Explanation
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Figure (a) shows what happens when a rod 'A' with a positive charge is brought near the cap. Electrons are attracted and get accumulated on 'C'. This leaves positive charges on 'L' and 'M'. The repulsion caused by the leaves diverges them. When the rod 'A' is taken away the electrons get distributed and hence the leaves close.
When a negatively charged rod 'B' is brought near the cap [Figure (b)], the free electrons in the metal are repelled. Thus all the electrons move away from the cap to 'L' and 'M'. Since both the leaves have negative charges they repel and diverge. The movement of electrons from the cap gives it an equal amount of positive charge. If the charged rod 'B' is taken away the electrons will get redistributed and the leaves collapse.
Charging by contact and conduction
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Touch the cap 'C' of the electroscope with a rod 'A' carrying a negative charge or with a rod 'B' carrying a positive charge. In both cases the leaves diverge.
Explanation
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Some of the charge on the rod is transferred by contact to 'C'. The charge spreads along the metal and the leaves will get negatively charged and repel each other.
Identifying the charge
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In order to identify the charge of a body we should use a charged electroscope. Let us say the electroscope is negatively charged.
Now bring the unknown charged body 'X' near the cap of the electroscope.
If the leaves diverge more the charge in 'X' must be negative. Suppose that the leaves close a little when 'X' is brought near 'C', then 'X' may have a positive charge or it may not have any charge.
In order to confirm the positive charge you must bring the rod 'X' near the cap of a positively charged electroscope.
If the leaves diverge more, then 'X' has positive charge.
Distribution of charge over surfaces
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Since the charges repel, the charge on a hollow or solid metal sphere will move away from the centre as far as it will go. Thus the charge on a conductor of any shape is always found on the surface and never inside the conductor. It has been found that a charged metal sphere has a uniform charge density all over its surface. A pear shaped conductor, however, has a high density at the pointed part.
Coulomb's Law
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The force of attraction or repulsion between two charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.
Sparking
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Dry air is an insulator. When two charged bodies are brought close, a very strong electric field is created between them. This field can pull out electrons from the air molecules, i.e, ionise the air present between in the gap. As a result there is a flow of charge between the two charged bodies seen as a flash and is accompanied with a crackling sound. This phenomenon of discharge is called sparking. Lighting is sparking on a very large scale.
