W

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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!
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
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Work done during volume changes
Expanding gas in a piston Force and pressure p
F/A gt 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.
Work done by system
Net work done by system equals enclose area
Work done on system
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! gt no 100 efficient
heat engines!
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High temperature Reservoir
High temperature Reservoir
QC
QH
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
• Carnot cycle is made with only reversible
  processes gt most efficient heat engine possible

p
V
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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.
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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
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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?