Incandescent Light Bulbs - Physics Application

Incandescent light bulbs are the most popular way to light houses in the world and this method was the first electrical method of lighting. In this report I will be studying this outdated bulb and the physics involved in artificial lighting and will be comparing it with more efficient lighting.

History

Before incandescent light bulbs, lighting methods were dangerous. Candles, oil lamps and torches were all fire hazards and relied on burning fossil fuels to emit light. This lighting was inconsistent and concentrated so you needed many candles to light a room. When Edison and Swan invented the incandescent light bulb in the late 1800s, it quickly became the most popular form of lighting and we haven't looked back, until now.

From the burning of fossil fuels came was realisation that the hotter the burning, the whiter the light and the more light emitted. From this realisation Edison was able to design a simple light bulb that is very similar to the one we use today. It works by heating up a substance so much that it glows and emits light.

Physics in Incandescent Light Bulbs

An electrical current goes through a contact at the base of the bulb. The electricity then goes up the wire and to the filament (a coiled wire that resists electric current) where it meets resistance (slowing down of electrons causing heat). The resistances causes the electrons to bump into the atoms and this vibrates that atom which gives off heat and the atom also gives off some light photons (light particles) to release energy by photonic emission (releasing of photons).

The temperature of the filament must get to 2200°C before it will emit visible light otherwise only infrared light that we cannot see is emitted. Because of this, the filament is made to provide maximum resistance to the electrons so maximum heat will be output and more light will be emitted.



Tungsten
Not all metals can go to 2200°C without melting though. Tungsten is used as the material for the filament as it has an abnormally high melting point. The tungsten is manufactured to be very thin as thin materials are better resistors as there is less room for electrons to move.
The tungsten is also double coiled (coiled up and then coiled again) to provide maximum surface area and to make sure that enough electricity goes through the wire in a small space as well as letting enough electricity go through the wire.
1 inch of tungsten would not provide enough resistance or heat up enough for light to be emitted. In a 60 watt bulb 2 metres of tungsten wire is coiled up into just 1 inch so enough electricity can go through and be resisted.

Argon

The problem with tungsten is that at 2200°C it would simply burn away and combust. To prevent combustion the tungsten is housed in an air tight glass bulb with argon gas which is used because it is a non reactive gas (its outer shell of electrons is full and it is stable). The argon gas not only stops the tungsten combusting (no oxygen available so can't combust), it also stops the tungsten evaporating at high temperatures because most of the evaporated tungsten hits an argon atom and rebounds back into the filament. Early light bulbs used a vacuum to stop combustion but this did not stop evaporation like argon did so the life span of the bulb was shortened as the filament would just evaporate away.

Alternatives

Carbon is a better element in theory than tungsten for the filament as it has an even higher melting point but it evaporates too quickly and so the lifespan of the bulb would be short.

Krypton is the ideal gas for the housing instead of argon but is much more expensive so it is not used in most incandescent light bulbs. It is used in bulbs that need a longer lifespan like traffic lights.

Efficiency

Incandescent bulbs are not very efficient and have a short life span. Despite this they are the most popular bulb as they are very cheap and easy to install.

Incandescent is defined as "emitting light as a result of being heated." This definition reveals why incandescent bulbs are inefficient with energy as they must heat up to produce light. The energy does not go directly into lighting. Furthermore, energy is wasted emitting light we can't see. Only 5% - 10% of the energy is output as light, the rest comes off as wasted heat and rays not in the visible light spectrum.
Energy Efficient Bulbs
Energy efficient bulbs are more efficient as they use less electricity/energy as they put out less wasted heat energy.

Compact Fluorescent lamps (CFLs) use heated mercury atoms to emit UV rays that make phosphor glow and give off light. Electricity is put through mercury atoms and this gives of UV light as they become excited. This UV light then hits the phosphor and glows giving off light.

CFLs are more efficient as they don't need to reach as high temperatures in mercury to emit the UV light so they have less wasted energy as heat.

CFLs are becoming more common but their toxic substances (mercury) make them risky. The initial cost of the a CFL bulb is high but the running costs are much lower as they use up to 80% less energy. They also last 6 times longer than Incandescent light bulbs.



Despite the fact incandescent bulbs are very inefficient today, they are still much more efficient that fossil fuels used in the past. The incandescent bulb has been the most popular bulb for too long. CFLs are far more efficient and considering 25% of electricity in some countries is used on lighting maybe it is time to switch to more efficient lighting.   Tim Armstrong