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Physics Question 38 – 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) : The outer body of an air craft is made of metal which protects persons sitting inside from lightning-strikes.\ Reason (R) : The electric field inside the cavity enclosed by a conductor is zero.\ In the light of the above statements, choose the most appropriate answer from the options given below :

Consider the behavior of conductors when subjected to external electric fields, especially in the context of electrostatic equilibrium.

Step 1: Evaluate Assertion (A)✦ Active

Assertion (A) states that the metal body of an aircraft protects persons inside from lightning strikes. This is a correct statement. Aircraft are designed to act as Faraday cages. When lightning strikes, the charge flows over the outer metallic surface, and the electric field inside the aircraft remains zero, thus protecting the occupants.

Step 2: Evaluate Reason (R)○ Expand

Reason (R) states that the electric field inside the cavity enclosed by a conductor is zero. This is a fundamental principle of electrostatics, known as electrostatic shielding or the Faraday cage effect. For a conductor in electrostatic equilibrium, any excess charge resides on its outer surface, and the electric field within its material and any internal cavities is zero.

Step 3: Determine Relationship between (A) and (R)○ Expand

Assertion (A) describes a practical application of electrostatic shielding (aircraft acting as a Faraday cage). Reason (R) provides the fundamental physical principle (electric field inside a conductor's cavity is zero) that explains why the aircraft's metal body protects its occupants. Therefore, Reason (R) is the correct explanation for Assertion (A).

💡 Teacher's Secret Hint

The 'because' test can be applied: 'Assertion (A) is true BECAUSE Reason (R) is true.' If it makes sense, R is the correct explanation.

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