17 September 2014

Lightning and Protection : Science

Lightning and Protection : NCERT / CBSE Revision Notes


Lightning is a natural phenomenon that has fascinated people for ages. Several people thought and researched about the cause of lightning and its process. Benjamin Franklin discovered that there is an electric discharge between clouds that produces a spark, and it is the electric spark between the clouds and the earth that appears as lightning. His famous kite experiment proved this fact.

The occurrence of lightning is as follows. The formation of clouds involves friction between water droplets in the atmosphere. The friction charges the particles in the atmosphere. By a process, not yet completely understood, among the positive and negative charges, the negative charges accumulate at the bottom of the cloud and the positive charges at the top. As the accumulation of the charges increases, the cloud will induce positive charges on the ground nearby. As the amount of charge increases, the negative charges on the cloud tend to make a path towards the ground, and it results in a narrow streak of electrical discharge, which we call lightning.

An electroscope is a device that detects whether an object is charged or not, and the type of charge on a body. It consists of a glass jar fitted with a cork lid and a metallic wire passing through it. There are two metallic strips at the bottom of the wire. The upper end of the wire is connected to a metal disc. When a positively charged body is brought in contact with the metal disc, the charge is transferred to the metal strips through the wire, and the strips diverge because both the strips acquire like charges and they repel. If the metal disc of the electroscope is touched with the hand, it loses its charge to the ground by transfer of charge through the human body. This is called earthing.

Must Read : Earthquakes

Protection from Lightning

Lightning is caused due to the heavy electric discharge from the clouds to the earth. If lightning is not controlled, it can cause heavy damage to life and property. Hence, protective measures are of utmost importance during lightning strikes. Interiors of buildings are safe places to protect ourselves from lightning strikes. Taking shelter in interiors and not staying in open areas is one of the best measures you can take. You should not take shelter under trees when in the open, or go to an open area if outside. If you are in a vehicle, you should not come out of it; you should also close the doors and windows of the vehicle.
Tall buildings are provided with lightning rods. A lightning rod is a metal rod that is erected at the top of a building. It is connected to a thick metallic plate, made of a good conducting material like copper and buried under the earth, through a thick copper wire. If lightning is likely to strike the building, it is attracted by the lightning rod, and the electrical discharge is transferred through the wire to the ground. Thus, it keeps the building safe.