GCSE Science/Uses of static electricity

GCSE Science/Electricity

There are several practical uses of static electricity in our daily life. We will look at three of them on this page. There are many, many more, but these are the easiest to understand. Let’s take a look at them.

Number One: The Photocopier
One example of the practical use of static electricity is a photocopier. A photocopier is a 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. Relates to xerography

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

Stage two
Paper is placed over the plate and a light shines 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 is 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. You could call this a template.

Stage three
Toner particles are sprayed 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 opposite charges attract. More light then allows the positive charge to escape (However the negative charge on the toner remains.)

Stage four
A sheet 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 through 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 is capable of producing 20 duplicate pages per minute (20ppm), which is approximately one page every three seconds.

Questions on Photocopiers
Q1) If the black areas of the image leave a positive charge in 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?

Number Two: 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 charged 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 - Because the charge is over the entire car part, the paint will stick to it.
 * Neater job - The paint is distributed evenly across the surface

Questions
Q1) If the paint droplets are given a positive charge, what charge should the car part be given?

A1) A negative charge as the positive will always be looking for a negative to balance it out.

Q2) Why does less paint fall on the floor? Give reasons for your answer

A2) This is because the oppositely charged particles are naturally attracted to each other and depending on the strength of the charge the range of attraction will increase/decrease respectively.

Q3) In the application of painting cars, wouldn't negatively charging the spray source and positively charging the object being sprayed also attract dust, dirt, and trash too?

Number Three: Pollution Control
Static electricity is used in pollution control by applying a static charge to dirt particles in the air and then collecting those charged particles on a plate or collector of the opposite electrical charge. Such devices are often called electrostatic precipitators. Out of the three this is the one people will know it most for.

Factories use static electricity to reduce pollution coming from their smoke stacks. They give the smoke an electric charge. When it passes by electrodes of the opposite charge, most of the smoke particles cling to the electrodes. This keeps the pollution from going out into the atmosphere.

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