Announcements 9/22/10
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HW solutions 1-8 posted… but running out of space
This is what I see when I grade clicker quizzes:
Quiz 6
No student assigned to transmitter 1B0E1C09
No student assigned to transmitter 1FC8F72
No student assigned to transmitter 8FF4FB8
No student assigned to transmitter 13149F98
No student assigned to transmitter 7586837
No student assigned to transmitter 1B5B3D7D
No student assigned to transmitter 141E909A
No student assigned to transmitter 13CA2FF6
Question 1 Average score: 1.00/ 1 ( 38 students)
Question 2 Average score: 1.76/ 2 ( 41 students)
Question 3 Average score: 1.00/ 1 ( 41 students)
Question 4 Average score: 1.49/ 2 ( 41 students)
Question 5 Average score: 1.00/ 1 ( 40 students)
Question 6 Average score: 1.00/ 1 ( 41 students)
Question 7 Average score: 1.00/ 1 ( 41 students)
Question 8 Average score: 1.00/ 1 ( 40 students)
Average score on quiz: 9.24/10
Deadline to get
your clicker
registered: start
of exam (next
week)
Quick writing
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When I expand a gas from one volume to
another, the final pressure will be greater if it
is expanded isothermally than if it is
expanded adiabatically. Explain why without
using equations.
Special Processes: Review
Constant volume
 Constant pressure
 Constant temperature
 No heat added
 General process?
a. Eint = ?
b. Won gas = ?
c. Qadded = ?
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Last Special Case: Cyclical Processes
P
State 1
V
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Eint = ?
net Wby gas or Won gas? |W| = ?
Qadded = ?
Engines: Energy Transformation
work
heat
engine
exhaust
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Qin = work by gas + Qout
Notation: Qh, Qc, Th, Tc,
|W|
Qh = |W| + Qc
Heat Engine Example
P
(a) - piston at
room temp and
atmospheric
pressure
a
V
Heat Engine Example
P
(a-b) - gas
heated to keep
volume
constant as
cart rolls onto
piston
b
a
V
Heat Engine Example
P
(b-c) - gas
heated to
increase
volume and lift
the cart
c
b
a
V
Heat Engine Example
P
(c-d) - gas
cooled to keep
volume
constant at
cart rolls off
piston
b
c
a
d
V
Heat Engine Example
P
(d-a) - gas
cooled to
reduce volume
back to initial
level
b
c
a
d
V
Efficiency
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How good is your engine?
Definition
Worked Problem
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A car engine produces power of 5000 W, at
20 cycles/second. Its efficiency is 20%. What
are |W|, Qh, and Qc per cycle?
What do those quantities represent?
Reading Quiz (graded)
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What is the name we use for the cycle which
models how a gas engine works?
a. Carnot cycle
b. Brayton cycle
c. Diesel cycle
d. Otto cycle
e. Tri cycle
Otto Cycle: Gas Engines
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(Starting at 2) Piston
is compressed
quickly
Heat is then added
quickly by igniting
fuel (const volume)
Piston then expands
quickly
Heat is then
expelled quickly (by
getting rid of old air)
→ In our approxiImage credit: http://www.grc.nasa.gov/WWW/K-12/airplane/otto.html
mation we pretend
like same air is
“Compression ratio” r = Vmax/Vmin
reused
Derived in book:
eOtto  1 
1
r
 1
Thought Question
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If I replaced all of the nitrogen (N2) in the air
with carbon dioxide (CO2), what do you think
would happen to the efficiency of car
engines?
a. They would get more efficient
b. They would get less efficient
c. The efficiency would not change
eOtto  1 
1
r
 1
Diesel Cycle: Diesel Engines
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What’s the main difference between gas and
diesel engines?
Change to our PV-diagram model
Diesel cycle details… done in HW problem
Worked problem: Class designed
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Make up a “three-legged cycle”.
What is the efficiency of this cycle?
Game plan:
a. Calculate Q for each leg
b. Calculate Qin, Qout, |W|
c. e = |W|/Qin
d. Test to make sure e < emax