Heat Engines
UCSD
Physics 12
(Many slides are from prof. Tom Murphy)
UCSD
Lecture 7
Physics 12
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Heat Engines
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Heat flows from Th to Tc, turning turbine along the way
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Heat Engines
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Hot source of energy
heat energy delivered to sink
Th
ΔQh heat energy delivered from source
externally delivered work:
ΔW = ΔQh – ΔQc
conservation of energy
ΔQc
efficiency =
Cold sink of energy
Tc
ΔW
work done
=
ΔQh heat supplied
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Physics 12
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Let’s extract a lot of
work, and deliver very
little heat to the sink
Th
ΔQh
In fact, let’s demand 100%
efficiency by sending no heat
to the sink: all converted
to useful work
ΔW = ΔQh – ΔQc
ΔQc
efficiency =
Tc
ΔW
work done
=
ΔQh heat supplied
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Power plants these days (almost all of which are heat-engines)
typically get no better than 33% overall efficiency
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Heat Pumps provide a means to very efficiently move heat
around, and work both in the winter and the summer
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Hot entity
(indoor air)
Th
heat energy delivered
ΔQh
heat energy extracted
ΔQc
Cold entity
(outside air or refrigerator)
Tc
Just a heat engine run
backwards…
delivered work:
ΔW = ΔQh – ΔQc
conservation of energy
efficiency =
ΔQh heat delivered
ΔW = work done
efficiency =
ΔQc heat extracted
ΔW = work done
(heat pump)
(refrigerator)
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Participation Question
(write on piece of paper with
name and hand in)
1.  A windmill puts out 1000 Watts of electrical power. It is
used to run a motor at 90% efficiency which lifts an
elevator system which has 80% efficiency? How much
energy is finally given to lifting after 10 seconds of
operating?
•  A. 7200 Joules
•  B. 10000 Joules
•  C. 720 Watts
•  D. None of the above
•  E. Can’t be calculated from these numbers
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Heat Engines
Physics 12
UCSD
Participation Question
(write on piece of paper with
name and hand in)
1.  If you open the door of your refrigerator and leave it
open, the kitchen will
A. Slowly cool down
B. Slowly heat up
C. Stay the same temperature
D. Can’t say from this info; depends upon EER
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