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- Why do light-bulb filaments usually blow when first switched on, instead of at other times when they have been on for a long time and at their hottest?
- Why is it that the filament is more likely to blow when switched on than when the light is being switched off?
- What causes the blackening on the inside of a light bulb?
- Then why choose tungsten as the filament material?
- Sometime there is a spark or arc when a light bulb burns out. Why's that?
- How does a halogen lamp work?
- Which saves energy, halogen or ordinary light bulb?
Written by: LEE Boon-ying
- Why do light-bulb filaments usually blow when first switched on, instead of at other times when they have been on for a long time and at their hottest?
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This is primarily because the highest current flows through the filament the moment it's switched on.
Let's re-visit some high-school science. The resistance of a material usually increases with temperature. For instance, for tungsten which light filaments are commonly made of, the resistance increases by about 0.6% for every degree rise in temperature. The filament of a lighted bulb can reach more than 2,000 degrees Celsius. This means that the filament's resistance rises from a few ohms when cold to tens of ohms when hot. Recalling the relationship I = V/R (I is the current, V the voltage and R the resistance), it is obvious that, for a given voltage V (normally 220 V in Hong Kong), the initial current is largest and amounts to more than 10 times the rated value. The initial heating is also highest (power P = IV = V2/R), resulting in thermal stress on the filament.
For light bulbs that have been used for some time, inevitably there is some part of the filament that is slightly thinner than the rest. This part will heat up faster as it has a higher resistance (P = I2R), making matters worse. This also causes further thinning of the already thinner part.
(Source: Wikipedia)
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- Why is it that the filament is more likely to blow when switched on than when the light is being switched off?
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When the light is switched off, the filament is in warm surroundings and cools down slowly, in contrast with the rapid heating experienced when being switched on. Hence the chance of blowing is lower when the light is being switched off.
The use of a dimmer switch reduces the initial current flowing through the filament and lessens the chance of blowing the bulb when you switch it on.
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- What causes the blackening on the inside of a light bulb?
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The blackening results from evaporation of tungsten and then deposition on the inside of the bulb. Evaporation also results in thinning of the filament with use.
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- Then why choose tungsten as the filament material?
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Tungsten is used because it has a high melting point (over 3,000 degrees Celsius) and stays strong even at high temperatures. Moreover, it evaporates less than other common metals (i.e. it has a very low vapour pressure).
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- Sometime there is a spark or arc when a light bulb burns out. Why's that?
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The spark or arc is actually an electrical discharge across the broken ends of the filament. The arc allows a large current to flow through, even for a very short time, and sometimes causes a fuse to blow or a circuit breaker to trip.
Tapping or shaking of a burnt light bulb may sometimes cause the broken ends of the filament to re-connect, extending the life of the bulb by a very short time.
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- How does a halogen lamp work?
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An ordinary light bulb contains an inert gas (such as argon or krypton) and nitrogen, which keep the tungsten filament from oxidizing (i.e. burning up). In contrast, a halogen bulb contains iodine (or occasionally bromine), which belongs to a highly reactive chemical family, the halogens.
Why choose a reactive material? The iodine inside a halogen bulb preserves the tungsten in a different way. It reacts with the evaporated tungsten atoms to become tungsten iodide, a gaseous compound. The compound then floats around inside the bulb until it hits the hot filament, whereupon the high temperature breaks it down into iodine vapour and metallic tungsten. This way, the filament remains intact and the 'recycling' process continues. For this reason, a halogen bulb lasts twice as long.
The chemical process enables the lamp to operate at a high temperature and emits bright, white light. The bulb casing thus is made of quartz to withstand the high temperature.
(Source: Wikipedia)
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- Which saves energy, halogen or ordinary light bulb?
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Like ordinary light bulbs, halogen lamps only use 10 to 12% of the energy in generating light (the rest is mostly emitted as heat), and are not as efficient as fluorescent lamps or compact fluorescent lamps.
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Reference:
- "What Einstein told his barber", R.L. Wolke, Dell Publishing, 2000.
- "New Scientist - Why don't penguins' feet freeze?", Profile Books, 2006.
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