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Electric Charge

  • Charge is the property associated with matter due to which it produces and experiences electrical and magnetic effects.
  • It is known that every atom is electrically neutral, containing as many electrons as the number of protons in the nucleus.
  • Charged particles can be created by disturbing the neutrality of an atom. Loss of electrons gives positive charge (as then np > ne) and gain of electrons gives negative charge (as then ne > np) to a particle.

Unit and dimensional formula

S.I. unit of charge is Ampere × sec = coulomb (C), smaller S.I. units are mC, μC.
C.G.S. unit of charge is Stat coulomb or e.s.u. Electromagnetic unit of charge is ab coulomb.
Dimensional formula, 36952.png

Properties of Charge

  • It can be transferred from one body to another.
  • Charge cannot exist without mass but reverse is not true.
  • It can neither be created nor be destroyed.
  • Independent of velocity of charged particle.
  • Electric charge produces electric field 36958.png, magnetic field 36968.png, and electromagnetic radiations.

Point charge

A finite size body may behave like a point charge if it produces an inverse square electric field. For example, an isolated charged sphere behave like a point charge at very large distance as well as very small distance close to its surface.

Charge on a conductor

The charge given to a conductor always resides on its outer surface. This is why a solid and hollow conducting sphere of same outer radius will hold maximum equal charge. If surface is uniform the charge distributes uniformly on the surface and for irregular surface the distribution of charge, i.e., charge density is not uniform. It is maximum where the radius of curvature is minimum and vice versa, i.e., σ ∝ (1/R). This is why charge leaks from sharp points (Fig. 1).
Fig. 1

Quantization of charge

If the charge of an electron (= 1.6 × 10–19C) is taken as elementary unit, i.e., quanta of charge, the charge on any body will be some integral multiple of e, i.e.,
Q = +ne with n = 1, 2, 3, . . .
Charge on a body can never be 36999.png, ±17.2e, or ±10–5e, etc.

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