The maximum torque acting on a coil having an effective area $0.04 m^2$ and carrying current of 100 μA is $4 × 10^{-8} N m$, when placed in a magnetic field. The magnitude of the magnetic field is

$10^{-2} Wb\, m^{-2}$

The correct answer is Option (2) → $10^{-2} Wb\, m^{-2}$

Given:

Effective area $A = 0.04 \, m^2$

Current $I = 100 \, \mu A = 100 \times 10^{-6} A$

Maximum torque $\tau_{max} = 4 \times 10^{-8} \, Nm$

Number of turns $N = 1$ (not mentioned, so assumed)

Formula:

$\tau_{max} = N I A B$

Substitute values:

$4 \times 10^{-8} = (1) \times (100 \times 10^{-6}) \times (0.04) \times B$

$4 \times 10^{-8} = 4 \times 10^{-6} \times B$

$B = \frac{4 \times 10^{-8}}{4 \times 10^{-6}}$

$B = 10^{-2} \, T$