Incandescent Lightbulbs

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Incandescent Lightbulbs
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Introductory Question

• An incandescent lightbulb contains some gas with
  the filament. How would removing the gas affect
  the bulbs energy efficiency?
• Make it more efficient
• Make it less efficient
• No change

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Observations about Lightbulbs

• Lightbulbs glow yellow-white
• They get very hot during operation
• You can feel heat radiating from them
• They eventually burn out
• They come in many wattages
• They come in many specialized types

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5 Questions about Lightbulbs

• How 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?

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Question 1

• How does a lightbulb produce light?

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An Incandescent Lightbulb

• Light is emitted by a hot tungsten filament
• Electric wires deliver power to the filament
• Glass bulb protects the filament
• Inert gas fill prolongs filaments life

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Question 2

• What determines a lightbulbs color?

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Thermal Radiation

• 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

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Black Body Spectrum

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

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Filament Temperature and Color

• Filament behaves as a (nearly) black body
• It emits a spectrum characteristic of its
  temperature
• Its visible efficiency increases with temperature
• Its life expectancy decreases with temperature
• Filaments 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

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Question 3

• What determines a lightbulbs brightness?

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Power and Light

• 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

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Three-Way Bulbs

• Two separate filaments
• One small, low-power filament
• One large, high-power filament
• Three light levels
• Low-power filament only
• High-power filament only
• Both filaments together

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Question 4

• Why do lightbulbs eventually burn out?

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Filament Requirements

• Filament requirements are challenging
• Filament must remain solid to high temperatures
• Filament must experience minimal sublimation
• Filament must be electrically conducting
• Tungsten metal is the best filament material
• Tungsten remains solid to 3422 C
• Tungsten sublimes relatively slowly at 2500 C
• Tungsten conducts electricity

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Tungstens Shortcomings

• Tungsten is reactive and burns in air
• Tungsten gradually sublimes, even at 2500 C
• Filament is encased in inert-gas-filled glass
  bulb
• Glass bulb keeps out oxygen
• Inert gas bounces tungsten atoms back onto
  filament
• Gas leads to convective heat loss
• Filament life is relatively short above 2500 C
• Sublimation and convection darken top of bulb

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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 efficient
• Make it less efficient
• No change

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Sealing Issues

• Atoms vibrate with thermal energy
• Their average separation increases with temp
• Solids expand when heated
• Some materials expand morethan others when
  heated
• To avoid stresses and fracture,glass and wires
  must expand equally

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Question 5

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

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Halogen Bulbs

• Halogen bulbs recycle tungsten onto filaments
• Bromine/iodine/oxygen gases added to a small bulb
• The bulbs envelope operates at high temperatures
• 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

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Specialized Bulbs

• Clear vs. Soft white bulbs glare control
• Long life bulbs underheated and inefficient
• Energy-saver bulbs underwattage and silly
• Rough service bulbs supported filament
• Krypton bulbs better sublimation protection
• Heat bulbs extra cool infrared emitters
• Photoflood bulbs extra hot blue emitters

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Summary about Lightbulbs

• Lightbulbs emit visible thermal radiation
• Most of their thermal radiation is not visible
• They fail when the filament sublimes away
• The glass envelope keeps oxygen out
• The inert gas fill lengthens the filament life