Case Study Question 1:
When an insulator is placed in an external field, the dipoles become aligned. Induced surface charges on the insulator establish a polarization field Ēi in its interior. The net field Ē in the insulator is the vector sum of Ē, and Ēi as shown in the figure.
On the application of external electric field, the effect of aligning the electric dipoles in the insulator is called
polarisation and the field Ē; is known as the polarisation field.
The dipole moment per unit volume of the dielectric is known as polarisation (P).
For linear isotropic dielectrics, P =χE, where χ = electrical susceptibility of the dielectric medium.
(i) Which among the following is an example of polar molecule?
(ii) When air is replaced by a dielectric medium of constant K, the maximum force of attraction between two charges separated by a distance
(a) increases K times
(b) remains unchanged
(c) decreases K times
(d) increases 2K times.
(iii) Which of the following is a dielectric?
(c) Antimony (Sb)
(d) None of these
(iv) For a polar molecule, which of the following statements is true ?
(a) The centre of gravity of electrons and protons coincide.
(b) The centre of gravity of electrons and protons do not coincide.
(c) The charge distribution is always symmetrical.
(d) The dipole moment is always zero.
(v) When a comb rubbed with dry hair attracts pieces of paper. This is because the
(a) comb polarizes the piece of paper
(b) comb induces a net dipole moment opposite to the direction of field
(c) electric field due to the comb is uniform
(d) comb induces a net dipole moment perpendicular to the direction of field
AnswerAnswers: (i) (d)
Explanation: In polar molecules the centre of positive and negative charges are separated even when there is no external field. Such molecule are have a permanent dipole moment. Ionic molecule like HCl is an example of polar molecule.
Explanation: As F’=F/K, Therefore the maximum force decreases by K times.
Explanation: A polar molecule is one in which the centre of gravity for positive and negative charges are separated.