Introduction
What is a Light Bulb?
A light bulb is a simple apparatus that converts electric energy into light energy. The law of conservation of energy says that energy can neither be created nor destroyed, it simply changes form. A light bulb is able to take the electricity supplied to it and change its form into something that can be used for illumination.
What is a Light Bulb made out of?
A bulb is made up of a positive and a negative terminals embedded inside glass, with a tungsten filament that joins the two. When electricity is supplied to the terminals, the flow of electrons heats up the thin filament in between. The electrons continue to bang against the filament until it heats up to the point that it begins to glow. This process happens extremely quickly.
What makes a Light Bulb glow?
The filament is encased inside transparent glass to let the light shine through and also to protect it from overheating. The air inside the glass is actually a small amount of gas that helps prevent the filament from becoming too hot and breaking. When the light bulb has fused it means that the thin coil inside has snapped and therefore electricity cannot flow completely through the circuit.
Task
For this WebQuest you will work in a small group to:
- Read the comprehension passage
- Answer the questions on the sheet provided by your teacher
- Participate in a whole class discussion to compare your answers with the other students
Process
There are two types of globes we can buy for our lights at home: the traditional incandescent globe and the more energy-efficient fluorescent globe.
Incandescent globes
The components of an incandescent globe are housed in a sealed glass case containing a gas called argon. The metal filament coil is about 2.5 cm long. It is made from 2 meters of extremely thin tungsten wire. To fit into the space, the fine strip of wire is wound into a tight coil which is then wound around itself to make an even tighter coil.
The filament is supported by 2 wires connected to a glass mount and 2 stiff contact wires that form part of a circuit.
The globe is connected to the circuit by 2 metal contacts, one at the foot of the globe and the other at the side.
Current flows from the circuit through one contact, up the stiff wire to the filament, then down the stiff wire to the other contact and back to the circuit.
As the electrons flow through the filament and crash into tungsten atoms, they release energy so the filament gets hot. The resistance of the coiled thin wire slows the flow of electrons and the energy that is released by the bombardment of atoms increases.
Only a little of the energy given off is light energy; 90% of it is released as heat. This is why incandescent globes get very hot. This is very inefficient and wastes a lot of energy.
Fluorescent globes
A fluorescent light globe is a sealed glass tube filled with argon and containing a small amount of mercury, a poisonous metal that is a liquid at room temperature. The glass tube can be a long strip, circular or coiled to fit in a standard lamp fitting.
There is an electrode at each end of the tube. When the globe is switched on, a large force between the two electrodes attracts electrons through the gas, from one electrode to the other. As the current flows, heat is produced, which turns the mercury into a gas. When electrons and argon atoms collide with the atoms of mercury gas, energy is released in the form of ultra violet light which the human eye cannot see.
However, the inside of the tube is coated with a layer of phosphor, a substance which can store energy and release it as light. The phosphor adopts the invisible ultra violet light and omits a bright visible light. The colour of the light can be varied by using different amounts of phosphor.
Most of the energy released in a fluorescent globe is converted to a visible light energy.
A ballast controls the flow of electrons through the gas. When a current flows through the gas, there is not much resistance to the flow of electrons and the current can build up. This would cause the globe to blow; however, the ballast corrects this problem.