Electric Charges and Fields || Electrostatics Notes || Physics || Class 12 || Chapter 1 Notes Class 12 ||

                            Charge

Charge is that property of an object by virtue of which it apply electrostatic force of interaction on other objects.

Charges are of two types

(i) Positive charge (+ve) 

(ii) Negative charge (-ve) 

Like charges repel and unlike charges attract each other.

              Quantization of Charge

Charge on any object can be an integer multiple of a smallest charge (e).

                   Q = ± n e

where, n = 1, 2, 3,……. and e = 1.6 * 10-19 C.

                 Conservation of Charge

Charge can neither be created nor be destroyed. but can be transferral from one object to another object. Recently a new particle has been discovered called ‘Quark’. It contains charge ± e / 3, ± 2e / 3. [The protons and neutrons are combination of other entities called quarks, which have charges 1 / 3 e. However, isolated quarks have not been observed, so, quantum of charge is still e. ]

       Coulomb’s Law of Electrostatics

Electrostatic force of interaction acting between two stationary charges is given by

where q1, q2 are magnitude of point charges, r is the distance between them and εo is permittivity of free space.

Here, 1 / 4πεo = (10^-7 N – s² / C²)C²

Substituting value of c = 2.99792458 × 10^8 m/s,

We get 1 / 4πεo = 8.99 x 10^9 N-m²/C²

In examples and problems we will often use the approximate value,

      1 / 4πεo = 9 * 10^ 9N-m²/C²

The value of εo is 8.85 * 10^-12 C² / N-m C².

If there is another medium between the point charges except air or vacuum, then εo is replaced by εo K or εo εr or ε  where K or εr is called dielectric constant or relative permittivity of the medium.

                   K = εr = ε / εo

where, ε = permittivity of the medium.

» For air or vacuum, K = 1

» For water, K = 81

» For metals, K = ∞

Coulomb’s Law in Vector Form 

The above equations give the Coulomb’s law in vector form.

Force on q1 due to q2 = –  Force on q2 due to q1

                      F 12 = – F 21

      F12  = q1 q2 / 4πε . r1 – r2 / |r1 – r2|³

The forces due to two point charges are parallel to the line joining point charges; such forces are called central forces and electrostatic forces are conservative forces.

                 

Electric Field

The space around an electric charge in which its influence can be experienced is known as electric field.

                     Electric Field Lines

“An electric field line is an imaginary line or curve drawn through a region of space so that its tangent at any point is in the direction of the electric field vector at that point. The relative closeness of the lines at some place give an idea about the intensity of electric field at that point.”

» Two lines can never intersect.

Electric field lines always begin on a positive charge and end on a negative charge and do not start or stop in mid space.

            Electric Field Intensity (E)

The electrostatic force acting per unit positive charge on a point in electric field is called electric field intensity at that point.

Its SI unit is NC-1 or Vim and its dimension is [MLT-3 A-1].

It is a vector quantity and its direction is in the direction of electrostatic force acting on positive charge.

Electric field intensity due to a point charge q at a distance r is given by