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Page 1: Incandescent Light Bulbs 1 Incandescent Light Bulbs.

Incandescent Light Bulbs 1

Incandescent Light Bulbs

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Question:

An incandescent light bulb 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

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Observations About Incandescent Light Bulbs

• Light bulbs glow yellow-white

• They get very hot during operation

• You can feel heat radiating from them

• They burn out

• They come in many wattages

• They come in many specialized types

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Thermal Radiation

• All materials contain electric charges

• Thermal energy makes charges accelerate

• Accelerating charges emit electromagnetic waves

• All materialsemit electro-magnetic waves (thermal radiation)

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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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Incandescent Bulbs

• Features:– Tungsten filament yields light– Electric wires deliver power– Glass bulb protects filament– Inert gas fill prolongs life

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Operation Issues, Part 1

• Filament temperature– Determines color temperature and efficiency– Higher temperature yields higher efficiency– Higher temperature shortens filament life

• Filament heating– Heats due to power lost by an electric current– Requires thinner filament at higher voltages

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Operation Issues, Part 2

• Filament reactivity– Tungsten is reactive– Tungsten needs protection from oxygen in air

• Filament sublimation– At high temperatures, tungsten atoms sublime

• Non-reactive gas limits sublimation– Gas bounces tungsten atoms back to filament– Gas leads to convective heat loss

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Question:

An incandescent light bulb 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

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Sealing Issues

• Atoms vibrate with thermal energy

• Average separation increases with temp

• Solids expand when heated

• Some materials expand morethan others when heated

• To avoid stress and fracture,glass and wires must expand equally

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Halogen Bulbs

• Features:– Bromine/Iodine/Oxygen gas added to bulb– Bulb has small, high temperature envelope

• Produces a filament recycling process

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Three-Way Bulbs

• Two separate filaments– One low-power filament– One high-power filament

• Three light levels– Low-power filament only– High-power filament only– Both filaments together

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Specialized Bulbs

• Clear vs. Soft white bulbs• Long life (high voltage) bulbs• Rough service bulbs• Energy-saver bulbs• Krypton bulbs• Heat bulbs• Photoflood bulbs

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Summary AboutIncandescent Light Bulbs

• Light Bulbs 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