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JEE Main 2025 Apr 2 Shift 2, Physics Q1: Electric Potential

JEE Main2025Apr 2, Shift 2Physics
Q.

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 :

  1. A

    (A) is correct but (R) is not correct

  2. B

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

  3. C

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

  4. D

    (A) is not correct but (R) is correct

Solution

In a polar dielectric the individual molecules carry permanent dipole moments, but in the absence of an external field thermal motion keeps them pointing in random directions. The vector sum of these randomly oriented dipoles averages to zero, so the net dipole moment of the bulk sample is .

Therefore Assertion (A) is wrong: the net dipole moment is zero without an external field. Reason (R) correctly states the underlying fact about random orientation.

(A) is incorrect, (R) is correct.

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