Uses of Static Electricity
The Electrostatic Precipitator
Electrostatic precipitators are used in industry for environmental protection. In factories, power plants and chemical plants smoke and other exhaust have to be treated to remove dust particles and other particulates before it is released into the atmosphere.
Highly positively charged wires are stretched across the centre of the chimney or exhaust vent to form a grid. The wires are charged to a positive charge of about 50,000V. As the smoke passes over the wires the particles carried in the smoke become positively charged. These particles are then attracted towards negatively charged collecting plates where they stick. Thus, the dust particles are separated from the smoke which then carries on up the chimney and into the atmosphere. The negatively charged collecting plates are periodically struck with a mechanical hammer to remove the dust particles which then drop into a collecting bin.
Remember, the principle would also be true if the wires were negatively charged and the collecting plates positively charged. As the process relies on the principle that opposite charges attract.
Paint Spraying/Crop Spraying
The principles of electrostatics are used are used by industry in the process of paint spraying for e.g. the automotive industry when spraying cars. The nozzle of the spray gun is given a charge. The paint droplets exiting the nozzle gain this charge. As the droplets all hold the same charge they repel each other so that they spread out into a fine mist. The object to be painted is grounded or earthed. The charged droplets are attracted to the grounded object, even the back of it due to the electrostatic attraction (remember charged objects are attracted to uncharged objects). This process requires less paint and gives a uniform finish.
The same principle is used to benefit farmers in crop spraying. If the fertilizer/pesticide is given the same charge the droplets repel to form a larger cloud thereby increasing the coverage. These are then attracted to the uncharged crops.
The Photocopier
Photocopiers consist of a drum or belt covered with a layer of photoconductive material. A photoconductive material conducts electricity when struck by light (photo meaning light and conductive referring to conducting electrons).
When a photocopier is started a high voltage wire distributes a positive charge evenly on the surface of the belt or drum. An intense beam of light moves across the sheet of paper to be copied placed on the glass surface. The dark areas on the paper absorb the light and the white areas reflect the light back onto the drum/belt below. The reflected light strikes the positively charged photoconductive material on the metal neutralising it (by producing electrons). The dark areas on the paper for e.g. the text or picture that do not reflect the light leave regions of positive charges on the surface of the belt/drum (like a shadow).
Negatively charged, dry black pigment particles known as toner are then spread over the drum/belt. These negatively charged particles are attracted to the positively charged regions on the drum/belt (remember opposite charges attract). A blank sheet of paper is passed over the charged wire making it positive and then passed over the drum/belt. The positively charged paper attracts the toner off the drum/belt. The paper plus toner is then heated and passed through rollers to melt and fuse the toner to the paper.