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