Electronics/Photocopier

The Photocopier
One example of the practical use of static electricity is a photocopier. A photocopier is a fairly complicated piece of equipment, but the basic principle of how it works is fairly simple. The best way to understand what is going on is to consider it as a stage by stage process.

Stage one
Positive charge is sprayed onto a plate from a high voltage power supply. The plate is connected to the earth but the charge does not have quite enough energy to flow away. (The plate is not a good conductor of electricity)

Stage two
Paper is placed over the plate and a light shone onto the paper. Where the paper is white the light is reflected onto the plate, where the paper is dark a shadow falls onto the plate. The light falling on the plate gives it just the extra energy needed to allow the charge to escape to earth. The plate becomes neutral where the paper was white but keeps its charge where the paper is black. The plate is now a copy of the paper with charges taking the place of ink.

Stage three
Toner particles are spayed through a negatively charged nozzle onto the plate. As the toner passes through the nozzle it picks up the charge so that each particle of toner becomes negatively charged. The now charged toner is attracted to the areas of positive charge because unlike charges attract. More light then allows the positive charge to escape (however, the negative charge on the toner remains).

Stage four
A piece of paper is given a very strong positive charge, and then placed in contact with the plate. The paper attracts the toner. The paper is then removed from the plate and passed through a heating unit. The heat melts the toner and bonds it to the paper.

In a real photocopier, there is no plate, just a large drum. As the drum rotates its surface goes through stages one to four. At the end of the sequence a scraper removes any toner left on the drum and the whole process is repeated with a new image. A good photocopier can process a page in less than a second.

Questions on Photocopiers
'Q1)'If the black areas of the image leave a positive charge on the plate what charge do the white areas of the image leave? (Be very careful, the answer may not be what you think!)

Q2) How does the light shining onto the charged plate allow it to lose its charge ?

Q3) Why is the toner given a negative charge ?

Q4) Why does the paper attract the toner ?

Spray painting car parts
When paint is sprayed from a paint gun, the painter normally needs to use a fair amount of skill to ensure the paint goes on evenly. By connecting the spray nozzle to a negative electrode, it is possible to charge each droplet of paint. If the car part is then given the opposite charge, the paint droplets will be attracted to the car body part. This has several advantages.
 * Less mess- The paint goes to the body part and not to the electrically neutral floor.
 * Less wastage - All the paint ends up on the body part, very little is wasted.
 * Less skill needed - The paint wraps around the part, because the charges all over the part attract the paint. The paint even ends up on the back!

Questions
Q5) If the paint droplets are given a positive charge, what charge should the car part be given?

Q6) Why does less paint fall on the floor?