Question

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. (A) is correct but (R) is not correct
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• D. (A) is not correct but (R) is correct

Hint:

When analyzing assertions and reasons in physics, remember that the reason should adequately explain the assertion. If not, mark them as correct but not the correct explanation.

Answer 1

The Correct Option is D

In considering the given Assertion (A) and Reason (R), we start by analyzing each statement:

Assertion (A): The statement mentions that the net dipole moment of a polar linear isotropic dielectric is nonzero even without an external electric field. This assertion implies that there is a preferential alignment of the dipoles resulting in a non-zero net dipole moment in the absence of an external force, which is contrary to the basic understanding of polar dielectrics.

Reason (R): It states that without an external electric field, the permanent dipoles in a polar dielectric are oriented randomly. This aligns with our understanding that in such a case, the random order of dipoles cancels out each other's effects, leading to a net dipole moment of zero.

Hence, upon evaluation:

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

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

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

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

We conclude that option (A) is not correct but (R) is correct is the most appropriate. The assertion (A) incorrectly suggests a net dipole without an external field, while the reason (R) accurately describes the dipole behavior.

Answer 2

We are given two statements related to the dipole moment of a polar dielectric substance:

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 the absence of an external electric field, the different permanent dipoles of a polar dielectric substance are oriented in random directions.

Concept Used:

Polar dielectric substances have permanent dipole moments due to their molecular structure. However, in the absence of an external electric field, the dipoles are oriented randomly. The random orientation causes their individual dipole moments to cancel out vectorially, resulting in a net dipole moment of zero. When an electric field is applied, these dipoles partially align with the field, producing a nonzero net dipole moment.

Step-by-Step Solution:

Step 1: Analyze the Assertion (A).

The statement says that even without an electric field, a polar dielectric has a nonzero net dipole moment. This is incorrect because, although individual molecules have permanent dipoles, their orientations are random, so the net dipole moment of the material as a whole is zero.

Step 2: Analyze the Reason (R).

The reason correctly states that in the absence of an external electric field, the permanent dipoles are randomly oriented, leading to mutual cancellation of dipole moments.

Step 3: Check the logical connection between (A) and (R).

Since the Reason (R) correctly explains that random orientation causes the net dipole moment to vanish, it contradicts the Assertion (A). Thus, (A) is false but (R) is true.

Final Computation & Result:

The correct evaluation is:

Assertion (A): False
Reason (R): True

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