incandescent lightbulbs

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Incandescent Lightbulbs. Introductory Question. An incandescent lightbulb contains some gas with the filament. How would removing the gas affect the bulb’s energy efficiency? Make it more efficient Make it less efficient No change. Observations about Lightbulbs. - PowerPoint PPT Presentation


  • Incandescent Lightbulbs

  • Introductory QuestionAn incandescent lightbulb contains some gas with the filament. How would removing the gas affect the bulbs energy efficiency?

    Make it more efficientMake it less efficientNo change

  • Observations about LightbulbsLightbulbs normally emit yellow-white lightThey get hot during operationYou can feel heat radiating from themThey eventually burn outThey come in many wattagesThey come in many specialized types

  • 5 Questions about LightbulbsHow does a lightbulb produce light?What determines a lightbulbs color?What determines a lightbulbs brightness?Why do lightbulbs eventually burn out?Are halogen bulbs really better?

  • An Incandescent LightbulbLight is emitted by a hot tungsten filamentthat is heated electricallyand that emits thermal radiationthat includes visible lightHeat is flowing from the hotter filament to its cooler surrounding as thermal radiation, that is partly visible light

  • Question 2What determines a lightbulbs color?

  • Thermal RadiationMaterials all emit thermal radiation becausethey contain electric chargesand thermal energy causes those charges accelerate.Accelerating charges emit electromagnetic wavesHotter temperatures yield shorter wavelengths

  • Black Body SpectrumThe spectrum and intensity of electromagnetic waves from a black body depend only on its temperature

  • Filament Temperature and ColorFilament behaves as a (nearly) black bodyIt emits a spectrum characteristic of its temperature,so its visible efficiency increases with temperature.Its life expectancy decreases with temperatureFilaments temperature is set by a power balancePower arrives as electricityand leaves as heat, some of which is radiation.Net power is zero when filament is about 2500 C

  • Question 3What determines a lightbulbs brightness?

  • Power and LightLightbulb filament maintains zero net powerIts thermal power out must equal electrical power inso its radiated power increases with electrical power.Higher wattage bulbsuse larger filaments with more surface area,obtain more electrical power,maintain the usual 2500 C filament temperature,and radiate more visible light overall.

  • Three-Way BulbsTwo separate filamentsOne small, low-power filamentOne large, high-power filamentThree light levelsLow-power filament onlyHigh-power filament onlyBoth filaments together

  • Question 4Why do lightbulbs eventually burn out?

  • Filament RequirementsFilament requirements are challengingIt must remain solid to high temperatures,experience minimal sublimationand be electrically conducting.Tungsten metal is the best filament materialTungsten remains solid to 3422 C,sublimes relatively slowly at 2500 C,and conducts electricity.

  • Tungstens Shortcomings Tungsten is burns in air and gradually sublimesFilament is encased in inert-gas-filled glass bulbto keep out oxygenand to bounce tungsten atoms back onto filament.Inert gas, however, leads to convective heat lossand filament is short-lived above 2500 CSublimation and convection darken top of bulb

  • Introductory Question (revisited)An incandescent lightbulb contains some gas with the filament. How would removing the gas affect the bulbs energy efficiency?

    Make it more efficientMake it less efficientNo change

  • Sealing IssuesAtoms vibrate with thermal energyAverage separations increase with temperatureSolids expand when heatedSome materials expand more with temperature than othersTo avoid stresses and fracture, glass and wires must expand equally

  • Question 5Are halogen bulbs really better?What about long-life bulbs?What about energy-saver bulbs?What about krypton bulbs?

  • Halogen BulbsHalogen bulbs recycle tungstenBromine/iodine/oxygen gases added to a small bulbthat operates at a high temperature throughout.Tungsten atoms sublime from the hot filamentbut then combine chemically with the gasesand soon redeposit on the filament to prolong its lifeFilament of halogen bulb can operate at 2800 CBulb offers better whiteness and energy efficiency

  • Summary about LightbulbsLightbulbs emit visible thermal radiationMost of their thermal radiation is not visibleThey fail when the filament sublimes awayThe glass envelope keeps oxygen outThe inert gas fill lengthens the filament life

    Demonstration: Clear-But-Dim Incandescent Light BulbDemonstration: Opening an Incandescent Light BulbDemonstration: Blackbodies are Really BlackDemonstration: An Incandescent Light Bulb at Various TemperaturesDemonstration: Different Wattage Bulbs - More of the Same LightDemonstration: A Three-Way BulbDemonstration: An Unprotected Filament Burns UpDemonstration: A Halogen Bulb


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