Why/How do lightbulbs fail?

Answer 1

There is a filament in the glass...the heat burns this through...the only reason why it lasts as long as it does is because the gas in it preserves it

Answer 2

Each time the bulb is turned on and off, strong forces (caused by heat) run through the filament, bending and contorting it microscopically in all sorts of wonderful ways. Eventually, the structure of the filament weakens too much, and it breaks. This usually happens right after turning the bulb on, because of the sudden jolt of heat.

Answer 3

Incandescent light bulbs consist of a glass enclosure (the "envelope, or bulb") which the bulb with an inert gas reduces evaporation of the filament and reduces the required strength of the glass. Inside of the bulb is a filament of tungsten wire, through which an electrical current is passed. The current heats the filament to an extremely high temperature (typically 2000 to 6000 K depending on the filament type, shape, and amount of current passed through). Heated electrons in the continuous energy bands of tungsten become excited and then transition to lower energy states of the solid. As they do, they release thermally equilibrated photons which have a black body spectrum. This spectrum, unlike those caused by non-equilibrium atomic or molecular transitions such as in a mercury-vapor lamp, is continuous, typically peaking in the visible light but also containing significant energy in the near-infrared wavelengths.

Incandescent light bulbs usually also contain a glass mount on the inside, which supports the filament and allows the electrical contacts to run through the envelope without gas/air leaks. Many arrangements of electrical contacts are used, such as a screw base (one or more contacts at the tip, one at the shell), a bayonet base (one or more contacts on the base, shell used as a contact or only used as a mechanical support), and for some lamps an electrical contact at either end of a tubular lamp. Contacts in the lamp socket allow the electrical current to pass through the filament. Power ratings range from about 0.1 watt to about 10,000 watts, and up. To improve the efficacy of the lamp, the filament usually consists of coils of fine wire, also known as a 'coiled coil'. For a 60 watt 120-volt lamp, the length of the filament is usually 6.5 feet or 2 metres.

One of the smallest problems of the standard electric light bulb is evaporation of the filament. The largest problem is the inevitable variations in resistivity along the filament cause non-uniform heating, with "hot spots" forming at points of higher resistivity. Thinning by evaporation increases resistivity. But hot spots evaporate faster, increasing their resistivity faster—a positive feedback which ends in the familiar tiny gap in an otherwise healthy-looking filament. Irving Langmuir suggested that an inert gas, instead of vacuum, would retard evaporation and still avoid combustion, and so ordinary incandescent light bulbs are now filled with nitrogen, argon, or krypton. However, a filament breaking in a gas-filled bulb can pull an electric arc, which may spread between the terminals and cause very heavy current flow; intentionally thin lead-in wires or more elaborate protection devices are therefore often used as fuses built into the light bulb. [1]

During ordinary operation, the tungsten of the filament evaporates; hotter, more-efficient filaments evaporate faster. Because of this, the lifetime of a filament lamp is a trade-off between efficiency and longevity. The trade-off is typically set to provide a lifetime of 750-1000 hours for ordinary lamps. See the section below, Voltage, light output, and lifetime, for a discussion of the trade-offs involved in setting a lamp life specification.

In a conventional (not halogen) lamp, the evaporated tungsten eventually condenses on the inner surface of the glass envelope, darkening it. For bulbs that contain a vacuum, the darkening is uniform across the entire surface of the envelope. When a filling of inert gas is used, the evaporated tungsten is carried in the thermal convection currents of the gas, depositing preferentially on the uppermost part of the envelope and blackening just that portion of the envelope.

Some old, high-powered lamps used in theatre, projection, searchlight, and lighthouse service with heavy, sturdy filaments contained loose tungsten powder within the envelope. From time to time, the operator would remove the bulb and shake it, allowing the tungsten powder to scrub off most of the tungsten that had condensed on the interior of the envelope, removing the blackening and brightening the lamp again.

When a light bulb envelope breaks while the lamp is on or if air leaks into the envelope, the hot tungsten filament reacts with the air, yielding an aerosol of brown tungsten nitride, brown tungsten dioxide, blue-violet tungsten pentoxide, and yellow tungsten trioxide which then deposits on the nearby surfaces or the bulb interior

Answer 4

The tungsten filament fails. I believe the mechanism is that it loses metal by sublimation.

Answer 5

the continous running of the bulb causes the filament to get thinner and ultimately it breaks when it cannot bear the heat produced.

Answer 6

A smal lamout of the filiment is burned off when the light is on. It gets thinner until it breaks.

Answer 7

the filament breaks

The bulb cracks releasing the gas inside.

Answer 8

Usually i have to reach up to the bulb. sometimes it just wants to fall out. kamikaze i guess.

Answer 9

check this out: http://www.howitworks.net/how-light-bulbs-work.html