Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R) .
Assertion (A) : Net dipole moment of a polar linear isotropic dielectric substance is not zero even in the absence of an external electric field.
Reason (R) : In absence of an external electric field, the different permanent dipoles of a polar dielectric substance are oriented in random directions.
In the light of the above statements, choose the most appropriate answer from the options given below :

Solution

Physics sir · 4 steps

Each molecule has its own dipole, but random directions make them cancel, so the net is zero.

1. Permanent dipoles. In a polar dielectric such as water, every molecule has its own permanent dipole moment. That part is true even with no field applied.
2. No external field. Thermal agitation keeps these dipoles pointing in random directions. For every dipole pointing one way there is, on average, one pointing the opposite way, so the vector sum is zero.
3. Check each statement. Assertion (A) says the net dipole moment is not zero without a field, so (A) is false. Reason (R) describes the random orientation correctly, so (R) is true.
4. So. (A) is not correct but (R) is correct, which is option (c). In fact, (R) is exactly the reason the net moment is zero, the opposite of what (A) claims.

The idea to keep. Polar molecules have individual dipoles, but a net dipole moment needs alignment. Without a field, random thermal orientation gives a total of zero. An external field is what produces a net polarisation.

Answer from the NTA final answer key. Solution by PrepWiser.