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Physics Question 41 – JEE-MAIN 2025

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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 :

Polar molecules possess inherent electric dipole moments due to asymmetric charge distribution.

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Ninja Strategy•Independent Truth Evaluation

Evaluate the truthfulness of Assertion (A) and Reason (R) independently first, then match the combination to the options.

Step 1: Analyze Assertion (A)✦ Active

Polar dielectric substances consist of molecules that inherently possess permanent electric dipole moments. However, in the absence of an external electric field, these individual molecular dipole moments are randomly oriented due to thermal agitation. Consequently, the vector sum of these random dipoles over a macroscopic volume averages to zero. Therefore, the net dipole moment of a polar dielectric substance in the absence of an external field is zero. Hence, Assertion (A) is incorrect.

Step 2: Analyze Reason (R)○ Expand

The statement correctly describes the behavior of permanent dipoles in a polar dielectric substance when no external electric field is present. Due to thermal energy, the dipoles are in constant random motion and are oriented in random directions. This random orientation is precisely why the net dipole moment averages to zero. Hence, Reason (R) is correct.

Step 3: Conclude based on A and R○ Expand

Based on the analysis, Assertion (A) is incorrect, and Reason (R) is correct. This combination matches option 4.

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