Case: Read the passage and answer the question.
Conductors contain mobile charge carriers. In metallic conductors, these charge carriers are electrons. In a metal, the outer (valence) electrons part away from their atoms and are free to move. These electrons are free within the metal but not free to leave the metal. The free electrons form a kind of ‘gas’; they collide with each other and with the ions and move randomly in different directions. In an external electric field, they drift against the direction of the field. The positive ions made up of the nuclei and the bound electrons remain held in their fixed positions.
A sensitive instrument is kept inside a closed hollow metal box. There is no charge inside the box. The box is placed in a strong external electric field, but the instrument stays unaffected. What is the reason for this?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The electric field inside a charge-free cavity of a conductor is zero.
The hollow space inside the box is a cavity in a conductor. The free electrons of the box move until the field inside the metal is zero.
Option (1) The box picks up a large net charge from the external field. — Incorrect. The external field only moves charges within the box. A neutral box stays neutral as a whole.
Option (2) Induced charges collect on the inner surface of the box and push the field out of the hollow space. — Incorrect. With no charge in the cavity, all the induced charges sit on the outer surface of the box. The inner surface carries no charge.
Option (3) The electric field inside a charge-free cavity of a conductor is zero. — Correct. Whatever the charges and fields outside, the field in a charge-free cavity of a conductor is zero. This is called electrostatic shielding, and it is used to protect sensitive instruments.
Option (4) The field inside the box becomes weaker, as in a dielectric, but it does not become zero. — Incorrect. A field that is reduced but not zero is found inside a dielectric. A metal box gives zero field in its cavity.
NCERT (2026-27):
Electrostatics of Conductors – “Whatever be the charge and field configuration outside, any cavity in a conductor remains shielded from outside electric influence: the field inside the cavity is always zero. This is known as electrostatic shielding. The effect can be made use of in protecting sensitive instruments from outside electrical influence.”