Question

Assertion: In the absence of an externally applied electric field, the displacement per unit volume of a polar dielectric material is always zero.

Reason: In polar dielectrics, each molecule has a permanent dipole moment but these arc randomly oriented in the absence of an externally applied electric field.

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not correct explanation of A

(c) A is true but R is false

(d) A and R are false

Answer 1

Correct option (a) Both A and R are true and R is the correct explanation of A

Explanation:

vector E = applied electric field and

vector P = polarisation or dipole moment per unit volume 

[vector P is the component of dipole moment of each molecule in direction vector E and n is the no. of molecules per unit volume]

Now in the absence of an electric field (vector E = 0) the polarisation vector P vanishes (because vector p is there if  vector E is there).

Therefore displacement  vector D = 0