1. The Laws of Thermodynamics
Physics 2053
Lecture Notes
The Laws of Thermodynamics (01 of 38)

2. 1) The First Law of Thermodynamics
Topics
1) The First Law of Thermodynamics
2) Work Done on a Gas
3) Pressure - Volume Graph
4) Thermodynamic Processes
5) The Second Law of Thermodynamics
6) Heat Engines
7) Carnot cycle
8) Entropy
The Laws of Thermodynamics (02 of 38)

3. First Law of Thermodynamics
System U Q W
Environment
DU = Q + W
The Laws of Thermodynamics (03 of 38)

4. Work Done on a Gas F W = F Dx DV = A Dx W= PA Dx Area Dx W = P DV DV
The Laws of Thermodynamics (04 of 38)

5. Pressure - Volume GraphT4 T3
Isotherms
(lines of constant
temperature) T2 P T1
Area under curve represents work
Pressure
Internal energy
is proportional
to temperature
Volume V
The Laws of Thermodynamics (05 of 38)

6. Thermodynamic Processes
A. Isobaric
B. Isovolumetric
C. Isothermal
D. Adiabatic
The Laws of Thermodynamics (06 of 38)

7. Thermodynamic Processes
Isobaric Expansion (Constant Pressure) P Po
a) W = PDV T4
b) DU increases T3 T2 T1
c) Q = DU + W DV V
DU = Q + W
The Laws of Thermodynamics (07 of 38)

8. Thermodynamic Processes
Isobaric Compression (Constant Pressure)
a) W = PDV P Po
b) DU decreases T4 T3
c) Q = -(DU + W) T2 T1 DV V
DU = Q + W
The Laws of Thermodynamics (08 of 38)

9. Thermodynamic Processes
Isovolumetric (Constant Volume)
Decrease in Pressure P
a) W = 0 Po Pf
b) DU decreases T3
c) Q = DU T2 T1 V
DU = Q + W
The Laws of Thermodynamics (09 of 38)

10. Thermodynamic Processes
Isovolumetric (Constant Volume)
Increase in Pressure
a) W = 0 P
b) DU increases Po Pf
c) Q = DU T3 T2 T1 V
DU = Q + W
The Laws of Thermodynamics (10 of 38)

11. Thermodynamic Processes
Isothermal (Constant Temperature)
Expansion Po Pf Vo Vf P
b) DU = 0 T3
c) Q = -W T2 T1 V
DU = Q + W
The Laws of Thermodynamics (11 of 38)

12. Thermodynamic Processes
Isothermal (Constant Temperature)
Compression Po Pf Vo Vf P
b) DU = 0 T3
c) Q = W T2 T1 V
DU = Q + W
The Laws of Thermodynamics (12 of 38)

13. Thermodynamic Processes
Adiabatic (No Heat Exchange)
Expansion P Po Pf Vo Vf
a) W = DU
b) DU decreases T2
c) Q = 0 T1 V
DU = Q + W
The Laws of Thermodynamics (13 of 38)

14. Thermodynamic Processes
Adiabatic (No Heat Exchange)
Compression P Po Pf Vo Vf
a) W = DU
b) DU increases T2
c) Q = 0 T1 V
DU = Q + W
The Laws of Thermodynamics (14 of 38)

15. Thermodynamic Processes
An ideal gas expands to 10 times its original volume,
maintaining a constant 440 K temperature. If the gas
does 3.3 kJ of work on its surroundings, how much heat
does it absorb? T P V Po Pf Vo Vf
In an isothermal process DU = 0
The Laws of Thermodynamics (15 of 38)

16. Thermodynamic Processes (n/a)Po
2Po Vo
2Vo
3Vo II
An ideal gas initially has pressure Po, at volume Vo and absolute temperature To. It then undergoes the following series of processes:
III I IV
I. Heated, at constant volume to pressure 2Po
II. Heated, at constant pressure to pressure 3Vo
III. Cooled, at constant volume to pressure Po
IV. Cooled, at constant pressure to volume Vo
The Laws of Thermodynamics (16 of 38)

17. Thermodynamic Processes (n/a)Po
2Po Vo
2Vo
3Vo
2To II
6To
III I To IV
3To
Find the temperature at each end point in terms of To
The Laws of Thermodynamics (17 of 38)

18. Thermodynamic Processes (n/a)II
2Po
III I
Find the net work done by the gas in terms of Po and Vo Po IV Vo
2Vo
3Vo
Net work equals net area under curve
The Laws of Thermodynamics (18 of 38)

19. Thermodynamic Processes (n/a)Po
2Po Vo
2Vo
3Vo II
III I
Find the net change in internal energy in terms of Po and Vo IV
The Laws of Thermodynamics (19 of 38)

20. The Second Law of Thermodynamics
The second law of thermodynamics is a statement about which processes occur and which do not. There are many ways to state the second law; here is one:
Heat will flow spontaneously from
a hot object to a cold object.
It will not flow spontaneously from
a cold object to a hot object.
The Laws of Thermodynamics (20 of 38)

