Incandescent with the filament. How would removing the gas affect - PDF document

Incandescent Lightbulbs 1 Incandescent Lightbulbs 2 Introductory Question An incandescent lightbulb contains some gas � Incandescent with the filament. How would removing the gas affect the bulb’s energy efficiency? Lightbulbs Make it more efficient A. Make it less efficient B. No change C. Incandescent Lightbulbs 3 Incandescent Lightbulbs 4 Observations about Lightbulbs 5 Questions about Lightbulbs � Lightbulbs glow yellow-white � How does a lightbulb produce light? � They get very hot during operation � What determines a lightbulb’s color? � You can feel heat radiating from them � What determines a lightbulb’s brightness? � They eventually burn out � Why do lightbulbs eventually “burn out”? � They come in many wattages � Are halogen bulbs really better? � They come in many specialized types Incandescent Lightbulbs 5 Incandescent Lightbulbs 6 Question 1 An Incandescent Lightbulb � How does a lightbulb produce light? � Light is emitted by a hot tungsten filament � Electric wires deliver power to the filament � Glass bulb protects the filament � Inert gas fill prolongs filament’s life 1

Incandescent Lightbulbs 7 Incandescent Lightbulbs 8 Question 2 Thermal Radiation � What determines a lightbulb’s color? � All materials emit thermal radiation � All materials contain electric charges � Thermal energy causes those charges accelerate � Accelerating charges emit electromagnetic waves � Hotter temperatures yield shorter wavelengths Incandescent Lightbulbs 9 Incandescent Lightbulbs 10 Black Body Spectrum Filament Temperature and Color � The spectrum and intensity of electromagnetic � Filament behaves as a (nearly) black body waves from a black body depend only on its � It emits a spectrum characteristic of its temperature temperature � Its visible efficiency increases with temperature � Its life expectancy decreases with temperature � Filament’s temperature is set by a power balance � Power arrives as electricity � Power leaves as heat, some of which is radiation � Net power is zero when filament is about 2500 °C Incandescent Lightbulbs 11 Incandescent Lightbulbs 12 Question 3 Power and Light � What determines a lightbulb’s brightness? � Lightbulb filament maintains zero net power � Its thermal power out must equal electrical power in � Its radiated power increases with its electrical power � Its radiated power increases with its surface area � Higher wattage bulbs � use larger filaments with more surface area � maintain the usual 2500 ° C filament temperature � and radiate more visible light as a result 2

Incandescent Lightbulbs 13 Incandescent Lightbulbs 14 Three-Way Bulbs Question 4 � Two separate filaments � Why do lightbulbs eventually “burn out”? � One small, low-power filament � One large, high-power filament � Three light levels � Low-power filament only � High-power filament only � Both filaments together Incandescent Lightbulbs 15 Incandescent Lightbulbs 16 Filament Requirements Tungsten’s Shortcomings � Filament requirements are challenging � Tungsten is reactive and burns in air � Filament must remain solid to high temperatures � Tungsten gradually sublimes, even at 2500 °C � Filament must experience minimal sublimation � Filament is encased in inert-gas-filled glass bulb � Filament must be electrically conducting � Glass bulb keeps out oxygen � Tungsten metal is the best filament material � Inert gas bounces tungsten atoms back onto filament � Tungsten remains solid to 3422 °C � Gas leads to convective heat loss � Tungsten sublimes relatively slowly at 2500 °C � Filament life is relatively short above 2500 °C � Tungsten conducts electricity � Sublimation and convection darken top of bulb Incandescent Lightbulbs 17 Incandescent Lightbulbs 18 Introductory Question (revisited) Sealing Issues An incandescent lightbulb contains some gas � Atoms vibrate with thermal energy � with the filament. How would removing the � Their average separation increases with temp gas affect the bulb’s energy efficiency? � Solids expand when heated � Some materials expand more Make it more efficient than others when heated A. Make it less efficient B. � To avoid stresses and fracture, No change glass and wires must expand equally C. 3

Incandescent Lightbulbs 19 Incandescent Lightbulbs 20 Question 5 Halogen Bulbs � Are halogen bulbs really better? � Halogen bulbs recycle tungsten onto filaments � What about “long-life” bulbs? � Bromine/iodine/oxygen gases added to a small bulb � What about “energy-saver” bulbs? � The bulb’s envelope operates at high temperatures � What about krypton bulbs? � Tungsten atoms sublime from the hot filament � but then combine chemically with the gases � and soon redeposit on the filament to prolong its life � Filament of halogen bulb can operate at 2800 °C � Bulb offers better whiteness and energy efficiency Incandescent Lightbulbs 21 Incandescent Lightbulbs 22 Specialized Bulbs Summary about Lightbulbs � Clear vs. Soft white bulbs – glare control � Lightbulbs emit visible thermal radiation � Long life bulbs – underheated and inefficient � Most of their thermal radiation is not visible � Energy-saver bulbs – underwattage and silly � They fail when the filament sublimes away � Rough service bulbs – supported filament � The glass envelope keeps oxygen out � Krypton bulbs – better sublimation protection � The inert gas fill lengthens the filament life � Heat bulbs – extra cool infrared emitters � Photoflood bulbs – extra hot blue emitters 4