Physics Topics can also be used to explain the behavior of complex systems, such as the stock market or the dynamics of traffic flow.
Lightning Definition, Explaination and Dangers
During rainy season, many times we see a flash of light in the clouds in the sky which is followed by a loud sound called thunder (garjan). The bright flash of light which we see in the clouds is called lightning.
Lightning is an electric discharge in the atmosphere between oppositely charged clouds (or between charged cloud and the earth). Lightning is actually a great electric spark in the sky. Lightning is produced by the electric charges in the sky. The electric nature of lightning was established by a scientist named Benjamin Franklin. We will now describe how a storm cloud gets electrically charged and then produces lightning.
A cloud is a visible mass of condensed water vapours floating in the atmosphere, high above the ground. A heavy, dark, rain cloud is called storm cloud. When a storm cloud develops in the sky, strong winds move upwards through the cloud and make the water drops present in the cloud to rub against one another. This rubbing together of water drops produces extremely large electric charges in the cloud due to friction. The small water drops acquire a positive charge and, being lighter, move to the upper part of the cloud with rising wind. On the other hand, the larger water drops acquire a negative charge and, being heavier, come down in the lower part of the cloud. In this way, the top of the cloud becomes positively charged whereas the bottom of the cloud becomes negatively charged (see Figure).
When the amount of opposite electric charges on the top and bottom of a storm cloud becomes extremely large, then electric charges start flowing with high speed through the air between them. When the positive and negative charges of a cloud meet, they produce an intense spark of electricity between the cloud in the sky. We see this electric spark as a flash of lightning is the sky. The electric sparks of lightning heat the nearby air in the sky to very high temperatures. Due to this heat, the air in the sky expands rapidly and produces a loud sound which we call thunder.
Lightning usually occurs within a cloud in the sky. It is called ‘sheet lightning’. Sometimes, however, lightning also occurs between a cloud and the earth (or tall objects of the earth). It is then called ‘fork lightning’. This is described below.
Storm clouds carry electric charges. Now, if a storm cloud having negative charges at its bottom passes over a tall building, it induces positive charges on the roof of the building (see Figure). When the electric charges on the bottom of the cloud become extremely large, then these tremendous electric charges present on the bottom of the charged cloud suddenly flow to the roof of the building and we see a flash of lightning coming towards the building. We say that lightning has struck the building (see Figure). Thus, lightning strikes the earth or its tall structures when electric charges flow between the cloud and the earth through a tall building, a tree or any other object.
Lightning strikes are more frequent in the hilly areas because in such areas clouds are comparatively closer to the ground than in the plains. In the plains, lightning usually strikes tall structures like tall buildings, factory chimneys, radio and TV transmission towers or big trees. This is because all these tall objects are closer to the charged clouds than the ground.
When we take off woollen or synthetic clothes (like polyester or nylon clothes), sometimes we hear a crackling sound. And if it is dark (as during night), we can even see tiny sparks. This happens as follows : When we take off a woollen (or synthetic) sweater, it rubs against our shirt. The rubbing together of sweater and shirt produces opposite electric charges on them. The discharge of these electric charges produces tiny sparks of light as well as crackling sound. In 1752 an American scientist, Benjamin Franklin, showed that the tiny sparks of light observed while putting off clothes and the lightning in the sky are essentially the same phenomenon (of electric discharge).
Sometimes we also see sparks on an electric pole when the naked electric wires fixed on the tall poles become loose and touch one another. Such sparks are quite common when a strong wind is blowing and shaking the over-head electric wires too much. Sometimes we also see electric sparks in our home when a plug is loose in its socket and a current is switched on.
Dangers of Lightning
A flash of lightning carries a lot of electric energy. When lightning strikes a building, its tremendous electric energy can set the building on fire or cause serious damage to its structure. When lightning strikes a tree, it can burn up the tree and damage it by its enormous electric energy. And when a person is hit by lightning during a thunderstorm, then the electric energy passes through the body of the person due to which the person gets severe burns and gets killed. Thus, when lightning strikes the earth, it can cause a lot of destruction by damaging property (buildings, etc.), trees and killing people. Since lightning strikes can destroy life and property, it is, therefore, necessary to take measures to protect ourselves and our buildings from the dangers of lightning. The damage caused to buildings and other tall structures by lightning can be prevented by installing lightning conductors on them. This is discussed below.
