Question

Calculate the torque acting upon the following structure carrying current $l$ due to the magnetic field $B$. Both magnetic field and the structure are in the plane of paper as shown.

Answer 1

$\begin{array}{c}\tau =B.i.A\\ =B.\left[I\right].\left[\pi R^2+\pi \left(4R^2\right)\right]\\ =B.I.\left(5\pi R^2\right)\\ Hence,\\ \tau =5BI\pi R^2\end{array}$