Transcript 151c15

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
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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
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Refrigeration: getting heat to flow
from cold to hot requires work!
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.
High temperature Reservoir
QH
W
QC
Low temperature Reservoir
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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
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Isobaric process: process at constant pressure
W = p(V2  V1)
Other processes:
W = area under the curve on a pressure-volume (p-V) diagram
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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 0.600 kg/m3.
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Indicator Diagrams: p-V diagrams used to analyze cyclic
processes which use a gas in a heat engine.
p
p
Work done
by system
V
V
p
p
Net work done
by system equals
enclose area
Work done
on system
V
V
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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!
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High temperature Reservoir
QH
High temperature Reservoir
QC
W
QC
Low temperature Reservoir
Low temperature Reservoir
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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
p
V
Carnot cycle is made with only
reversible processes => “most efficient
heat engine possible”
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The most efficient engine cycle operating
between two specified temperatures:
Carnot Cycle
p
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.
a
b
d
c
V
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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 :
QC
W QH  QC
Eff 

 1
QH
QH
QH
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For the Carnot Engine only:
Q is proportional to T for both isothermal processes, so
TC QC

TH QH
so
Eff carnot  1 
TC
TH
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?
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