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MCQEasyJEE 2025Gauss's Law Applications

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): The outer body of an aircraft 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:

  • A

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

  • B

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

  • C

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

  • D

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

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given:

  • Assertion (A): The outer body of an aircraft 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.

Find: Determine which option correctly relates Assertion (A) and Reason (R).

The aircraft body is metallic, so it behaves as a conductor. During a lightning strike, charge redistributes over the outer surface of the conductor. Due to electrostatic shielding, the electric field inside the cavity enclosed by a conductor is zero.

Therefore, passengers sitting inside remain protected because the lightning current travels along the outer metal surface without producing an electric field inside the enclosed region.

Thus, both Assertion (A) and Reason (R) are correct, and Reason (R) is the correct explanation of Assertion (A).

The correct option is A.

Step-by-step Explanation

Given:

  • Assertion (A): The outer body of an aircraft 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.

Find: Check the truth of both statements and whether the reason explains the assertion.

  1. The metal body of an aircraft offers protection against lightning because lightning travels along the surface of a conductor without penetrating the interior.
  2. This is the Faraday cage effect.
  3. The electric field inside a conductor in electrostatic equilibrium is zero, and hence the electric field inside the enclosed cavity remains zero.
  4. Because of this shielding, the inside of the aircraft remains protected from the external electrical discharge.

Hence, Assertion (A) is correct. Hence, Reason (R) is also correct. Further, Reason (R) correctly explains Assertion (A).

Therefore, the correct option is A.

Common mistakes

  • Confusing the electric field inside the conductor material with the field outside it. The protection works because charges rearrange on the outer surface and shield the interior. Always use the idea of electrostatic shielding for the enclosed region.

  • Thinking that lightning must enter the cabin because the aircraft is struck directly. This is incorrect because the metal body provides a conducting path around the passengers. Focus on current flowing over the outer surface.

  • Assuming Assertion (A) is true only because metal is strong or thick. Mechanical strength is not the reason here; the relevant property is electrical conduction and charge redistribution.

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