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rubbing a glass rod with a silk cloth is an example of charging by

Charging by friction

Explanation:

Rubbing a glass rod with a silk clothe is known as charging by friction.

Charging by friction is also known as rubbing. Here, a glass rod comes into contact with the electron pool of silk and it looses electrons to the rod. Other examples are pencil and paper, fur and glass rod etc.

Atoms are structured in a way in which they have a tiny nucleus which is positively charged and a dense pool or cloud of electrons orbiting around the centralised nucleus. The electrons can readily be removed and replaced by materials coming in contact with one another.

Some substances have a higher affinity for electrons and would readily want to draw electrons to themselves. A triboelectric series offers a peep into some of these substances. The higher a material is up the series, the higher its affinity. On the series, silk shows more affinity than a glass rod. In essence, when a glass rod is rubbed against silk, their atomic space interacts and electrons are gained or lost. The silk gains electron and becomes negatively charged. The glass rod is left with and imbalance of charge in it, and it becomes positively charged.

Charging by induction does not involve contact between two substances. Here, a charged body is only brought close enough to an uncharged one. They do not touch.

Charging by conduction involves a charged body coming in contact with an uncharged body. Therefore, the charged body charges the uncharged one and electrons are exchanged.

These charging methods finds very useful application in nature and industrial scope.