4.Charging by Contact

Objects become electrically charged in either of two ways: by contact or by induction.

A charged object transfers electric charge to an object with lesser charge if the two touch. When this happens, a charge flows from the first to the second object for a brief time. Charges in motion form an electric current. When charge flows between objects in contact, the amount of charge that an object receives depends on its ability to store charge. The ability to store charge is called capacitance and is measured in units called farads.

Charging by contact can be demonstrated by touching an uncharged electroscope with a charged comb. An electroscope is a device that contains two strips of metal foil, called leaves, that hang from one end of a metal rod. A metal ball is at the other end of the rod. When the charged comb touches the ball, some of the charges on the comb flow to the leaves, which separate because they now hold like charges and repel each other. If the comb is removed, the leaves remain apart because they retain their charges. The electroscope has thus been charged by contact with the comb.

This flow of charge between objects with different amounts of charge will occur whenever possible. However, it requires a pathway for the electric charge to move along. Some materials, called conductors, allow an electric current to flow through them easily. Other materials, called insulators, strongly resist the passage of an electric current.

Under normal conditions, air is an insulator. However, if an object gains a large enough charge of static electricity, part of the charge may jump, or discharge, through the air to another object without touching it directly. When the charge is large enough, the air becomes a conductor. Lightning is an example of a discharge. B.

5.Coulomb’s Law

Objects with opposite charges attract each other, and objects with similar charges repel each other. Coulomb’s law, formulated by French physicist Charles Augustin de Coulomb during the late 18th century, quantifies the strength of the attraction or repulsion. This law states that the force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. The greater the charges on the objects, the larger the force between them; the greater the distance between the objects, the lesser the force between them. The unit of electric charge, also named after Coulomb, is equal to the combined charges of 6.24 × 1018 protons (or electrons).

If two charged objects in contact have the same capacitance, they divide the charge evenly. Suppose, for example, that one object has a charge of +4 coulombs and the other a charge of +8 coulombs. When they touch, charge will flow from the 8-coulomb object to the 4-coulomb object until each has a charge of +6 coulombs. If each object originally had a charge of +6 coulombs, no charge would flow between them.

If two objects have different capacitances, they divide the charge in proportion to their capacitances. If an object with a capacitance of 10 farads touches an object with a capacitance of 5 farads, the 10-farad object will end up with twice the amount of charge of the 5-farad object. Suppose that the objects are oppositely charged and that one has a charge of +20 coulombs and the other a charge of -8 coulombs. Their total charge is therefore +12 coulombs. After they touch, the 10-farad object will have a charge of +8 coulombs and the 5-farad object will have +4 coulombs.