# Lightbulbs. How does a lightbulb work? Lightbulbs. Electromagnetic radiation. End of semester grade policy. Electric Current

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2 Electromagnetic radiation EM radiation also travels in waves, like sound But light can travel through a vacuum so what exactly is waving? EM radiation is a periodic modulation of electric field What is Electric field? Light is periodic modulation of electric field Strong field Electric field Strong field Electric field l Electric field l l Color of light depends on its wavelength. Electric field exists everywhere in space Describes the force on a charged particle at each point in space Vector has a magnitude and direction Units: Newtons/Coulomb Contains energy Created by charges (and created in other ways) Analogy: Like gravitational field describes the force on a particle with mass Balloon demo The electromagnetic spectrum See this next week: All EM radiation is a periodic modulation of an electric field light is just one part of the electromagnetic spectrum with specific range of l (and f) light is the strongest part of the EM spectrum emitted by the sun It is the part of the EM that our eyes can detect Electromagnetic radiation Light travels in waves, like sound Speed of sound, in air = 330 m/s Frequency of concert A = 440 Hz Wavelength of concert A = 0.75 m Speed of light, in vacuum = 3 X10 8 m/s Frequency of red light = 4.6 X Hz Wavelength of red light = a) 0.75 m b) 7.5 X 10-5 m c) 6.5 X 10-7 m d) 1.38 X m incoming white light beam light made up of many colors IR UV The spectrum of white light? A spectrometer measures the spectrum (range of wavelengths or frequencies) in light White light Defined as the spectrum of EM radiation emitted by the sun All visible l present with roughly equal intensity LED demo Understanding a spectrum: IR UV B A C Wavelength For this spectrum, rank power of light at each color: a. C greater than B greater than A b. A greater than B greater than C c. C greater than A greater than B d. B greater than A greater than C e. Cannot tell from this data. An object is giving off light with this spectrum. What color in the emitted light has the most power? a. IR, b. Yellow/green c. UV, d. red, e. blue, detector angle (wavelength or color) 2

3 Blackbody spectrum Everything that has a non-zero temperature emits EM radiation The spectrum of EM radiation coming from a black object is called the blackbody spectrum. BB spectrum determined by temperature only. The temperature of the object affects both - The total power of EM radiation emitted by the object - The range of wavelengths emitted (the spectrum) Blackbody spectrum and temperature Look at light bulb with variac to control how much electrical power goes into it. If I put half as much electrical power into it, what will happen? a. color will change, get whiter, brightness decrease b. color will stay the same, brightness decrease c. color will get redder, brightness decrease d. color will get redder, brightness the same e. color will get whiter, brightness the same. How does temperature affect emitted power (brightness)? Stefan-Boltzman law gives total electromagnetic power (energy/second) out of a hot object at temperature T Two 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? Area of surface Temperature of object (in Kelvin!) Stefan-Boltzmann constant, = 5.67 x 10-8 J/(s m 2 K 4 ) e = emissivity ; how well the light gets out a) The same amount b) Twice as much c) Half as much d) Four times as much e) Sixteen times as much. Comments on Stefan-Boltzman Law The Kelvin Temperature Scale: Links T to motion 1. Emissivity (e) Property of surface, Scale of 0 to 1. 1 (BB) means radiates well, 0 is lousy. Room temp 65F 18C 291K area, a e = T in Kelvin (= T Celsius +273) same T so same color. 3. Emitted power rises very quickly with temperature area, a e = degree Kelvin = 1 degree Celsius = 9/5 degree Fahrenheit 0 degree Celsius = 273 K All motion stops 3

4 Power Electric Current POWER (W) A particular light bulb s filament is at 2000 C. What is its temperature in Kelvin? a) 2000 K b) 2273 K c) 1727 K d) 2500 K A particular light bulb s filament is at 2000 C. What is its temperature in Farenheit? a) 2000 F b) 3632 F c) 1240 F d) 3246 F How does temperature affect emitted power (brightness)? Emitted power rises rapidly with temperature TEMPERATURE (K) P = e T 4 a = 5.67 x 10-8 J/(s m 2 K 4 ). T in Kelvin (= T Celsius +273) If I raise temperature of heating coil on stove from 450K to 500K, by what fraction will power emitted by coil increase? Power at 500K Power at 450K (note: emissivity and coil area 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 larger Blackbody spectrum Everything that has a non-zero temperature emits EM radiation The spectrum of EM radiation coming from a heated object is called the blackbody spectrum. (Although it isn t black!) The temperature of the object affects both - The total power of EM radiation emitted by the object (P T 4 ) - The range of wavelengths emitted (the spectrum) An efficient light bulb: Why are light bulbs inefficient? How does an incandescent light bulb work: - Electric current flows through filament - Filament gets hot - Hot filament emits EM radiation - Electrical energy EM radiation energy Electrical power in 100% conversion EM energy at visible l But..visible light is only a very small part of the EM spectrum Only 18% EM radiation from bulb is visible Rest is IR radiation (heat) Why are lightbulbs inefficient? Recall: Blackbody spectrum UV Wavelength (color) Higher temperature - More power ( T 4 ) - Peak power at shorter wavelengths - Lightbulb filament isn t hot enough to efficiently produce visible light IR Sun: T = 5800C Max power in visible where eye works Light bulb: T = 2500 C Max power in infrared (IR) where eye not sensitive 4

5 Power 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 Joules/Kelvin), c is the speed of light (3 x 10 8 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 l peak = hc/(2.82 kt) Identify spectrum of radiation given off by person (37 C) and a block of barely frozen ice (0 C). 3 wavelength a typical spectrum l peak = hc/(2.82 kt) shifts to shorter wavelength as T increases. See spectral simulator for homework Wavelength 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. 1 2 Thermal imaging Look at BB spectrum for a person (~ 313 K) No visible radiation emitted (too cold) all in the IR Not detected by the eye, but detected by an IR (thermal imaging) camera This remote temperature sensor works by measuring the infrared spectrum of the thing I point it at. Thermal imaging photography using non-visible EM radiation Try it out with hot plate 5

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