Announcements 9/12/12 Prayer Register your clickers by today at 5 pm!

Unregistered numbers from Monday: 3368A2F9, 7A8E14E

The Far Side

From warmup

Extra time on?a. emissivity (several people)

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

conduction convection radiation

Clicker question

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

From warmup

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

Burning ants with magnifying glassa. 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

“white point”

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

Clicker question

From warmup

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

A

hotT1

cold

L

2 1Q T TP kA

t L

dQ dTkA

dt dxReally: 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?

Demo

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. largeb. small

Clicker 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?

Clicker 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

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

LR

k

A TP

R

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

From warmup

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 variablesc. Because they look like this

http://en.wikipedia.org/wiki/Convection-diffusion_equation

d. (not quite correct) Convection is a theory that shows how conduction works in fluids.

Convection

Youtube: http://www.youtube.com/watch?v=7xWWowXtuvA