Mystery Clicker Number: 54161check if belongs to you. Get LOTS of points if it does!!

NEW SEAT AND GROUP ASSIGNMENTS TODAY!!. CHECK PRINTOUTS TO SEE WHERE YOU BELONG.

MINOR CHANGES TO FIX PROBLEMS OF LAST THURSDAY.

Reading quiz on chapter 8.1 air-cleaners, on thurs.

• How they work? What determines color? • Why do they “burn out”? • Why are fluorescent lights are more efficient? • Why physics makes it hard to improve efficiency?

Electrons

ElectricCurrent

Hot filament & everything else gives off“electromagnetic energy”

Today: 1) finish Lightbulbs

2) infrared radiation- all the “light” we don’t see.

3) Greenhouse effect-The physics of global warming.What’s behind CO2 emissionsdebate. (the basic physics, not the complexatmospheric modeling stuff)

max power in infrared (IR) where eye not sensitive

power

wavelength(= color)

IR UVVisible

sun

light bulb

max power in visiblewhere eye works.

longer shorter matches book, but not most places, like simulator

The funny shape of the black body spectrumThe shape of the spectrum for the thermal radiation of an object is rather complicated, but with some effort it can be understood and a formula derived that describes it. That is too advanced for this class, but in case you are interested, the formula for the amount of power at each frequency of an electromagnetic wave is given by P = (8h/c3)3 (eh/kT -1)-1, where k is the Boltzmann constant (= 1.38 x 10-23 Joules/Kelvin), c is the speed of light (3 x 108 m/s), h is Planck’s constant (6.626 x10-34 J∙s), and the temperature T is in units of Kelvin. From this equation it is possible to show that the wavelength where the power is highest is given by peak = hc/(2.82 kT)

frequencywavelength

detected pow

er

a typical spectrum

peak = hc/(2.82 kT)so shifts to shorter wavelengthas T increases.

just make brief note to this slide if they want to look at it on website

See spectral simulator forhomework

power(different scalefrom sun!)

Spectra of radiation given off by person and spectra of blockof barely frozen ice (32 F, 0 C).

a. 2 is person, 1 is ice, b. 1 is person, 2 is ice, c. none, because ice gives off no thermal radiation, d. 1 is both ice and person because they are almost identical e. 3 is person, one of the others is ice. ans. b. both person and ice give off IR, but so cold no visible. Person a bit hotter than ice, so more radiation, also hotter so shifted towards visible.

wavelength (= color)

IR Visible

1

2

3

power(different scalefrom sun!)

wavelength (= color)

IR Visible

1

2 ice (32 F, 0 C).

to check precise numbers: amount of power goes as T4, so 273 k = ice, 273 +37= 310 K = person. T4p/T4ice= (310/273)4 = Pperson/Pice =1.66 times more power from person.Peak wavelength goes as 1/T, so moved over about 20% (310/273 = 1.2)

person

in notes, don’t go through in class.

power(different scalefrom sun!)

wavelength (= color)

IR Visible

1

2

Spectra of radiation given off by person and spectra of block of barely frozen ice (32 F, 0 C).

look at amount of radiation (IR) to measure temperature.remote temperature sensor-- looks at IR to measure temp., demos.heat metal, look with IR sensitive camera.

Everything giving off electromagnetic radiation all thetime! People, floor, rocks, … Unless very hot, in IR.

Measure hot plate temperature withremote temp sensor. T= 300 degrees F.

Now we put a piece of glassin the way. What will temp sensorread? (decide as a group, and be prepared toexplain your answer)a. same temp., b. slightly less, c. about 73 degrees F, d. much colder than 73 F.

ans. c. 73 (room temp). Temp sensor gun detects the IRcoming from the glass. Amount tells it glass is room temp.Visible light (red laser beam) goes through glass but IR doesnot.

What would the ideal temperature be for a bulb filament?

a. Temp of standard bulb filament (2500 C).b. Temp of the sun (5800 C).c. Hotter than the sun.

Why?

The higher the temperature, the less power being wasted asIR light, so hotter means more visible light produced per Watt of electric power used. (More EFFICIENT). show simulator

Temperature of sun is spectrum best matched to eye.So temperature at least as hot as sun would be best.Colder means lots of power wasted as infrared. Heats butdoes not illuminate.

Hotter would produce bunch of UV light … bad for eyes/skin.

Back to light bulbs.

max power in infrared (IR) where eye not sensitive, wasted.

power

wavelength(= color)

IR UVVisible

sun 5800 C

light bulb 2500 C

max power in visiblewhere eye works.

If hotter is better, why are filaments in standard bulb not heated to 5800 C?a. because they melt if heated up any hotterb. because they vaporize if heated up any hotterc. because they oxidize (“burn up”) if heated any hotterd. would be too expensive to build

Sublimation: going into a vaporturns out to be limit before melting.So tungsten better than carbon.

Why not c ? Demo of filament with bulb removed.It burns up very quickly!

Solution- put inside bulb- inert (no oxygen) gas. demo

N, Ar

With no oxygen in bulb, can heat a lot farther until: Melting: (Edison thought was biggest problem so used carbon, melts at highest temp. But more sublimation so bulbs burnt out faster.) So not a, but if picked, as smart as T. E.

“sublimation”, direct solid to gas- Examples: • cold dry sunny day in Boulder-

snow vanishes before melting

• “Dry ice” carbon dioxide..

vapor

solid

ans. b. Light bulb filaments vaporize (“sublimation”) too fast at temperatures above 2500 C. Tungsten melts a bit lower than carbonused by Edison, but has lower sublimation rate, so bulbs last longer.

vapor

solid

Rate of sublimation increases rapidly as gets hotter.Hotter T, more visible light = better electrical to visiblelight conversion (“efficiency”), but faster sublimation.

