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 Andersonb. Michael Collinsc. Colin Fluckigerd. Cody Heffnere. Brian Jacksonf. Matthew Mathesong. Jacob Pearyh. Christopher Read

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 heatb. high specific heat

Thought question

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 0C water?

Reading quiz (graded)

Which of the following is not a way heat can be transferred?

a. conductionb. convectionc. perpetuationd. radiation

Reading quiz (graded)

Which of the following does not continuously emit electromagnetic waves?

a. A light bulb which is turned onb. A light bulb which is turned offc. A neutron stard. An ice cubee. None of the above

Blackbody Radiation

Hot objects glow! a. That glow carries away

energy

Surroundings also glow!a. That glow adds energy

4heat energy

timelost objectP e AT

4gained surroundingsP e AT

Demo

Burning ants with magnifying glassa. 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 wavelengthsa. Greenhouse effect

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

Thought question

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

A

T2

hotT1

cold

L

2 1Q T TP kA

t L

dQ dTkA

dt dxReally:

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 ºCb. 50 ºC ≤ T < 75 ºCc. 75 ºC ≤ T < 100 ºCd. 100 ºC ≤ T < 150 ºCe. T = 150 ºC

“Steady state” vs. “Thermal equilibrium”

Thermal Conductivity

Some Thermal Conductivities (from your textbook)

Material k (J/s∙m∙C)Copper 397Aluminum 238Iron 79.5Glass 0.84Wood 0.10Air 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 fatc. short and thind. 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 lessb. More than 10 but not greater than 25c. More than 25 but less than 35 d. 35 Wattse. More than 35 Watts

T2

hotT1

coldiron

Cu

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/mC; kCu = 397 W/mC)

a. Piron < Pcopper

b. Piron > Pcopper

c. Piron = Pcopper

T2

hotT1

cold

iron Cu

R-values

Some R-values (from your textbook)

Material R (ft2Fhr/Btu)Brick, 4” thick 4Styrofoam, 1” thick 5Fiberglass insulation, 3.5” thick 10.9Drywall, 0.5” thick 0.45

Convection

Demo: dye in tube