Transcript Physics 123 “Majors” Section Unit 1

Announcements 9/12/12
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
Prayer
Register your clickers by today at 5 pm! Unregistered
numbers from Monday: 3368A2F9, 7A8E14E
The Far Side
From warmup
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Extra time on?
a. emissivity (several people)
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Other comments?
a. You know you're a physics major when spell check
underlines more words that it doesn't know, than
words you don't know.
Heat Transfer
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conduction
convection
radiation
Clicker question
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Which of the following does not continuously
emit electromagnetic waves?
a. A light bulb which is turned on
b. A light bulb which is turned off
c. A star
d. A hot electric stove burner
e. None of the above
Blackbody Radiation
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Hot objects glow!
a. That glow carries away
energy
heat energy
time
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
4
Plost  e ATobject
Surroundings also glow!
a. That glow adds energy
4
Pgained  e ATsurroundings
From warmup
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If the temperature of a "black body"
doubles, how much does its rate of energy
emission change?
a. It would be multiplied by a factor of 16
because P is proportional to T^4.
Demo
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Burning ants with magnifying glass
a. OK, not really
Color of emission
“white point”
Chromaticity
diagram
You’ll learn/derive the
equation in Phys 360,
if you take it. Some
results:
Area ~ T4
Peak l ~ 1/T
More on Emissivity
“Fudge factor” between 0 and 1
 Different for different surfaces
a. 0.05 for “highly polished aluminum”
b. 0.8 for “anodized aluminum”
 Same as “absorptivity”
a. Why?
 Different for different wavelengths
a. Greenhouse effect
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Clicker question
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A metal sphere is heated to 1200 K, and puts out
1000 W of radiation energy. If it is cooled to 600
K, it will put out ______ W of radiation energy.
(Don’t worry about heat absorbed from
surroundings. Assume emissivity is the same for the
two temperatures.)
a. 31.25
b. 62.5
c. 125
d. 250
e. 500
From warmup
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To decrease the energy lost by conduction
from a hot object to a cold one, what can
you change?
a. You can increase the thickness or
decrease the cross-sectional area of
whatever connects the two objects. Or,
perhaps the simpler option, add insulation
in between, particularly one such as
fiberglass.
Thermal Conduction
T2
hot
T1
cold
A
L
Q
 T2  T1 
P
 kA 

t
L


Really:
dQ
dT
 kA
dt
dx
Warning: what is
meant by time?
“Steady state” vs. “Thermal equilibrium”
Thermal Conductivity
Some Thermal Conductivities
(from your textbook)
Material
Copper
Aluminum
Iron
Glass
Wood
Air
k (J/s∙m∙C)
397
238
79.5
0.84
0.10
0.0234
What “feels” colder, a metal car or a wooden box?
Demo
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Boiling water in a paper cup
Clicker: To prevent the cup from catching on
fire, would you like it to have a large or small
thermal conductivity?
a. large
b. small
Clicker question
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If I heat the left end of an iron rod such that its
temperature is a constant 200 degrees C, and I
put the right end in ice water, what will the
temperature of the middle of the rod be when the
rod approaches “steady state”?
a. 0 ºC
b. 50 ºC
c. 100 ºC
d. 150 ºC
e. 200 ºC
What if left half of rod is iron, but the right half is copper?
Clicker question
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If I heat one end of an iron rod to 150 degrees C and I
put the other end in ice water, I get a heat flow of 10 J/s
through it. If I do the same with a particular copper rod,
I get 25 J/s. If I stick the two of them together, side by
side, how much heat will flow through the combined rod?
a. 10 Watts or less
b. More than 10 but not greater than 25
c. More than 25 but less than 35
d. 35 Watts
e. More than 35 Watts
T2
hot
iron
Cu
T1
cold
Clicker question
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I put an iron rod and a copper rod end-to-end to
form one long rod. The total heat flow through the
combined rod is 100 J/s. How does the heat flow
(J/s) through the iron compare to the heat flow
through the copper? (kiron = 79.5 W/mC; kCu = 397
W/mC)
a. Piron < Pcopper
b. Piron > Pcopper
c. Piron = Pcopper
T2
hot
T1
cold
iron
Cu
R-values
L
R
k
AT
P
R
Some R-values
(from your textbook)
Material
Brick, 4” thick
Styrofoam, 1” thick
Fiberglass insulation,
3.5” thick
Drywall, 0.5” thick
R (ft2 F hr/Btu)
4
5
Yuck!
10.9
0.45
Why useful: R values of wall materials add
Worked Problem
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You foolishly decide to build the walls of your new
house out of solid aluminum (k = 238 W/mC), 5 cm
thick. As a result, in the wintertime heat leaks out like
a sieve. How much money will this cost you each day?
The inside temp is 70 F (21.1 C), the average
outside temperature is 25 F (-3.9 C). The surface
area is 280 m2. The gas company charges you $0.89
per “therm” (1.055  108 J). Only count heat loss
through conduction.
Class survey: guess the answer
Answer: $27,288
Quick Discussion
Material
Air
Fiberglass
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k (J/s∙m∙C)
0.0234
0.045
If air is such a poor thermal conductor, why
is it beneficial to use fiberglass insulation in
your attic?
From warmup
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Why do you think there are no equations in the
"convection" section of the book?
a. It is too complicated for a simple equation to
explain it.
b. There are too many unknown variables
c. Because they look like this
http://en.wikipedia.org/wiki/Convectiondiffusion_equation
d. (not quite correct) Convection is a theory that
shows how conduction works in fluids.
Convection
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Youtube:
http://www.youtube.com/watch?v=7xWWowXtuvA