Incandescent Lightbulbs

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

Observations about Lightbulbs  Lightbulbs normally emit yellow- white light  They get 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

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?

An Incandescent Lightbulb  Light is emitted by a hot tungsten filament that is heated electrically and that emits thermal radiation that includes visible light  Heat is flowing from the hotter filament to its cooler surrounding as thermal radiation, that is partly visible light

Question 2  What determines a lightbulb’s color?

Thermal Radiation  Materials all emit thermal radiation because they contain electric charges and thermal energy causes those charges accelerate. Accelerating charges emit electromagnetic waves  Hotter temperatures yield shorter wavelengths

Black Body Spectrum  The spectrum and intensity of electromagnetic waves from a black body depend only on its temperature

Filament Temperature and Color  Filament behaves as a (nearly) black body It emits a spectrum characteristic of its temperature, so 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 and leaves as heat, some of which is radiation. Net power is zero when filament is about 2500 °C

Question 3  What determines a lightbulb’s brightness?

Power and Light  Lightbulb filament maintains zero net power Its thermal power out must equal electrical power in so its radiated power increases with electrical power.  Higher wattage bulbs use 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 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

Question 4  Why do lightbulbs eventually “burn out”?

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

Tungsten’s Shortcomings  Tungsten is burns in air and gradually sublimes  Filament is encased in inert-gas-filled glass bulb to keep out oxygen and to bounce tungsten atoms back onto filament. Inert gas, however, leads to convective heat loss and filament is short-lived above 2500 °C  Sublimation 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 bulb’s energy efficiency? A. Make it more efficient B. Make it less efficient C. No change

Sealing Issues  Atoms vibrate with thermal energy  Average separations increase with temperature  Solids expand when heated  Some materials expand more with temperature than others  To avoid stresses and fracture, glass and wires must expand equally

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

Halogen Bulbs  Halogen bulbs recycle tungsten Bromine/iodine/oxygen gases added to a small bulb that operates at a high temperature throughout. 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

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