Problem

A rectangular coil of area

5.0 \times 10^{-4}\,\text{m}^2

and 60 turns is pivoted about one of its vertical sides. The coil is in a radial horizontal magnetic field of

9 \times 10^{-3}\,\text{T}

. What is the torsional constant of the spring connected to the coil if a current of

0.20\,\text{mA}

produces an angular deflection of

18^\circ