The mutual inductance of two co-axial coils is 2 H. If the current in one coil changes uniformly from zero to 1.0 A in 100 ms, the emf induced in the other coil is

-20 V

The correct answer is Option (4) → -20 V

Given:

Mutual inductance $M = 2 \, H$

Current change: $\Delta I = 1.0 \, A$

Time interval: $\Delta t = 100 \, ms = 100 \times 10^{-3} \, s = 0.1 \, s$

Formula:

$\varepsilon = -M \cdot \frac{\Delta I}{\Delta t}$

Substitute values:

$\varepsilon = -2 \cdot \frac{1}{0.1}$

$\varepsilon = -20 \, V$

The negative sign indicates the direction of induced emf according to Lenz’s law.

Note: The answer provided follows the official NTA answer key, where the induced emf has been taken with its sign according to Lenz’s law, giving −20 V. However, the wording of the question creates a possible ambiguity because it does not clearly specify whether the signed value of emf or only its magnitude is required. If the question is interpreted in terms of magnitude only, then 20 V can also be considered a logically valid answer.