1Homework 8 is now availableHomework is due Saturday at noon.Reminder: CULearn is UNREALIABLE. Get your work in early!Exam resultsUse vast quantities of power- ~25% of all power for lighting.Incandescent light wastes 88% of that!!• How do 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? Electric Current flows, filament heats up, electrons shaking give off electromagnetic wavesElectricCurrentHot filament gives off“electromagnetic energy”This week: Lightbulbsprismincoming light beamLook at light bulb … What comes out? What is light? (new form of energy)White light is made up of many different colors.Some “colors” you can’t even see.Electromagnetic radiationLight travels in waves, like soundRecall: sound is periodic modulation of air pressureHigh pressureLow pressureElectromagnetic radiationLight travels in waves, like soundLight is periodic modulation of “electric field”Strong fieldColor of light depends on its wavelength.2Electromagnetic radiationLight travels in waves, like soundSpeed of sound, in air= 330 m/sFrequency of concert A= 440 HzWavelength of concert A= 0.75 mSpeed of light, in vacuum= 3 X108m/sFrequency of red light= 4.6 X 1014Hz Wavelength of red light =a) 0.75 mb) 7.5 X 10-5mc) 6.5 X 10-7md) 1.38 X 1023mElectromagnetic radiationLight travels in waves, like soundSpeed of light, in vacuum= 3 X108m/sFrequency of red light= 4.6 X 1014Hz Wavelength of red light =a) 0.75 mb) 7.5 X 10-5mc) 6.5 X 10-7md) 1.38 X 1023mAnswer: l = c/n= (3 X 108)/(4.6 X 1014)= 6.5 X 10-7m= 650 nmA spectrometer can measure the spectrum of a light bulb.(We’ll have one on Thursday).incoming light beamlight made up of many colorsThe power that the spectrometer detects will depend on the color of the light.a typical spectrumdetector angle(color)detected powerIRUVVisibledetector angle(color= wavelength)detected powerIRUVVisibleABCFor this spectrum, rank power of light at each color: a. C greater than B greater than Ab. A greater than B greater than Cc. C greater than A greater than Bd. B greater than A greater than Ce. Cannot tell from this data. Understanding a spectrum: Answer is d. B has most power. Followed by A then C. For an object giving off light with this spectrum, what color has there most power: a. IR, b. yellow, c.UV, d. red, e. blue,ans. b. yellow at pos. BThe pattern of light coming from a heated object is called the “blackbody spectrum.” (Although it isn’t black!)Go to the blackbody spectrum simulationincoming light beamlight made up of many colorsLight bulb produces lots of light beyond the red … Infrared light (IR) has most power and we can’t even see it. A lightbulb wastes lots of power since only 18% of electromagnetic radiation given off is visible color.Look at light bulb with variac to control how muchelectrical power goes into it. If I put half as much electrical power into it, what will happen?a. color will change, get whiter, brightness decreaseb. color will stay the same, brightness decreasec. color will get redder, brightness decreased. color will get redder, brightness the samee. color will get whiter, brightness the same.c. color will get redder, brightness decreaseWhy? Less energy into it, it heats up less.Temperature affects both color and brightness.Lower temperature is redder and dimmer.3Figuring out how much power given temperature: Stefan-Boltzman law gives total electromagnetic energy per second (energy/second) out of a hot object like filament.Power = e X  X T4 X a Temperature affects both color and brightness.Area of surfaceTemperature of object (in Kelvin!)Stefan-Boltzmann constant =  = 5.67 x 10-8J/(s m2K4)e = “emissivity”; how well the light gets outTwo burners on the stove are at the same temperature, but the left-hand burner has twice the area. How much more infrared radiation is it putting out?a) The same amountb) Twice as muchc) Half as muchd) Four times as muche) Sixteen times as much.Answer b: twice as much; radiation is linear in area.Power = e X  X T4 X a Figuring out how much power given temperature: Stefan-Boltzman law gives total electromagnetic energy per second (energy/second) out of a hot object like filament.Power = emissivity x Boltzmann’s Constant x (Temp)4x (area of surface)P = eT4a Power goes up very fast with Temperature.e is “emissivity”, property of surface, 0 to 1.1 means radiates well, 0 is lousy. T in Kelvin (= T Celsius +273)area, ae = 0.9area, ae = 0.3same T so same color.Temperature affects both color and brightness.Absolute(Kelvin)Fahrenheit(degrees)Room TempWater freezesDry IceAir freezesDeep space, 3 KAbsolute zero!All motion stops-273 oC11025020015010050-60-4600earth3001 Kelvin = 1 degree Celsius = 9/5 degree FahrenheitKelvin temperature scaleA particular light bulb’s filament is at 2000 C. What is its temperature in Kelvin?a) 2000 Kb) 2273 Kc) 1727 Kd) 2500 KAnswer b: 2000 C = (2000 + 273) KPower = emissivity x Boltzmann’s Constant x (Temp)4x (area of surface)P = eT4a  = 5.67 x 10-8J/(s m2K4). T in Kelvin (= T Celsius +273)Power goes up very fast with Temperature.Temperature effects both color and brightness.01002003004005006007000 50 100 150 200 250 300 350TEMPERATUREPOWERIf I raise temperature of heating coil on stove from 450K to 500K, how much more will be power emitted by coil? =Power at 500KPower at 450K(note: emissivity and area of the coil will cancel!)a Power is 1.11 times larger.b. Power is 250 times larger.c. Power is 1.52 times larger.d. Power is 5.0 times larger.e. Power is 50 times larger4Power = emissivity x Boltzmann’s Constant x (Temp)4x (area of surface)P = eT4a  = 5.67 x 10-8J/(s m2K4). T in Kelvin (= T Celsius +273)Power goes up very fast with Temperature.Temperature effects both color and brightness.01002003004005006007000 50 100 150 200 250 300 350TEMPERATURE (Kelvin)POWERIf raise temperature of heating coil on stove from 450K to 500K, how much more will be power emitted by coil? =Power at 500KPower at 450K(note: emissivity and are of the coil will cancel!)Power at 500K = e(500 K)4aPower at 450K e(450 K)4a = 6.25E10 K4 = 1.524.10E10 K4Temp increase by ~11%, Power increase by