21. The Second Law of Thermodynamics
Direction of Time
The Laws of Thermodynamics (21 of 38)

22. Heat Engines
In a heat engine; mechanical energy can be obtained from thermal energy when heat flows from a higher temperature to a lower temperature.
Work done by engine:
High Temp. Th Qh
Thermal efficiency:
Engine W Qc
Low Temp. Tc
The Laws of Thermodynamics (22 of 38)

23. Work done by engine each cycle
Heat Engines
Th= 550 K Tc
Engine Qh Qc W
= 630 J
= 920 J
For the engine
Work done by engine each cycle
The efficiency of the engine
The Laws of Thermodynamics (23 of 38)

24. P V The Carnot Cycle Isothermal Expansion Th Adiabatic Expansion
Adiabatic Compression Tc V
Isothermal Compression
The Laws of Thermodynamics (24 of 38)

25. P Work V The Carnot Cycle High Temp. Th Qh Th Engine W Qc Low Temp. Tc
Carnot efficiency:
The Laws of Thermodynamics (25 of 38)

26. Work done by engine each cycle
The Carnot Cycle
Th= 550 K Tc
Engine Qh Qc W
= 470 J
= 890 J
For the engine
Work done by engine each cycle
The efficiency of the engine
The Laws of Thermodynamics (26 of 38)

27. Temperature of the cool reservoir based on a ratio
The Carnot Cycle
Th= 550 K Qh
= 890 J
Temperature of the cool reservoir based on a ratio
Engine
W = 420 J Qc
= 470 J Tc
The engine undergoes 22 cycles per second,
its mechanical power output (N/A)
The Laws of Thermodynamics (27 of 38)

28. A carnot engine absorbs 900 J of heat each cycle and provides 350 J
The Carnot Cycle Th
A carnot engine absorbs 900 J of
heat each cycle and provides 350 J
of work Qh
= 900 J
Engine
W = 350 J
The efficiency of the engine Qc Tc
The heat ejected each cycle
The Laws of Thermodynamics (28 of 38)

29. A carnot engine absorbs 900 J of heat each cycle and provides 350 J
The Carnot Cycle Th
A carnot engine absorbs 900 J of
heat each cycle and provides 350 J
of work Qh
= 900 J
Engine
W = 350 J
The engine ejects heat at 10 oC
The temperature of the hot reservoir Qc
=550 J
Tc = 283 K
The Laws of Thermodynamics (29 of 38)

30. The Carnot Cycle
Th= 650 K
Tc= 300 K
Engine Qh Qc
W = ?
= 400 J
A carnot engine operates between a hot reservoir at 650 K and a cold reservoir at 300 K. If it absorbs 400 J of heat at the hot reservoir, how much work does it deliver? =
The Laws of Thermodynamics (30 of 38)

31. Entropy
The Laws of Thermodynamics (31 of 38)

32. Another statement of the second law of thermodynamics:
Entropy
Definition of the change in entropy S when an amount of heat Q is added:DD
Another statement of the second law of thermodynamics:
The total entropy of an isolated system never decreases.
When an irreversible process occurs in a closed system,
the entropy S of the system always increases: it never
decreases.
The Laws of Thermodynamics (32 of 38)

33. Natural processes tend to move toward a state of greater disorder.
Entropy
Entropy is a measure of the disorder of a system. This gives us yet another statement of the second law:
Natural processes tend to move toward
a state of greater disorder.
Example: If you put milk and sugar in your coffee and stir it, you wind up with coffee that is uniformly milky and sweet.
No amount of stirring will get the milk and sugar to come back out of solution.
The Laws of Thermodynamics (33 of 38)

34. Thermal equilibrium is a similar process –
Entropy
Another example: when a tornado hits a building, there is major damage.
You never see a tornado approach a pile of rubble and leave a building behind when it passes.
Thermal equilibrium is a similar process –
the uniform final state has more disorder than the separate temperatures in the initial state.
The Laws of Thermodynamics (34 of 38)

35. Another consequence of the second law:
Entropy
Another consequence of the second law:
In any natural process, some energy
becomes unavailable to do useful work.
If we look at the universe as a whole, it seems inevitable that, as more and more energy is converted to unavailable forms, the ability to do work anywhere will gradually vanish. This is called the heat death of the universe.
The Laws of Thermodynamics (35 of 38)

36. SUMMARY First Law of Thermo Thermo Processes
Isothermal = temp is constant
Adiabatic = no heat exchanged
Isobaric = pressure is constant
Isochoric (isovolumetric) = volume is constant

37. SUMMARY cont.
Heat engines change heat into useful work (requires a temperature difference)
Efficiency of a heat engine:
Carnot effeciecy:

38. Second law of thermodynamics:
Summary
Second law of thermodynamics:
heat flows spontaneously from a hot object to a cold one, but not the reverse
Thermal energy cannot be changed entirely to work
natural processes tend to increase entropy.
Change in entropy:
Entropy is a measure of disorder.
As time goes on, less and less energy is available to do useful work.
The Laws of Thermodynamics (37 of 38)

39. END