Light bulbs today use tungsten, least sublimation rate.

Eventually sublimates away. Black stuff on glass.When all the way through, filament breaks offrattles around in bulb.

Not “Burned out”, actually “evaporated out”!

T = 2500 C

Wastes 88% of power!!tradeoff between efficiencyand filament lifetime

run hotter, evaporate away much sooner

How to get more efficient light bulbs?

1. Halogen lamps- run hotterrun hotter, undue sublimation. Incredibly clever!!

hot filament

glass envelope(cooler) halogen molecule

picks up tungsten off cool glass

carries it back to center,releases it to hot filament

2. Florescent lights even better. Will learn about at end of term.

skip in class, come back to if time

III. The greenhouse effect. Why the temperature of the earth depends on CO2 and H2Oand other “greenhouse gases” in atmosphere.Simple picture- why these cause heating. (life depends on)

Exactly how temp changes with more CO2? Complicated.Depends on how atmosphere and world changes with heating.

Car sitting in bright sun. Inside gets much hotter thanthe air next to it. Why?

a. car absorbs light energy from sun better than the pavement.b. sunlight causes chemical reactions in car materials that give off heat.c. electrical appliances such as clock that run all the time incar causes it to heat up.d. windows let in energy but do not let it escape.e. none of the above make sense, must be different explanation.

ans. d- greenhouse effect. Sunlight passes through glass to inside of car, IR absorbed by glass and part reemitted back into car … not able to escape directly.

How possible? Electromagnetic energy changes color! Visible light from sun, IR light off hot car materials. IR also off hot ground, but it goes through air, but not glass. Remember temperature sensor

pow

er

wavelength

IRUV

Visible

sun

100 F car inside

energy from sun mostly visible light. Goes through window

Some energy leaves car when IR radiation emitted to outside by glass and car frame. Amount of energy given off by glass depends on its temperature.

Why not “hot car seat effect”?same physics used to keep green houses warmbefore cars- so “green-house effect”.

energy from sun mostly visible light. Goes through window

Glass keeps IRfrom escaping.Glass warms up some,gives off some IR, ½sent back into house.

IR emitted by inside plants, etc hits glass and is absorbed

Glass warms up emits IR … ½ radiation to outside, ½ radiation to inside Energy absorbed by glass = Energy emitted by glassBut some energy goes back into greenhouse!

Temperature of earth- conservation of energySunlight putting energy into earth - light energy into thermalKeep adding energy, keeps getting hotter. Cons. of energy!As Earth heats, IR radiation out increases until balance… Power in from sun = Power out from radiation into space. Knowing this can calculate Temperature of earth!

How hot would earth be if all IR got out like shown?

Powerin = solar power/m2 at earth x area of sunlight intersected by earth x fraction of sunlight absorbed by earth

= 1380 W/m2 x πR2earth x 0.7 =

= 1.22 x 1017 Watts(!)

Stefan-Boltzmann Law says Powerout = Power radiated to space by Earth

with surface temperature T = T4 x surface area of Earth

x fraction of IR emitted that makes it to space = 5.67 x 10-8 J/(s m2 K4) x T4 x 4πR2

earth m2 x fraction of IR escaping = 2.89 x 107 W/K4 x T4 x fraction of IR escaping

Greenhouse effect and temperature of earth.

Powerin from Sun = Powerout from earth

1.22 x 1017 Watts = 2.89 x 107 W/K4 x Tavg4

Solve to get Tavg = sqrt (sqrt 4.22 x 109 K4) = 255 K = -18 C ~ 0 F

Brrr!

How hot would earth be if all IR got out like shown?

Earth not that cold becauseof greenhouse effect. Atmosphereacts like the glass.Not all IR gets out.

Why doesn’t all of the IR get out of atmosphere?Certain types of molecules “greenhouse gases” absorb IR and send it back towards earth. Carbon dioxide, H2O, Not oxygen or nitrogen. Only part of IR getting out,

Earth’s surface temperature rises untilPower in from sun = Power out to space .T up from 255 K (0 F) to 285 K (54 F)

When the concentration of greenhouse gases goes up, the total power emitted by earth to space, a: goes up, b. goes down, c. stays the same

c. stays the same. It always has to just balance theamount coming in from sun or earth will rapidly heatup. Effect of greenhouse gases is to block some going from ground to space, so need more leaving ground to have same amount get into space.

power in =power out

go to simulation. show if no greenhouse. what temp has to beadd greenhouse gas, see temp rise.complicated stuff, clouds, etc.

How to calculate temperature when greenhouse effect?

Stefan-Boltzmann Law says Power out to space = 2.89 x 107 W/K4 x T4 x fraction of IR escaping

if greenhouse gases absorb and emit IR such that only 61% of power radiated by earth’s surface gets into space.So Pout = 0.61 x 2.89 x 107 W/K4 x T4. To conserve energy Pin = Pout, which means T is higher than before.

1.22 x 1017 W = 0.61 x 2.89 x 107 W/K4 x T4, solving for T, get T = (6.92 x 109)1/4 = sqrt (sqrt(6.92 x 109)) = 288 K.

Powerin = solar power/m2 at earth x area of sunlight intersected by earth x fraction of sunlight absorbed by earth

= 1380 W/m2 x πR2earth x 0.7 =

= 1.22 x 1017 Watts(!)

Temperature of surface of earth has to be value so that Power out to space = Power in from sun

What is the effect of clouds?a. Decrease the temperature of the groundb. Increase the temperature of the groundc. No effect on the temperature of the ground

Issues: 1. Clouds reflect part of sunlight back to space. 2. Clouds absorb IR radiation from earth and send it back to ground.

Net global effect is cooling by a few decrees C.