How Light Bulbs Work Lighting up Our Lives
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Research for the torch (second IA) (secondary research)
How Light Bulbs Work – Lighting up Our Lives
The Power
Okay. Light bulbs (known as incandescent) are really quite simple, and simply brilliant. The bulb has two metal contacts at the bottom of the base where they get their power from. These touch the electrical circuit in the fitting attached to your mains electricity, or any number of batteries, if we’re talking flashlights. The electrical charge used to light the bulb travels through it from one contact to the other in a loop. After hitting one contact the current goes up a wire to a filament, which is held on a supporting glass mount in the bulbs’ centre, then travels across it, down another wire identical to the first, and on to the other contact.
A Fundamental Filament
This filament is central in importance to the light bulb as well as central in position. It is made out of tungsten, which is a metal with an extremely high melting point, and it certainly needs one. After the light bulb is switched on, the tungsten filament is heated to between an incredible 2,200 and 2,500 degrees Centigrade! As well as its’ own properties, to further stop it burning up; the glass bulb does not contain any oxygen, but instead holds an inert gas called argon or a mixture of argon and nitrogen for all regular bulbs or krypton/xenon instead of the argon for more expensive premium models. (What about halogen bulbs? We’ll get to them later).
The filament is also tremendously long and thin. For example; in a standard 60 watt bulb the tungsten wire is over six feet long, but at the same time, it is less than one-hundredth of an inch in diameter. So how does it fit into such a tiny space? It’s double coiled, that’s how. Wound up tight to produce a first coil, then this coil is re-wound again to make the smaller than an inch filament that can be seen inside the bulb.
Good Vibrations
So the electrical charge heats up the filament to produce the light. How? The electrons that make up the electricity current rocket along, slamming into the tungsten atoms and causing them to vibrate. This friction produces heat or thermal energy, which is captured and then released by the electrons in the form of photons (light). Most of these are unfortunately in the lower end of the spectrum (known as infrared) and are invisible to humans. But the hotter the filament, the higher wavelength visible photons are emitted which we can see, and the brighter the light from the bulb.
Size Matters
Higher wattage bulbs have longer filaments, so they produce more light from having more atoms to vibrate, and conversely, low voltage light bulbs have shorter coils so the light is dimmer.
In a three-way light bulb, there are two filaments present with one being larger than the other. When the setting is on low, then only the small filament is used, so the light is dim. If the setting is put to medium, then only the larger filament has current travelling through it, and the smaller one is cut off from the flow. When the setting is on high; both filaments are in use together and the light is therefore very bright. To control this there are three connections (for the three operating modes). One each for both filaments exclusive use, and a third to be shared. A complicated switch controls the delivery of current.
Nothing Lasts Forever
So the tungsten filament is under tremendous strain, and won’t last. As the bulb is used for more and more hours the vibration and white-hot temperatures begin to take their toll. Increasingly the atoms from the coil will shake so much they will start to lose contact with each other and begin splitting away from their brethren.
In old vacuum light bulbs, they would shoot off into the space inside the bulb until they hit the glass. But with the argon or krypton inside them, modern bulbs last longer as many tungsten atoms hit the gas atoms and bounce around randomly, hopefully to reattach themselves to the filament if they get near enough to do so. Krypton atoms have more mass than those of argon and get more hits, but krypton is much rarer, so you have to pay for the benefit.
Though these light bulbs last longer, sooner or later the filament begins to disintegrate as the tell-tale darkening of the glass bulb increases (being caused by the errant tungsten) and your bulb will blow.
Often it might make the sound, ‘plink’, a few times and start flickering first, only to settle down again as if teasing you, so you decide not to change it after all and put the replacement you’ve just rummaged around for back again. Until next time you turn the lights on, that is, when it decides to give out after all.
Despite your suspicions to the contrary, however, the bulb is not getting its’ revenge on you for using it when not absolutely needed, or acting out of spite against humanity in general. It really can’t help it. A weak old bulb is at correct operating temperature when turned on for a while, but can’t reach it uniformly all the way along the filament when first switched on. There is always a surge of electricity drawn by a light bulb being turned on because of there being less electrical resistance in the tungsten when it is cooler. This resistance increases as the bulb heats up, but the weak spots in the filament heat up quicker than the rest (they have less surface area due to the evaporated tungsten atoms) and the funnelling effect will cause these weaker areas to melt or snap due to the increased vibration.
So most light bulbs may not last that long, but they are relatively cheap and very plentiful.
Hello to Halogen
A halogen bulb works differently. It still has the same tungsten filament inside it as do the others, but here chemistry is employed in addition to physics to prolong its’ working life.
Inside these light bulbs, there is a halogen gas (almost always iodine) present, mixed in with the argon or krypton. This new gas reacts with the vaporized tungsten that collects on the glass to form chemical compounds called metal halides. These then leave the inner surface of the bulb in a constant recycling process and return to near the filament where the increased heat breaks down the halide into its constituent parts. The tungsten molecules are now given to return to the filament, and the iodine molecules are free again to join up with any more ejected tungsten.
