8.5 Some uses of static electricity effects

e.g. paint spraying, particle anti-pollution and photocopiers

(a) Electrostatic sprayers are used in industry to give a thin even coating of whatever is needed to coat the surface e.g. using an electrostatic spray gun to paint the body of a car or any other object that can be given a static charge.

An electrostatic paint sprayer (spray gun) gives the tiny droplets of paint a negative static charge. The object to be painted/coated is given a positive static charge.

As you operate the spray gun, each tiny paint drop is repelled by any other (like charges repel) giving a fine and even dispersion of paint in the spray. When the spray comes near the car body the negative droplets are attracted to the positive surface of the car body and the charges neutralised as a fine and even layer of paint forms on the surface.

 The positive electric field of the bicycle frame interacts with the electric field of the paint droplets.

Electric field lines and spraying

This enables the coating to be highly controlled and the spray easily surrounds any shape so you don't get shadow regions of no or little paint and neither do you get excess paint thickness either, so very little paint is wasted.

The same principle applies to insecticide sprayers which work in a similar way to protect crops from pests.

However you can't give the plants a static charge directly! and for environmental reasons, you want to minimise the amount of insecticide used - minimising harmful pollution side-effects to other animals or plants.

To spray the crops, you use a low-flying aircraft fitted with a spray gun connected to a reservoir tank of the insecticide.

Instead the droplet-particles of the insecticide are given a static charge which makes the like-charged particles repel and spread out more evenly compared to a conventional multi-nozzle spray system.

Then, on spraying onto the plant crops, the insecticide droplets induce the opposite charge on the plant leaves (induction).

So the insecticide is attracted evenly all over the surface of the plant and coats in a much more even manner.

Its the same principle as a rubbed balloon sticking to the ceiling or spraying the car with paint described in (a)!

(b) Trapping pollution particles in smoke

Removal of harmful particulates

Static electricity in the form of charged plates across which a large negative potential difference (50,000 V) is applied can be used to minimise the dust and smoke particles in discharges from factory chimneys.

These consist of fine solid particles - some potentially harmful

The waste gases pass through a negatively charged metal mesh. This large static charge induces a negative charge on the smoke particles or dust. Further up the chimney the negative dust/smoke particle are attracted to the oppositely charged positive plates (again a very high, but positive voltage) and removed from the air before the smoke leaves the chimney. The statically attracted particles collect in the L shaped trays below the positive plates.

Every so often the collection plates have to be shaken to remove the collected particles of dust and soot and the trays emptied. This process is called electrostatic precipitation and the apparatus is called the electrostatic precipitator (electrostatic smoke precipitator, to be precise).

Static electric field removing fine particles from smoke

(c) Photocopiers and printers use static electricity to copy images onto a statically charged plate before printing them.

Laser printers use static electricity to print out documents.

Inside the printer casing is an image drum that is initially given an overall positive charge.

A laser is shone onto the drum producing a negative image on parts of the drum corresponding to the original document or file.

The toner, the powdered ink in the printer cartridge, is given a positive charge.

Therefore the ink powder is attracted to, and sticks to, the negative areas of the image drum.

Then negatively charged printer paper is rolled across the image drum.

Because the negative charge on the paper is stronger than on the image drum, the positively charged ink powder is transferred from the image drum to the paper,

The paper is then heated to fuse the ink powder to the paper, hence visually reproducing the contents of the original document or file.