Announcements 9/14/11 Prayer Remember, HW solutions are being posted in hallway

around corner from where you turn in the homework, right next to the CSR office.

Calvin andHobbes

Video Boiling water in a paper cup

Reading quiz (graded) Which of the following is not a way heat

can be transferred? a. conductionb. convectionc. liquificationd. radiation

Thought question (ungraded) 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 stard. A hot electric stove burnere. 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 glass

a. OK, not really

Color of emission

You’ll learn/derive the equation in Phys 360, if you take it. Some results: Area ~ T4

Peak ~ 1/TChromaticitydiagram

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 from 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)

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

Thermal Conduction

A

hotT1

cold

L

2 1Q T TP kAt L

dQ dTkAdt dx

Really: Warning: what is meant by time?

“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

What “feels” colder, a metal car or a wooden box?

Thought question 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 ºCb. 50 ºCc. 100 ºCd. 150 ºCe. 200 ºC

What if left half of rod is iron, but the right half is copper?

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 < Pcopperb. Piron > Pcopperc. Piron = Pcopper

T2

hotT1

cold

iron Cu

R-values

Some R-values (from your textbook)

Material R (ft2 F hr/Btu)Brick, 4” thick 4Styrofoam, 1” thick 5Fiberglass insulation, 3.5” thick 10.9Drywall, 0.5” thick 0.45

LRk

A TPR

Why useful: R values of wall materials add

Yuck!

Worked Problem You foolishly decide to build the walls of your new

house out of solid aluminum (k = 238 W/mC), 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.

Answer: $27,288

Class survey: guess the answer

Quick Discussion

If air is such a poor thermal conductor, why is it beneficial to use fiberglass insulation in your attic?

Material k (J/s∙m∙C)Air 0.0234Fiberglass 0.045

Convection

Demo: dye in tube