahrenheit

10. 18-6 Thermal expansion
ƒ Thermal expansions
-Expansion slots for sections of the
bridge
-Dental cavity filling: thermal
expansion property should be
matched.
-Bimetal strip
ƒ Linear expansion
ƒ Volume expansion

11. SP 18-2
ƒ A oil trucker loaded 37,000 L of diesel
fuel in Las Vegas and delivered his
entire load in Payson, UT (23 oC lower).
ƒ β of diesel fuel = 9.5 × 10-4 /oC
ƒ α of steel truck tank = 11 × 10-6 /oC
ƒ How many liters did he deliver?

12. 18-7 Temperature and Heat
Heat is the energy transferred between
system and environment
because of the temperature difference
a) System is hotter than environment
Æ System loses heat (Q<0)
b) Same temperature (in equilibrium)
Æ No heat flows (Q=0)
c) System is colder than environment
Æ System gains heat (Q>0)

13. 18-8 The absorption of heat by
solids and liquids

14. ƒ Heat Capacity C
ƒ Specific Heat (heat capacity per unit mass)
ƒ Molar Specific Heat (heat capacity per one mole)
• 1 mol = 6.02 X 1023
ƒ An important point:
• constant pressure vs. constant volume
ƒ Heat of Transformation
- Heat of vaporization
- Lv=539 cal/g = 40.7 kJ/mol = 2256 kJ/kg (water)
- Heat of fusion
- LF=79.5 cal/g = 6.01 kJ/mol = 333 kJ/kg (water)

15. Molar Specific Heat
Problem 23

16. SP 18-3

17. 18-9 A closer look at Heat and
Work

18. P-V plot

19. Problem 43

20. 18-10 The first law of Thermodynamics
18-11 Special cases
Adiabatic process:
no heat transfer
Constant volume process:
no work

21. Cyclical process = [Q=W]
Returns to the same state:
If the system received a
certain amount of heat,
the same amount of work
has been done by the
system.
Internal energy remains
the same after a cycle.

22. Problem 48 cyclic process

23. Free expansion = [pext=0:W=0]
No heat transfer.
No external pressure.
No work in expansion.
No equilibrium during
the expansion

24. SP 18-5
Boiling water Æ steam
Constant pressure (1atm)
Work done by the gas?

25. 18-12 Heat Transfer
Mechanisms: conduction
Problem 61

26. Conduction through a composite slab
P must be invariant
(energy conservation)
Parallel combination:P=P1+P2 energy is
transferred through two channels

27. Convection
Difference in temperature
Æ
Difference in pressure
Æ
Fluid can flow
(from high pressure to low pressure)
Æ
Heat is transferred

28. Radiation
Electromagnetic wave
e.g. radiation from the Sun
Any object with T > 0 K radiates!

29. SP 18-7
ƒ Ts = -23 oC; Ta = + 6.0 oC
Water: m = 4.5 g, A = 9.0 cm2, d = 5.0 mm
ε = 0.90
ƒ Find the time required for the water
to freeze via radiation. Can the
freezing be accomplished during one
night?