incandescent lightbulbs
DESCRIPTION
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 PresentationTRANSCRIPT
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 efficientB. Make it less efficientC. 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 hotterfilament to its cooler surroundingas thermal radiation, that is partlyvisible 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 efficientB. Make it less efficientC. 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