Try this one, it's so cool :D

Two equal metal round plates are placed on small distance $d$ in a magnetic field of induction $B$ whose field lines are parallel with disc's axis . They are rotateing with same angular veocity $\omega$ but in the oposite direction. If the centers of plates are connected with a thin conductive thread and charge density of plate's is $\sigma(r)$ where $r$ is distance from plates axis find $\sigma(1 m)$ .

$B=0.1T$

$\omega=1 \frac{rad}{s}$

$d=10^{-12}m$