THEORY OF ELECTRIC CHARGE AND FORCE

Electric charge is an entity such that a body acquiring electric charge is capable of attracting small objects like bits of paper.

SI Unit: Coulomb Electrostatic unit (esu): Stat coulomb Electromagnetic unit (emu): Ab coulomb

CONCEPT: A body is charged by friction due to transfer of electrons, which are material particles. Hence a body acquiring –ve charge becomes heavier due to gain of electrons and the body acquiring +ve charge becomes lighter due to loss of electrons.

Quantisation of charge: Charge carried by a body is not of continuous nature but of discrete nature. It is an integral multiple of the magnitude of charge on an electron. Hence charge is said to be quantised.

Conservation of charge: Net charge on an isolated system can neither be created nor destroyed. The net charge on an isolated system always remains constant. There is no exception to this law.

Invariance of charge: The magnitude of charge remains the same when converted from one inertial frame to another. It means that there is no effect of speed of charge on its magnitude.

Coulomb’s Law : Two stationary point charges attract or repel each other with a force, which is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them.

Limitations of Coulomb’s law: It is applicable only to point charges and charges at rest.

Concept: Dielectric constant of a conductor is infinite.

Concept: Minimum value of dielectric constant is 1 (for free space) and for any other medium, K > 1.

Concept: When a dielectric slab is introduced between two charges, then the force between them decreases.

Electrostatic and Gravitational forces

Similarities

i) Both obey inverse square law. ii) Both are central forces. iii) Both are conservative in nature.

Differences

i) Gravitational force is always attractive where as electrostatic force can be both attractive and repulsive. ii) The electrostatic force depends on the medium in which the charges are placed whereas the gravitational force is independent of the medium.

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