Electric Discharge. How does it work?

Many of the everyday effects of electrostatics involve a charged object losing its charge and being neutralised. This is called: electric discharge. Some common examples are discussed below.

Earthing/ Grounding

The earth is a relatively good conductor and, because of its size, it can receive or give up a large number of electrons without becoming appreciably charged. If a negatively charged conductor is connected to the earth, surplus electrons on the object will drain off to the earth until the object has discharged completely and is neutral. Similarly, a positively charged object connected to the earth will attract electrons up from the earth until the object is neutralised. Both of these situations are examples of discharge by earthing or grounding.

Atmospheric discharge from the surface of a conductor

Because similar charges repel each other, the charge on a conductor spreads over the entire surface. The distribution of charge over the surface of a sphere is uniform, but for other shapes, the charge tends to be concentrated near any sharply contoured features. For example, the greatest concentration of charge occurs around sharp, pointed areas on a conductor’s surface.

All air contains some positive and some negative ions. Moist, humid air contains many more of these ions. When ionised air comes into contact with the surface of a charged conductor, oppositely charged ions are attracted towards the conductor, and similarly charged ions are repelled. Electrons are easily transferred to and from highly charged areas of the conductor, and the conductor will discharge as a result.

Sparks and Arcing

When an object has an excess of electrons, they exert strong forces of repulsion on each other and move as far away from each other as possible. If such a charged object approaches another conductor, some electrons may even jump across the gap of air between the two. This type of discharge is called a spark.

As electrons jump across the gap, a cracking sound is heard and a small flash of light is often seen. The electrons ionise the air and produce a great quantity of heat. This heat causes the air to expand rapidly and thereby to produce a compression wave that spreads out at the speed of sound and is heard as cracking noise. The heat energy is also capable of causing the air molecules to produce light energy, which we see as a flash. This type of spark will be familiar if you have ever walked around on a rug and then touched a metal doorknob.

Friction between rubber tyres and the road causes vehicles to be electrically charged. A gasoline truck uses a metal chain to allow this charge to drain off to ground. Otherwise, the heat created by a spark discharge might ignite flammable vapour from the truck and cause an explosion. More recent tyres have carbon in the rubber that acts as a conductor so that static charges do not accumulate on the vehicle.

When a large number of electrons jump across a gap between two conductors, a tremendous amount of heat is generated. The temperatures produced can be sufficient to weld metals together. A device that welds metals by means of the heat generated by forming a large current across a gap in a conductor is called “arc welder”.

Lightning

Lightning is the discharge of electrons occurring between two charged clouds or between a charged cloud and the earth. Lightning usually strikes the highest point, generally, chimneys or very tall trees and the current passes to the earth through the path of least resistance. In the process, considerable heat is produced. Such heat may split building or set them on fire.

Rapid heating and formation of large raindrops from smaller ones in the atmosphere cause clouds to become electrically charged. A charged cloud induces a strong opposite charge on the surface of the earth directly beneath it. If the charge on the cloud increases beyond a certain point, a gigantic spark discharge occurs in the form of lightning.

Surplus electrons from a negatively charged cloud may jump across the air gap to Earth, or electrons may jump from the ground across the air gap to neutralise the deficit of electrons on a positively charged cloud. Lightning stroke may also travel between two oppositely charged clouds, or between two opposite charge centre in the same cloud. The path way becomes ionised and guides the main charge flow.

The dangers presented by lightning are immense. The discharge takes the shortest path to earth and therefore usually strikes the tallest conductor in the vicinity. To prevent buildings from being stricken by lightning, conductors are erected on top of buildings. The conductors could be a thick copper strip fixed to an outside wall, reaching the highest point of the building and ending in several sharp spikes.

When a negatively charged thundercloud passes overhead, it acts inductively on the conductor charging the points positively and the earthed plate negatively. The negative charge of the plate flows onto the earth. Negative ions are attached to the spikes and become discharged by giving up their electrons. The electrons pass down the conductor to the earth. Positive ions are repelled upwards from the spikes and spread out to form a space charge. In this way, the building is protected.

Properties of a lightening

• Length: from 150m to about 3km
• Duration: from 0.0002s to as much as 1.6s
• Width: from 1cm to about 30cm
• Temperature: up to 30000°C
• Electricity: Up to 200C (coulombs) of charge transferred, with a power up to many billions of kilowatts.

Lightning rods are not as a rule, needed in cities. Tall buildings have a sharp, pointed feature that acts as areas of high concentration of induced charge. As result, lightning rarely strikes shorter objects in urban areas.

Note: As a result of the high temperatures created by lightning strikes, oxygen and nitrogen in the atmosphere combine chemically to produce nitrates. These nitrates fall to earth with the rain and replenish our supply of natural fertiliser. Ozone which protects the earth from harmful cosmic radiation from outer space is also produced from oxygen atoms in the atmosphere during lightning storms.

Electrostatic generators

The van de Graaf generator was invented in 1931 by Robert Van de Graaf, an American Physicist working at the Massachusetts Institute of Technology.

The van de Graaf generator produces a continuous supply of charge on a large metal dome when a rubber belt is driven by an electric motor or by hand

1. Demonstration

In Figure (a) sparks jump between the dome and the discharging sphere. Electrons flow around a complete path (circuit) from the dome. Can you trace it? In (b) why does the ‘hair’ stand on end? In (c) the ‘windmill’ revolves due to the reaction that arises from the electric wind caused by the action at point’s effect.

The body on the insulating stool first gets charged by touching the dome and then lights a neon lamp. The dome can be discharged harmlessly by bringing your elbow close to it.

• Action

Initially, a positive charge is produced on the motor-driven Perspex roller due to it rubbing the belt. This induces a negative charge on the ‘comb’ of metal points P (figure-a) which are sprayed off by action points onto the outside of the belt and carried upwards. A positive charge is then induced in the comb of metal points Q and a negative charge is repelled to the dome.