Announcements 9/15/10

There still are students with unregistered clickers.
Here are the students who didn’t get any points for
either of the last two clicker quizzes. If you were here
and did the quizzes, that means your clickers are not
registered. (Sorry to name names.)
a. Liz Anderson
b. Michael Collins
c. Colin Fluckiger
d. Cody Heffner
e. Brian Jackson
f. Matthew Matheson
g. Jacob Peary
h. Christopher Read
Thought question

The fact that desert sand is very hot in the
day and very cold at night is evidence that
sand has a:
a. low specific heat
b. high specific heat
Quick Writing:

Ralph's professor stated "If you add an ice
cube to a glass of water, the temperature of
the water does not necessarily decrease."
That seems bizarre to him, because ice is
obviously used to cool down water! Can you
help him understand why his professor’s
comment is true?
a. Hint: What happens if you add ice to 0C
water?
Reading quiz (graded)

Which of the following is not a way heat can
be transferred?
a. conduction
b. convection
c. perpetuation
d. radiation
Reading quiz (graded)

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 neutron star
d. An ice cube
e. None of the above
Blackbody Radiation

Hot objects glow!
a. That glow carries away
energy
heat energy
time


4
Plost  e ATobject
Surroundings also glow!
a. That glow adds energy
Pgained  e
4
ATsurroundings
Demo

Burning ants with magnifying glass
a. OK, not really
Color of emission
You’ll learn/derive
the equation in
Phys 360, if you
take it.
Area ~ T4
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

Thought question

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 by surroundings. Assume emissivity
is the same for the two temperatures.)
a. 31.25
b. 62.5
c. 125
d. 250
e. 500
Reading quiz (graded)

The rate that heat is conducted
through a wall (temperature T1 on one
side, T2 on other side) is proportional
to…
a. T2-T1
b. (T2-T1)2
c. (T2-T1)1/2
d. log(T2-T1)
e. exp(T2-T1)
Thermal Conduction
T2
hot
T1
cold
A
L
Q
 T2  T1 
P
 kA 

t
L


Really:
dQ
dT
 kA
dt
dx
Thought question

If I heat one end of an iron rod such that its
temperature is a constant 150 degrees C, and I
put the other end in ice water, what will the
temperature of the middle of the rod when the rod
approaches “steady state”?
a. 0 ºC ≤ T < 50 ºC
b. 50 ºC ≤ T < 75 ºC
c. 75 ºC ≤ T < 100 ºC
d. 100 ºC ≤ T < 150 ºC
e. T = 150 ºC
“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
Thought Question

You put the end of a rod in a fire and the
other end in a tub of water. The rod that
would heat the water fastest will be:
a. short and fat
b. long and fat
c. short and thin
d. long and thin
Thought question:

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
Thought question:

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
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
10.9
0.45
Convection

Demo: dye in tube