Electric Field and Applications of Electrostatics
Senior 2 · Physics
Electric Field and Applications of Electrostatics
Lesson 5 of 5
25 min
Rwanda's hills and frequent thunderstorms make lightning a serious danger, and every year it kills people and livestock. Lightning arrestors on schools, churches and health centres are one of the simplest life-saving devices in physics. The same ideas also paint cars, copy and print documents, clean smoke from factory chimneys and produce the millions of volts used in research. Each one is an application of what you learned in this unit.
How lightning happens. Inside a thundercloud, collisions between ice particles and water droplets separate charge: the base of the cloud usually becomes negative. This induces a positive charge on the ground, on trees and on buildings beneath it. When the field between cloud and ground becomes strong enough to ionise the air, a conducting path forms and a huge current flows: lightning. It heats the air so fast that the air expands explosively: thunder.
The lightning arrestor (lightning conductor) has three parts: one or more pointed metal rods fixed above the highest point of the building; a thick copper strip running down the outside wall; and a metal plate or rod buried deep in moist earth.
Staying safe: when you hear thunder, go inside a building or a metal-bodied vehicle. Avoid open fields, hilltops, water and tall isolated trees, and do not shelter under a tree.
The paint droplets are given the same charge (say, positive) as they leave the spray gun. Because like charges repel, the droplets spread into a fine, even mist. The object being painted, such as a car body or a bicycle frame, is earthed or given the opposite charge, so the droplets are attracted to it. They even curve round to coat the back and hidden corners, and very little paint is wasted.
The heart of a photocopier is a drum coated with a photoconductor such as selenium: an insulator in the dark, but a conductor when light falls on it.
A laser printer works in the same way, except that the charge picture is not made by projecting a page. A laser beam, switched on and off by the computer, sweeps across the charged drum and "writes" the image dot by dot, discharging the drum where the page should stay white. Toner, paper and fusing then follow as in a photocopier.
A Van de Graaff generator builds up a very high voltage, often hundreds of thousands of volts.
Van de Graaff generators are used to accelerate charged particles for nuclear-physics research and, in schools, to make hair stand on end and to produce sparks.
Coal-fired power stations, cement works and other factories produce smoke full of fine dust that harms people's lungs. An electrostatic precipitator removes it:
Large precipitators remove over 99% of the particles from chimney gases.
Electrostatics can also be dangerous. Fuel flowing through a pipe can charge it, and a spark near petrol vapour can cause an explosion. That is why fuel tankers are earthed with a cable before fuel is transferred, and why aircraft are earthed when they are refuelled.
Tip
Explain every application with the same three ideas: (1) like charges repel, unlike attract; (2) charge collects at points and escapes from them (action of points); (3) charge on a conductor lives on its outer surface. Name the idea you are using in each step; examiners reward the physics, not the description of the machine.
State two ways in which a lightning arrestor protects a building.
Solution: (i) By the action of points its sharp rods release ions that partly neutralise the charge of the cloud, reducing the chance of a strike. (ii) If lightning strikes, it provides a low-resistance path that carries the current safely to earth instead of through the building.
Why must the bottom of a lightning arrestor be buried in moist earth rather than in dry sand?
Solution: Moist earth conducts well, so the charge flows away easily. Dry sand conducts poorly: charge could build up and the current could jump into the building or to people nearby.
In a photocopier, why does toner stick only to the parts of the drum that match the writing on the page?
Solution: Light from the white parts of the page makes the photoconductor conduct, so the charge leaks away there. The dark writing sends no light, so those areas stay positively charged, and only they attract the negatively charged toner.
Why is the dome of a Van de Graaff generator large, hollow and smooth?
Solution: Hollow: charge delivered inside moves to the outer surface, so more charge can always be added. Large and smooth: the charge is spread out with no sharp points, so the field at the surface stays below the value that would cause point discharge, and the dome can reach a very high potential.
In an electrostatic precipitator, why are the dust particles given a charge?
Solution: So that they are attracted to the oppositely charged (earthed) collecting plates and stick to them, instead of going out of the chimney.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
