wwwcoloradoeduphysicsphetactivitiesPhys1010A by fionan


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    Reading quiz on chapter 8.1 air-cleaners, on thurs.
                Today: 1) finish Lightbulbs
•     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?

    Hot filament & everything      2) infrared radiation-
    else gives off                 all the “light” we don’t see.
    “electromagnetic energy”

                                3) Greenhouse effect-
                                The physics of global warming.
     Electric                   What’s behind CO2 emissions
     Current                    debate.
        Electrons               (the basic physics, not the complex
                                atmospheric modeling stuff)
                               IR    Visible   UV

                                       max power in visible
                                       where eye works.

                              light bulb
         longer                                     shorter
                                                     matches book,
                          (= color)
                                                    but not most
        max power in infrared (IR)                  places, like
        where eye not sensitive                     simulator
                        just make brief note to this slide if they want to look at it on website

                          The funny shape of the black body spectrum
   The 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 = (8h/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)
detected power

                                                        peak = hc/(2.82 kT)
                                                        so shifts to shorter wavelength
                                                        as T increases.
                         wavelength                     See spectral simulator for
                 a typical spectrum
Spectra of radiation given off by person and spectra of block
of barely frozen ice (32 F, 0 C).                   IR    Visible

                                         1                        3
(different scale
from sun!)                          2

a.   2 is person, 1 is ice,             wavelength (=
b.   1 is person, 2 is ice,             color)
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
         in notes, don’t go through in class.
                                                                 IR   Visible

power                                               1
(different scale                                        person
from sun!)                                      2
                            ice (32 F, 0 C).

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

power                                             1
(different scale
from sun!)                                    2

                           wavelength (= color)
  Everything giving off electromagnetic radiation all the
  time! People, floor, rocks, … Unless very hot, in IR.
       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.
                     Measure hot plate temperature with
                     remote temp sensor. T= 300 degrees F.

Now we put a piece of glass
in the way. What will temp sensor
read? (decide as a group, and be prepared to
explain 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 IR
coming from the glass. Amount tells it glass is room temp.
Visible light (red laser beam) goes through glass but IR does
            Back to light bulbs.
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.
The higher the temperature, the less power being wasted as
IR 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 but
does not illuminate.

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

                        sun 5800 C
                                     max power in visible
                                     where eye works.

                           light bulb 2500 C

                          (= color)

        max power in infrared (IR)
        where eye not sensitive, wasted.
If hotter is better, why are filaments in standard bulb not
heated to 5800 C?
a. because they melt if heated up any hotter
b. because they vaporize if heated up any hotter
c. because they oxidize (“burn up”) if heated any hotter
d. would be too expensive to build
Why not c ? Demo of filament with bulb removed.
It burns up very quickly!

Solution- put inside bulb-
inert (no oxygen) gas. demo
 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                    N, Ar
 burnt out faster.) So not a, but
 if picked, as smart as T. E.

 Sublimation: going into a vapor
 turns out to be limit before melting.
 So tungsten better than carbon.
ans. b. Light bulb filaments vaporize (“sublimation”) too fast at
temperatures above 2500 C. Tungsten melts a bit lower than carbon
used by Edison, but has lower sublimation rate, so bulbs last longer.

                                “sublimation”, direct solid to gas-
                                •   cold dry sunny day in Boulder-
                                    snow vanishes before melting

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


      Light bulbs today use tungsten, least sublimation rate.

                                    T = 2500 C
                                       Wastes 88% of power!!
                                       tradeoff between efficiency
                                       and filament lifetime

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

Not “Burned out”, actually “evaporated out”!
     run hotter, evaporate away much sooner
skip in class, come back to if time
    How to get more efficient light bulbs?

    1. Halogen lamps- run 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.
III. The greenhouse effect.
Why the temperature of the earth depends on CO2 and H2O
and 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 than
  the 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 in
car 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

 energy from sun
 mostly visible light.
 Goes through window
                                                    IR Visible

Some energy leaves car when             power
IR radiation emitted to outside                 100 F car inside
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 warm
     before cars- so “green-house effect”.

                                       Glass keeps IR
                                       from escaping.
    energy from sun
    mostly visible light.
                                       Glass warms up some,
    Goes through window                gives off some IR, ½
                                       sent 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 glass
           But some energy goes back into greenhouse!
         Temperature of earth- conservation of energy
Sunlight putting energy into earth - light energy into thermal
Keep 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!
                         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 4πR2earth m2 x fraction of IR escaping
     = 2.89 x 107 W/K4 x T4 x fraction of IR escaping
How hot would earth be if all IR got out like shown?
          Greenhouse effect and temperature of earth.
  How hot would earth be if all IR got out like shown?
 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

   Earth not that cold because
   of greenhouse effect. Atmosphere
   acts 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 until
                        Power 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 the
         amount coming in from sun or earth will rapidly heat
         up. 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 be
add greenhouse gas, see temp rise.
complicated stuff, clouds, etc.
      How to calculate temperature when greenhouse effect?
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

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.
What is the effect of clouds?
a. Decrease the temperature of the ground
b. Increase the temperature of the ground
c. No effect on the temperature of the ground

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.

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