Most of us has only seen the photocopier from outside, or just used it without any knowledge of the internal working. The working of photocopier depends on static electricity.
What is static electricity?
You might have heard about atom. An atom contains a nucleus. Inside the nucleus there are protons and neutrons. Protons are positively charged while neutrons are neutral. Electrons move in shells around the nucleus, and they are negatively charged. They move around the nucleus due to the attraction between opposite charges.
So static electricity works on charges. There are two rules that are always used:
- Like charges repels.
- Unlike charges attract.
If you bring two positively charged rods close to each other they will repel due to like charges. If a negatively charged rod is brought close to a positively charged rod, both the rods will attract due to unlike charges.
Electrical Insulators and Electrical Conductors
|Electrical Insulators||Electrical Conductors|
Charged particles are not free to move about.
Charged particles are free to move about.
It has less ability to conduct electricity.
It has high ability to conduct electricity.
It can be charged by friction.
It can be charged by induction.
Glass, Perspex, Silk, and Wool are some examples.
Copper, Iron, Steel, and Graphite are some examples.
Charging of insulators by friction:
As we know that in insulators electrons are not free to move about so they remain at the surface. Balloon and hair on the human head are insulators. If we rub the balloon against the hair, electrons will transfer from the hair to the balloon. Whenever two insulating material surfaces are rubbed against each other, electrons transfer from one material to another due to friction. As the electrons are lost from the hair, hair will become positively charged, and balloon due to gain of electrons will become negatively charged. Since both of them will have opposite charges, balloon will attract towards the hair.
Charging of conductors by induction:
Induction is the process of charging a conductor without contact between the conductor and the charging body.
Charging a single metal conductor by induction involves four steps:
1) Bring a positively charged rod (or negatively charged) near a metal conductor. The negative charges on the conductor will gather at the left side close to the rod due to attraction between opposite charges while the positive charges will gather at the right side.
2) Connect the right side of the metal conductor with the earth by touching it with you hand or a wire. This will neutralize the right side as electrons will be transferred from the earth to the conductor.
3) With the glass rod still in place, remove your hand from the conductor. This stops the earthing process.
4) Remove the glass rod. The negative charges redistribute themselves throughout the conductor. The conductor is now negatively charged.
Charging two metal spheres by induction includes three steps:
- Bring a charged rod near two conductors(metal spheres) that are touching each other.
- Due to charged rod, attraction and repulsion of charges will take place and both the conductors will have different charges.
- Remove the rod and separate the conductors to charge them.
Now you have gained an idea about how static electricity works. This will help you in understanding the working of a photocopier. There are nine steps after which you will know about the internal working of a photocopier.
- Inside the photocopier, there is a metal drum that is coated with selenium. Selenium is a photoconductor, which means it only conducts electricity in the presence of light.
- The drum is charged positively on it's surface. This is done by rotating the drum near a highly-charged wire.
- The original image to be photocopied is placed on a sheet of clear glass above the drum.
- An intense beam of light is shone on the image.
- The darker areas of the image reflects less light. The corresponding regions on the drum remain insulating, and the positive charges remain on the surface of the drum.
- The lighter areas of the image reflect more light onto the drum below. These areas of the drum become conducting and are discharged.
- As the drum continues rolling, the positively-charged image on the drum attracts the negatively-charged toner powder in the toner cartridge.
- A positively-charged sheet of paper is passed over the drum surface. Negatively-charged toner powder is attracted away from the drum and onto the sheet of paper.
- The sheet of paper is heated and pressed. This fuses the toner to the sheet of paper permanently.