This is known as the halogen cycle. The reaction only works successfully on the glass itself though, rather than the bulb’s inner space, once the tungsten has condensed and will not take place if the glass is not hot enough. Therefore halogen light bulbs have to be smaller (which increases the heat); handmade of a special higher-grade glass known as ‘hard glass’ or of quartz to allow them not to break at this extra high temperature.
Halogen bulbs cost more, but may have a lifetime of up to triple a normal light bulb of the same wattage, and at the same time be anything up to a fifth more efficient at producing light.
Long Lifers and Energy Misers
Long life light bulbs certainly last a very long time, so it might be argued that halogen is a waste of money. This is absolutely not true. A lot of these ‘long lifers’ are actually quite inefficient.
To burn longer they burn cooler, and at lower temperatures; a smaller percentage of energy is given off as light rather than wasted heat by the tungsten.
All light bulbs waste energy by giving off infrared light so the ‘energy misers’ out there on the shelves waiting for your basket to pass by may be considered a worthwhile option. But don’t be too hasty in gathering them up either. For some of these are not as good value as the cheap and cheerful regular guys. It might be claimed that a 55 watt can replace a 60, or a 90 will do for a 100 watt, but this is not necessarily so. Many (not all) of these ‘misers’ are more mean and miserly with light than with energy, and although it may not be noticed, sometimes produce less light by such a percentage factor that actually causes them to use more watts of power for a given unit of light if you chase this down the comparison scale.
Light Bulbs Rule OK?
So that is how they work. Not encyclopedic maybe, but a brief tour of a subject that matters to all; the incandescent electric light bulb. Still going strong after more than 120 years. Fluorescent lighting and LED‘s (Light Emitting Diodes) with their ‘cold light’ technology may be pushing more and more at the margins of their rule, but traditional light bulbs are still kings in our culture today.
How they work
A flashlight (usually called a torch outside North America) is a hand-held electric-powered light source. Usually the light source is a small incandescent light bulb or light-emitting diode (LED). Typical flashlight designs consist of the light source mounted in a parabolic or other shaped reflector, a transparent lens to protect the light source from damage and debris, a power source (typically electric batteries), and an electric power switch.
While most flashlights are hand-held, there are head or helmet-mounted flashlights designed for miners and campers and battery-powered lights for bicycles. Some flashlights are powered by hand-cranked dynamos or electromagnetic induction or are recharged by solar power.
The name flashlight is used mainly in the United States and Canada. In other English-speaking countries, the more common term is torch or electric torch.
The circuit
Have you ever taken an electric torch to pieces to find out how it works? Look at the diagram below which shows the arrangement of parts inside one kind of torch:
Structure of an electric torch
Why did the designer choose this particular combination of materials? The metal parts of the torch must conduct electric current if the torch is to function, but they must also be able to stand up to physical forces. The spring holding the cells in place should stay springy, while the parts of the switch must make good electrical contact and be undamaged by repeated use.
The lamp and reflector make up an optical system, often intended to focus the light into a narrow beam. The plastic casing is an electrical insulator. Its shape and colour are important in making the torch attractive and easy to handle and use.
A torch is a simple product, but a lot of thought is needed to make sure that it will work well. Can you think of other things which the designer should consider?
A different way of describing the torch is by using a circuit diagram in which the parts of the torch are represented by symbols:
Circuit diagram of an electric torch
There are two electric cells ('batteries'), a switch and a lamp (the torch bulb). The lines in the diagram represent the metal conductors which connect the system together.
A circuit is a closed conducting path. In the torch, closing the switch completes the circuit and allows current to flow. Torches sometimes fail when the metal parts of the switch do no make proper contact, or when the lamp filament is 'blown'. In either case, the circuit is incomplete.
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Current
An electric current is a flow of charged particles. Inside a copper wire, current is carried by small negatively-charged particles, called electrons. The electrons drift in random directions until a current starts to flow. When this happens, electrons start to move in the same direction. The size of the current depends on the number of electrons passing per second.
Current is represented by the symbol I, and is measured in amperes, or 'amps', A. One ampere is a flow of 6.24x1018 electrons per second past any point in a wire. That's more than six million million million electrons passing per second. This is a lot of electrons, but electrons are very small and each carries a very tiny charge.
In electronic circuits, currents are most often measured in milliamps, mA, that is, thousandths of an amp.
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Voltage
In the torch circuit, what causes the current to flow? The answer is that the cells provide a 'push' which makes the current flow round the circuit. When the cells are new, enough current flows to light the lamp brightly. On the other hand, if the cells have been used for some time, they may be 'flat' and the lamp glows dimly or not at all.
Each cell provides a push, called its potential difference, or voltage. This is represented by the symbol V , and is measured in volts, V.
Typically, each cell provides 1.5V. Two cells connected one after another, in series, provide 3V, while three cells would provide 4.5V:
Cells connected in series
Which arrangement would make the lamp glow most brightly? Lamps are designed to work with a particular voltage, but, other things being equal, the bigger the voltage, the brighter the lamp.
Strictly speaking, a battery consists of two or more cells. These can be connected in series, as is usual in a torch circuit, but it is also possible to connect the cells in parallel, like this: