Announcements 9/19/12


Prayer
Exam 1 starts a week from Saturday!
a. A will send out a link for a doodle.com
survey about exam review times; please
vote for times before Friday
Frank &
Ernest
From warmup

Extra time on?
a. a variety of things; no single thing in
particular

Other comments?
a. Why do Physicists always seem to enjoy
writing laws with the most confusing
wording they could possibly have used?
b. This may not be part of this lecture, but I
don’t understand how to find work when it's
an adiabatic process.
Demo/Video


Demo: Adiabatic cotton burner, again
Video: Mist from pressurized bottle
Special Processes: Review
Constant volume (isovolumetric)
 Constant pressure (isobaric)
 Constant temperature (isothermal)
 No heat added (adiabatic)
 General process?
P
a. Eint = ?
A
b. Won gas = ?
c. Qadded = ?

B
V
Last Special Case: Cyclical Processes
P
State 1
V



Eint = ?
Is it Wby gas or Won gas?
Qadded = ?
Engines: Energy Transformation
work
heat in
(higher T)
engine
exhaust out
(lower T)



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



How good is your engine?
Definition
Second Law (Kelvin-Plank form)
From warmup

Give your own example of some process or event where the
first law of thermodynamics is not violated, but the event
will not happen because it violates the second law of
thermodynamics.
a. An atomic bomb unexploding… The second law defines
time, in a way. It says that a more chaotic situation is
the future, the order in the past. If a process is
reversed, then it follows the first law, but not the
second, as it would result in an increase of energy
decrease of entropy.
b. My examples:
– All of the hot molecules in the air in my room
suddenly congregate right near my body and cause
me to spontaneously combust.
– A ball on the floor suddenly gathers kinetic energy
from the surrounding molecules in the floor. This
causes the floor to cool down a small amount and the
ball leaps into the air.
Worked Problem


A car engine produces 5000 W of power, at
20 cycles/second. Its efficiency is 20%. What
are |Wnet|, Qh, and Qc per cycle?
What do those quantities represent?
Otto Cycle: Gas Engines
Let’s ignore intake and exhaust strokes

A to B: Piston is compressed quickly

B to C: Heat is then added quickly by
igniting fuel (const volume)

C to D: Piston then expands quickly

D to A: Heat is then expelled quickly
(exhaust valve opened)
Warmup:

Work is done on the gas from A to B. How can an engine work if
we are doing work on it instead of getting work out?
a. We do a lot less work on it (area below lower curve) than we
get out (upper curve).
 In fact, what is the net work?
Otto Cycle: Gas Engines
Analysis:
→ We pretend heat is added
without changing gas
Wnet = Qh – Qc
e = Wnet/Qh
“Compression ratio” r = Vmax/Vmin
Derived in book:
eOtto  1 
1
r
 1
Diesel Cycle: Diesel Engines



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
A problem like this will be on the exam



Make up a “three-legged cycle”.
What is the efficiency of this cycle?
Game plan:
a. Calculate Q for each leg
b. Calculate Qh, Qc, |W|
c. e = |W|/Qh
d. [Next class period: Test to make sure e < emax]
Clicker question:

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