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MCQEasyJEE 2025Electric Dipole

JEE Physics 2025 Question with Solution

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:

  • A

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

  • B

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

  • C

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

  • D

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

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: An assertion-reason question about a polar linear isotropic dielectric substance.

Find: Which option correctly evaluates Assertion (A) and Reason (R).

A polar dielectric has permanent molecular dipoles, but in the absence of an external electric field these dipoles are oriented randomly. Because of this random orientation, their vector sum becomes zero for the substance as a whole.

So, Assertion (A) is false because it says the net dipole moment is non-zero even without an external electric field.

Reason (R) is true because it correctly states that the permanent dipoles are oriented in random directions in the absence of an external electric field.

Therefore, (A) is false but (R) is true.

The solution states that the correct option is D. However, the provided options contain a discrepancy because the visible option text matching the conclusion is listed as both C and D in the recorded data. Since the solution explicitly concludes The Correct Option is D, the answer is taken as D.

Detailed Explanation

Given:

  • 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.

Find: The most appropriate option.

Step 1: Examine Assertion (A).

Although molecules of a polar dielectric possess permanent dipole moments, the material as a whole does not have a non-zero net dipole moment when no external electric field is applied. The molecular dipoles do not remain aligned in one preferred direction on their own.

So Assertion (A) is incorrect.

Step 2: Examine Reason (R).

The reason correctly describes the physical situation. In the absence of an external electric field, permanent dipoles are randomly oriented.

Because dipole moment is a vector quantity, random orientation causes cancellation in the bulk material, giving zero net dipole moment.

So Reason (R) is correct.

Step 3: Draw the conclusion.

Since Assertion (A) is false and Reason (R) is true, the correct statement is:

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

Hence, the correct option is D according to the solution.

Common mistakes

  • Confusing permanent molecular dipole moment with net dipole moment of the whole dielectric. Individual molecules may be polar, but the bulk material can still have zero net dipole moment because the dipoles are randomly oriented. Always distinguish microscopic dipoles from the macroscopic resultant.

  • Assuming that a polar dielectric must automatically have a non-zero net dipole moment even without an external field. This is wrong because random orientation cancels the vector sum. A non-zero net dipole moment appears only when an external electric field partially aligns the dipoles.

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