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

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Incandescent Lightbulbs 1

Incandescent Lightbulbs

Incandescent Lightbulbs 2

Introductory Question

• An incandescent lightbulb contains some gas

with the filament. How would removing the gas affect the bulb’s energy efficiency?

A.

Make it more efficient

B.

Make it less efficient

C.

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 lightbulb’s color? What determines a lightbulb’s 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 filament’s life

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Question 2

What determines a lightbulb’s 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

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

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Question 3

What determines a lightbulb’s 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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Tungsten’s 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 bulb’s energy efficiency?

A.

Make it more efficient

B.

Make it less efficient

C.

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 more

than 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 bulb’s 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