Revision Notes on Electrostatic

Electrostatic Force and Electrostatic field:-

Electrostatic:- It is a branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges with no acceleration.

Coulomb’s Law:- It states that the electro-static force of attraction or repulsion between two charged bodies is directly proportional to the product of their charges and varies inversely as the square of the distance between the two bodies.

          F = Kq1q2/r2

          Here, K = 1/4πε0 = 9×109 Nm2C-2 (in free space)

Relative Permittivity (εr):-

 The relative permittivity (εr) of a medium is defined as the ratio between its permittivity of the medium (ε) and the permittivity (ε0) of the free space.

         εr = ε/ε0

Coulomb force in vector form:- The force on charge q1 due to q2 is,

       \vec{F_{12}} = [q_{1}q_{2}/r^{2}]\hat{r_{21}}

       If q1q2>0, R.H.S is positive.

If q1q2<0, a negative sign from q1q2 will change \hat{r_{21}} and \hat{r_{12}}.  The relation will again be true, since, in that case  have same directions.

Unit of Charge:-

       C.G.S, q = ±1 stat-coulomb

       S.I, q = ±1 Coulomb

Relation between coulomb and stat-coulomb:-

        1 coulomb = 3×109 stat-coulomb

        1 coulomb =(1/10) ab-coulomb (e.m.u of charge)

Dielectric constant:- The dielectric constant (εr) of a medium can be defined as the ratio of the force between two charges separated by some distance apart in free space to the force between the same two charges separated by the same distance apart in that medium.

          So, εr = ε/ε0 = F1/F2

Here, F1 and F2 are the magnitudes of the force between them in free space and in a medium respectively.

Charges:-

?         Line charge, λ = q/L

        Surface charge, σ = q/A

        Volume charge, ρ = q/V

Electric field (\vec{E}) :- The strength of an electric field is measured by the force experienced by a unit positive charge placed at that point. The direction of field is given by the direction of motion of a unit positive charge if it were free to move.

          \vec{E} = \vec{F}/q = Kq/r^{2}

Unit of Electric field:-

         E = [Newton/Coulomb] or [Joule/(Coulomb) (meter)]

Electric lines of force:- An electric line of force is defined as the path, straight or curved, along which a unit positive charge is urged to move when free to do so in an electric field. The direction of motion of unit positive charge gives the direction of line of force.

Electric Lines of Force

     Properties:-

 (a) The lines of force are directed away from a positively charged conductor and are directed towards a negatively charged conductor.

  (b) A line of force starts from a positive charge and ends on a negative charge. This signifies line of force starts from higher potential and ends on lower potential.

Electric field intensity due to a point charge:- E = (1/4πε0) (q/r2)