AIIMS 2007 Physics Conductors and Insulators Assertion Reason Question

To address the given question regarding the assertion and reason about electric fields in conductors, let's break down both the assertion and the reason, and then analyze the options.

Assertion: In a cavity within a conductor, the electric field is zero.
Reason: Charges in a conductor reside only at its surface.

Explanation of the Assertion

In electrostatics, one of the fundamental properties of conductors is that when they are in electrostatic equilibrium, the electric field inside the conductor is zero. This is due to the free movement of charge carriers (usually electrons) within the conductor.

When a conductor is charged, the free electrons rearrange themselves in such a way that they create an electric field that cancels any external electric field within the bulk of the conductor. As a result, the electric field inside any cavity (or hollow space) within the conductor also becomes zero.

Explanation of the Reason

The reason states that charges in a conductor reside only at its surface. This is indeed true. When a conductor is charged, the excess charge distributes itself uniformly on the outer surface of the conductor. This happens because like charges repel each other, and they move to the surface where they are farthest apart from one another. The interior of the conductor remains free of excess charge in electrostatic equilibrium.

Conclusion on the Correct Answer

Given the explanations above:

• The assertion is true because the electric field within a cavity in a conductor is indeed zero.
• The reason is also true because charges do reside only on the surface of a conductor.

Furthermore, the reason correctly explains why the assertion is true: the presence of surface charges generates an electric field that cancels out within the conductor and its cavities.

Thus, the correct answer is A: Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Clarification of Other Options

• Option B: Both Assertion and Reason are true but Reason is not the correct explanation of Assertion. This option is incorrect because the reason does indeed explain the assertion.
• Option C: Assertion is true but Reason is false. This option is incorrect because both the assertion and the reason are true.
• Option D: Both Assertion and Reason are false. This option is incorrect because both statements are indeed true.

Relevant Concepts

1. Gauss's Law: An important principle in electrostatics that states that the electric flux through a closed surface is proportional to the enclosed charge. For a conductor in electrostatic equilibrium, the field inside (including cavities) is zero.

2. Electrostatic Shielding: This phenomenon occurs because the electric field inside a conductor is zero, providing protection to any interior cavity from external electric fields.

In summary, both the assertion and the reason are true statements in the context of electrostatics, and the reason effectively explains the assertion, affirming that option A is the correct choice.