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Gauss Law

  • Inside a conductor, electrostatic field is zero. In static solution, the free charges have so distributed themselves that the electric field is zero everywhere inside.

  • At the surface of a charged conductor, electrostatic field must be normal to the surface at every point.

  • The interior of a conductor can have no excess charge in the static situation.

  • Electrostatic potential is constant throughout the volume of the conductor and has the same value (as inside) on its surface.

  • Electric field at the surface of a charged conductor is given by,

  • Electrostatic shielding − It is the phenomenon of protecting a certain region of space from external electric field.

Dielectrics and polarisation

  • Non-polar dielectrics − The centre of positive charge coincides with centre of negative charge in the molecule.


  • Polar dielectrics − The centres of positive and negative charges do not coincide because of the asymmetric shape of the molecules.

Non-Polar Dielectrics

  • When a non-polar dielectric is held in an external electric field , the centre of positive charge in each molecule is pulled in the direction of and the negative charge centre is pulled in a direction opposite to .

  • The two centres of positive and negative charges in the molecule are separated. The molecules get distorted. The non-polar molecule gets polarised or a tiny dipole moment is imparted to each molecule.

Polar Dielectrics

  • When no external field is applied, the different permanent dipoles of such a dielectric are oriented randomly. Therefore, the total dipole moment is zero.

  • When an external electric field is applied, the individual dipole moments tend to align with the field.

  • A net dipole moment in the direction of the external field is developed i.e., the dielectric is polarised.

  • Thus, each molecule becomes a tiny electric dipole, with a dipole moment parallel to the external field and proportional to it. Induced dipole moment P acquired by the molecule may be written as

P = αε0E0

Where, α is a constant of proportionality and is called atomic/molecular polarisibility

Consider any small volume element in the interior of the slab, shown in dotted in the above figure. Inside the dotted portion, there is no net charge density. The negative ends of the dipoles remain unneutralised at the surface AB and positive ends of the dipole remain unneutralised at the surface CD.

They set up an electric field opposite to .

Effective electric field in a polarised dielectric = E = E0 EP

Here, E is called the reduced value of the electric field.


  • A capacitor is a system of two conductors separated by an insulator. The conductors have charges Q and Q, with potential difference V = V1 V2 between them.

  • The electric field in the region between the conductors is proportional to the charge Q.

  • If potential difference V is the work done per unit positive charge in taking a small test charge from the conductor 2 to 1 against the field, then V is proportional to Q and the ratio Q/V is a constant.

C =

The constant C is called the capacitance of the capacitor.

  • Capacitance C depends on shape, size and separation of the system of two conduct…

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