Electrostatics

Electrostatics is the study of electric charges at rest and so it is also called static electricity.

Electric Charge

Electric charge, like mass, is one of the fundamental attributes of the particle of which the matter is made. Charge is the physical property of certain fundamental particles (like electron, proton) by virtue of which they interact with the other similar fundamental particles.

To distinguish the nature of interaction charges are divided into two parts (i) positive (ii) negative. Like charges repel and unlike charges attract. SI unit of charge is coulomb and CGS unit is esu.
1C = 3 x 109esu.

Magnitude of the smallest known charge is e = 1.6 x 10-19C (charge of one electron or proton).

Note : True test of electrification is repulsion and not attraction as attraction may also take place between a charged and an uncharged body and also between two oppositely charged bodies.

Charging of the body

Basically charging can be done by two methods; by
(i) Conduction
(ii) Induction

Ordinarily, matter contains equal number of protons and electrons. A body can be charged by the transfer of electrons or redistribution of electrons.

The process of charging from an already charged body can happen either by conduction or induction.Conduction from a charged body, involves transfer of like charges. A positively charged body can create more bodies, which are positively charged, but the sum of the total charge on all positively charged bodies will be the same as the earlier sum.
Induction is a process by which a charged body accomplishes the creation of other charged bodies, without touching them or losing its own charge. An uncharged conductor gets charged by induction due to redistribution of its own charge.

Properties of Charge

(i) Quantization of charge
Charge exists in discrete packets rather than in continuous amount.
i.e. charge on any body is the integral multiple of the charge on an electron
=> Q = ± ne, where n = 0, 1, 2, ……………..

(ii) Conservation of charge
Charge is conserved, i.e. total charge on an isolated system is constant. By isolated system, we mean here a system through the boundary of which no charge is allowed to escape or enter. This does not require that the amount of positive and negative charges separately conserved; only their algebraic sum is conserved.

(iii) Charges on a conductor
Static charges reside on the surface of the conductor.

(iv) Distribution of charges
The concentration of the charges is maximum on a surface with greater curvature.

Exercise : When a piece of polythene is rubbed with wool, a charge of – 2 × 10–7 C is developed on polythene. What is the amount of mass, which is transferred to polythene ?

(a)  5.69 × 10–19 kg            (b)  6.25 × 10–19 kg

(c)  9.63 × 10–19 kg             (d)  11.38 × 10–19 kg

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