Lightning conductor is a device used to protect a building from the effects of lightning. The tall buildings (and other tall structures) are protected from lightning strikes by using a device called lightning conductor. A lightning conductor is made of a thick strip of metal (usually of copper). The top end of lightning conductor is pointed like a sharp spike (or spikes) and it is fixed above the highest point of the building (see Figure). From the top of the building, the thick metal strip runs along the outer wall of the building to the ground. The lower end of metal strip is joined to a metal plate and buried deep in the ground near the base of the building (see Figure).
If lightning strikes, it will hit the top of the lightning conductor rather than the building. The electric energy of lightning passes through the metal strip and gets discharged safely into the ground through the buried metal plate. Since no electric energy produced by lightning passes through the building, no damage is caused to it. Thus, lightning can be discharged harmlessly into the ground (or earth) through the lightning conductor fitted on tall buildings and other tall structures like factory chimneys, radio and TV transmission towers and monuments like Qutab Minar. We know that metals are good conductors of electric charges (or electricity). So, a lightning conductor made of a metal works by conducting the electric energy of lightning into the earth. We can see the lightning conductors fixed on many tall buildings, factory chimneys, etc. Lightning conductor was invented by Benjamin Franklin.
If a tall building is not protected with a lightning conductor, then the tremendous electric energy produced during lightning would pass through the walls of the building, causing damage to the material of the walls and making the walls unsafe. It can even set the building on fire. A lightning conductor protects a tall building against lightning by providing its electric energy an easy path to go to the ground. Thus, the function of a lightning conductor is to conduct any lightning strikes safely to the earth (without causing any damage to the building).
Measures to Protect Ourselves from Lightning
We can take the following measures (or steps) to protect ourselves from lightning strikes during a thunderstorm:
(i) No open space is safe during lightning and thunderstorm. A house (or any other building) is a safe place during lightning. So, if we are in an open space (such as a park, playground, or road, etc.), we should rush to a safer place like a house or some other building nearby on hearing the thunder and observing the lightning in the sky.
(ii) Open vehicles like motorbikes, scooters, tractors, and construction machinery (like earth movers, etc.) are not safe during lightning and thunderstorm. So, we should leave such open vehicles during lightning and take shelter inside a house (or some other building). If, however, we are travelling by a covered vehicle like a car (or bus) when thunderstorm and lightning occur, then we are safe inside the car (or bus) with windows and doors of the vehicle closed.
(iii) If a person is in open space when thunderstorm and lightning begin, and there is no suitable shelter available nearby, then the following precautions should be taken for protection from lightning :
(a) When in open space, a person should never stand under a tree to take shelter during a thunderstorm because there is danger of lightning striking the tree and burning it up. This lightning can also pass through the body of the person standing under the tree and kill him. If, however, a person is in a forest, he should take shelter under a short tree because a short tree is less likely to be hit by lightning. On the other hand, a tall tree (being nearer to the thunderclouds) is more likely to be hit by lightning.
(b) When in open space, a person should not lie on the ground during the thunderstorm and lightning. A person should squat low on the ground during lightning. The person should place his hands on his knees with his head between the hands (as shown in Figure). This position will make the person the shortest object around which is unlikely to be hit by lightning. The person should also stay away from electric poles, telephone poles and other metal objects during lightning.
(iv) We should avoid raising an umbrella over our head during lightning.
This is because lightning may strike the top end of the metal rod of umbrella (held high over the head) and harm us.
(v) The TV antennas and dish antennas fixed on tall buildings are especially prone to lightning strikes. We should, therefore, switch off our TV sets during frequent lightning otherwise TV sets may get burnt.
(vi) Lightning can strike metal pipes (such as water pipes) fixed in buildings. So, during a thunderstorm when lightning is taking place, we should avoid touching the metal pipes fixed in a house or building.