Laws of Electrostatics

     The two fundamental laws of electronics are as bellow:
1) Like charges repel each other and unlike charges attract each other.
      The law can be demonstrated by another simple experiment. The ebonite rod becomes negatively charged when rubbed against fur clot. Now, if glass rod is rubbed against fur cloth, it gets positively charged. And if they are brought near each other, they try to attract each other. While two ebonite rods after rubbing against fur cloth, brought nearby, try to repel each other. This shows that like charges repel while unlike charges attract each other.
2) Coulomb's Inverse Square Law.
      The law states that the mechanical force, attraction or repulsion, between the two small charged bodies is,
i) Directly proportional to the product of the charges present on the bodies.
ii) Inversely proportional to the square of the distance between the bodies and
iii) Depends upon the nature of the medium surrounding the bodies.
      The Fig 1. shows two point charges, separates by distance 'd' meters. Te charges are Q1 and Q2 coulombs and K is the constant of proportionality.

      According to Coulomb's law, force between the charges can be mathematically expressed as

       The constant of proportionality, K depends on the surrounding medium and is given by,

      Where     ε0 = Absolute permittivity of the medium = ε0εr
       ε0 = Permittivity of free space   and εr = Relative permittivity of the medium

      For air,   εr= 1
      The concept of permittivity is discussed later.

Note : thus, one coulomb of charge may be defined as that charge, which, wen placed in the air or vacuum at a distance of one meter away from an equal and similar charge, is repelled by a force of 9 x 10 N.

Read also
Electrostatic Field 
Electric Flux
Electric Flux Density


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