Contents
Physics Topics are often described using mathematical equations, making them precise and quantifiable.
Why Charges Accumulate at Sharp Points? And What is a Lightning Conductor?
We know that if a conductor with a sharp point be charged, the surface density of charge at the pointed part becomes very high. It will then induce unlike charges of the air molecules and dust particles in the vicinity which will be attracted towards it. Due to this attraction they come in contact with the sharp end and their unlike charges get neutralised.
Only the particles having charges similar to the sharp end retain their charge and are therefore repelled strongly by the pointed end. As a result of both these processes (attraction and repulsion), the conductor gradually loses its charge through the pointed end. This is known as discharging action of points. So a conductor should be round and without any sharp end to retain its charge for a long time.
Lightning Conductor or Lightning Arrester
In 1752 AD, Benjamin Franklin experimentally proved the exis-tence of charges in the clouds and in the atmosphere. Scientists found that cosmic rays, ultraviolet rays and the rays emitted from radioactive substances on earth, charge the air particles and water drops of the clouds. Moreover, due to mutual friction of the clouds the water drops in it become charged. Both types of charges, positive and negative, are produced. A flash of lightning is nothing but a discharge of electricity along an air-track in the sky.
Such discharges are possible because of enormous difference in potential that may exist between a charged cloud and the earth or in between two oppositely charged clouds. The air, in the path of the lightning, is heated up due to the discharge and expands suddenly. This sudden expansion highly reduces the pressure of this area and as a result, the surrounding air rushes there with a tremendous force. The report of the thunder is due to these sudden expansions and contractions of the air.
A highly charged cloud usually causes electrostatic induction on earth. So the potential difference between cloud and ground may become high enough to start an electric discharge. This is known as lightning. The sound heard just after a lightning is called thunderclap.
During lightning, a high current is set up from the cloud to the ground. This current follows the least resistive path. For this, thunder generally strikes on tall buildings or trees. To protect highrise buildings from thunderbolt, lightning conductor is used. It consists of a long and thick copper strip. The upper end of it is designed with sharp points and the lower end is fixed to a metal plate buried deep in the ground [Fig.].
The lightning conductor protects the building using the dis-charging action of sharp points of the conductor. When a charged cloud passes above the points of the lightning conduc-tor, it induces unlike charge on the sharp points. The like charge induced on the other end passes to the earth. The discharging action of the sharp points partially neutralises the charge on the cloud. Hence possibility of lightning is reduced markedly. For this reason, the lightning conductor is also called lightning arrester.
Sometimes even after the discharging action of the sharp points, a discharge may take place between the charged cloud and the building. Then the thick copper strip provides the path of least resistance for the charge to flow to the earth without damaging the building. For this reason copper strips with sharp points are used as lightning conductors.
Necessary qualities of a good lightning conductor:
A good lightning conductor should have the following properties:
- The copper strip should not melt due to heat evolved during lightning discharge.
- The upper part of the copper strip should be provided with sufficient number of sharp points.
- No discontinuity should exist in the copper strip. The lower end of it should be buried deep in the ground.
Safe shelters and unsafe places during lightning:
Buildings on metal frame, houses fitted with lightning conduc-tors are the safest place during lightning. Staying in a car of metal frame with windows closed also offers adequate protec-tion, if it is connected to the earth.
On the other hand, stray tall trees, telegraph and telephone posts, high fences of mud etc. are unsafe places at the time of lightning.
It may be noted that, although lightning and thunder take place simultaneously, the sound of thunder reaches us much later, because the velocity of sound is much less than that of light. So, if a man hears the sound of thunder, there will be no chance of his being struck by lightning.