Lightning

Objective

This topic gives an overview of; 

  • Lightning
  • Charging by rubbing
  • Types of Charges and their Interaction
  • Transfer of Charge
  • The Story of Lightning

Lightning

You might have seen sparks on a electric pole when wires become loose. This phenomenon is quite common when a wind is blowing and shacking the wires. You might also have seen sparks when a plug is loose in its socket. Lightning is also an electric spark, but on a huge scale.

In ancient times people did not understand the cause of these sparks. They were, therefore, afraid of lightning and thought that the wrath of gods was visiting them. Now, of course, we understand that lightning is caused by the accumulation of charges in the clouds. We need not be afraid of lightning, but we have to take precautions to protect ourselves from the deadly sparks.

The Sparks that the Greeks Knew About

The ancient Greeks knew as early as 600 B.C. that when amber (amber is a kind of resin) was rubbed with fur, it attracted light objects such as hair. You might have seen that when you take off woollen or polyester clothes, your hair stands on ends. If you take off these clothes in the dark, you see even a spark and hear crackling sound. In 1752 Benjamin Franklin, an American scientist, showed that lightning and the spark from your clothes are essentially the same phenomena. However, this realisation took 2000 years.

We shall now study some properties of electric charges. We shall also see how they are related to the lightning in the sky.

Let us perform some activities to understand the nature of electric charges. But recall first what you might have played as a game. When you rub a plastic scale on your dry hair, the scale can attract very small pieces of paper.

Charging by rubbing

Take a used ballpen refill and rub it vigorously with a piece of polythene. Bring it close to small pieces of paper. Take care not to touch the rubbed end of the refill with your hand or with a metallic object. Repeat the activity with small pieces of dry leaf, husk and mustard seeds. Record your observations.

When a plastic refill is rubbed with polythene, it acquires a small electric charge. Similarly, when a plastic comb is rubbed with dry hair, it acquires a small charge. These objects are called charged objects. In the process of charging the refill and the plastic comb, polythene and hair also get charged.

Let's try to charge some other objects that are familiar to you. Collect the objects and the materials. Try to charge each by rubbing with the materials mentioned in the Table. Record your findings.

Objects rubbed Materials used for rubbing Attracts/does not attract pieces of paper Charged /not charged
Refill Polythene, woollen cloth  Attracts   Charged
Balloon Polythene, woollen cloth  Attracts   Charged
Eraser Wool Attracts Charged
Steel spoon Polythene, woollen cloth
 
Does not attract not charged

Types of Charges and their Interaction

Inflate two balloons. Hang them in such a way that they do not touch each other. Rub both the balloons with a woollen cloth and release them. A charged balloon repelled a charged balloon.

Now let us repeat this activity with the used pen refills. Rub one refill with polythene. Place it carefully in a glass tumbler using the tumbler as a stand. Rub the other refill also with polythene. Bring it close to the charged refill. Be careful not to touch the charged end with your hand. The two  repel each other.

Rub a refill and place it gently in a glass tumbler as before. Bring an inflated charged balloon near the refill and observe.The balloon and refill get attracted. 

Let's summarise the observations:

  • A charged balloon repelled a charged balloon.
  • A charged refill repelled a charged refill. 
  • But a charged balloon attracted a charged refill.

Does it indicate that the charge on the balloon is of a different kind from the charge on the refill? Can we say then, that there are two kinds of charges? Can we also say that the charges of the same kind repel each other, while charges of different kind attract each other?

It is a convention to call the charge acquired by a glass rod when it is rubbed with silk as positive. The other kind of charge is said to be negative.

It is observed that when a charged glass rod is brought near a charged plastic straw rubbed with polythene there is attraction between the two. A plastic straw would carry a negative charge.

The electrical charges generated by rubbing are static. They do not move by themselves. When charges move, they constitute an electric current. The current in a circuit which makes a bulb glow, or the current that makes a wire hot, is nothing but a motion of charges.

Transfer of Charge

Take an empty jam bottle. Take a piece of cardboard slightly bigger in size than the mouth of the bottle. Pierce a hole in it so that a metal paper clip could be inserted. Open out paper clip. Cut two strips of aluminium foil about 4 cm 1 cm each. Hang them on the paper clip as shown. Insert the paper clip in the cardboard lid so that it is perpendicular to it. Charge a refill and touch it with the end of the paper clip. Observe what happens. The Foil strips repel each other.  Touch now, other charged bodies with the end of the paper clip.  Foil strips behave in the same way in all cases. This apparatus  can be used to detect whether a body is charged or not.

The aluminium foil strips receive the same charge from the charged refill through the paper clip (remember that metals are good conductors of electricity). The strips carrying similar charges repel each other and they become wide open. Such a device can be used to test whether an object is carrying charge or not. This device is known as electroscope.

Thus, we find that electrical charge can be transferred from a charged object to another through a metal conductor.

Touch the end of the paper clip gently with hand and you will find a change in the foil strips. They come back to their original state. Repeat charging of foil strips and touching the paper clip. Every time you will find that the foil strips collapse as soon as you touch the paperclip with hand. The reason is that the foil strips lose charge to the earth through your body. We say that the foil strips are discharged. The process of transferring of charge from a charged object to the earth is called earthing.

Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electrical current.

The Story of Lightning

It is now possible to explain lightning in terms of the charges produced by rubbing.During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These vigorous movements cause separation of charges. By a process, not yet completely understood, the positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges. There is accumulation of positive charges near the ground also. When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound. We see streaks as lightning . The process is called an electric discharge.

The process of electric discharge can occur between two or more clouds, or between clouds and the earth. Today we need not get frightened by lightning like the ancient people did. Now we understand the basic phenomenon. Scientists are trying hard to improve our understanding. However, lightning strike could destroy life and property. It is, therefore, necessary to take measures to protect ourselves.

Summary

  • Some objects can be charged by rubbing with other objects.
  • There are two kinds of charges - positive charge and negative charge.
  • Like charges repel and unlike charges attract each other.
  • The electrical charges produced by rubbing are called static charges.
  • When charges move, they constitute an electric current.
  • An electroscope may be used to detect whether a body is charged or not.
  • The process of transfer of charge from a charged object to the earth is called earthing.
  • The process of electric discharge between clouds and the earth or between different clouds causes lightning.

 

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