1. Chapter 15: Thermodynamics
Thermodynamics: how heat is converted to and from other forms of energy, especially mechanical energy.
Heat engine: a process or system which converts heat into mechanical energy.
High temperature Reservoir
1. Heat (QH) is absorbed from a source at high temperature.
2. Mechanical work (W) is done (by converting some of the absorbed heat to mechanical work).
3. Heat (QC) is given off at a lower temperature QH W QC
Low temperature Reservoir

2. The first law of thermodynamics:
Energy is Conserved!
Net heat input = change in internal energy + net work output
Q = U + W
Cyclic Processes:
repeating process in which the system or heat engine returns to the starting point (same thermodynamic state) each cycle.
A Cyclic Process is necessary for most practical heat engines.
Over each complete cycle
U = 0
net heat input = net work output

3. Refrigeration: getting heat to flow from cold to hot requires work!
High temperature Reservoir QH
1. Heat (QC) is absorbed from a source at low temperature.
2. Mechanical work (W) is done on the system (work is input).
3. Heat (QH) is given off to the higher temperature reservoir. W QC
Low temperature Reservoir

4. Work done during volume changes
Expanding gas in a piston
Force and pressure
p = F/A => F = pA
Work = force x distance
W = F s = pA s
but A s is just the extra volume of gas, so
W = pV

5. Isobaric process: process at constant pressure
W = p(V2  V1)
Other processes:
W = area under the curve on a pressure-volume (p-V) diagram

6. Example 15.1: The heat of vaporization of water at atmospheric pressure is Lv = 2260 kJ/kg. How much of this heat represents work done to expand the water into steam against the pressure of the atmosphere? At T = 100 ºC an p = 1 atm, the density of water is 1.00x103 kg/m3 and the density of steam is kg/m3.

7. Indicator Diagrams: p-V diagrams used to analyze cyclic processes which use a gas in a heat engine.
Work done by system p p V
Net work done by system equals enclose area
Work done on system V

8. The Second Law of Thermodynamics
The Natural tendency of all physical systems is towards “disorder” (increasing entropy)
The entropy of a closed system can never decrease!
The natural direction of heat flow is from a reservoir of internal energy at a high temperature to a reservoir of energy at a low temperature.
Heat flow from Hot to Cold!
Major Consequence:
It is impossible to construct a heat engine which operates in a cycle that does nothing other than take in heat from a source and perform an equivalent amount of work!
=> no 100% efficient heat engines!

9. High temperature Reservoir High temperature Reservoir QC QHW QC
Low temperature Reservoir
Low temperature Reservoir

10. The Carnot Engine Cycle some types of processes
Isobaric process: occurs at constant pressure
Isochoric or Isovolumetric process: occurs at constant volume
Isothermal process: occurs at constant temperature
Adiabatic process: occurs with no heat transfer
Carnot cycle is made with only reversible processes => “most efficient heat engine possible” p V

11. a-b : Isothermal Expansion at TH. |QH| = proportional to TH d c
The most efficient engine cycle operating between two specified temperatures: Carnot Cycle
a-b : Isothermal Expansion at TH.
|QH| = proportional to TH
(absolute temperature!)
b-c : Adiabatic Expansion to TC.
c-d : Isothermal Compression at TC.
|QC| proportional to TC
d-a : Adiabatic Compression to TH. p V

12. Engine Efficiency
net mechanical work comes from net transfer of heat
W = QH  QC
Efficiency is the effectiveness with which supplied heat QH is converted to work :

13. For the Carnot Engine only:
Q is proportional to T for both isothermal processes, so
Example: Steam enters a steam turbine at 570 ºC and emerges into partial vacuum at 95 ºC . What is the upper limit to the efficiency of this engine?