I'm confused about the premise of this homework problem:
A thin, non-conducting horizontal disc lies in the $x$-$y$ plane. The disc has a mass $m$ and a total charge $q$ distributed uniformly over its surface. The disc can freely rotate about its axis.
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The disc is initially stationary. Then, at time $t=0 ,$ a magnetic field $\vec{B}$ directed in the direction of the $z$-axis (so perpendicular to the disc's plane) is switched on.Problem: Find the disc's angular velocity $\omega \left( t \right)$ as function of time, assuming that $\mathrm{B}=kt ,$ where $t$ is time.
This problem implies that the magnetic field, $\vec{B} ,$ causes a torque, making the disc rotate. However, I don't understand how the magnetic field would cause the disc to rotate at all.
I have studied electromagnetic induction, and I know that an electromotive force (emf) will be generated due to the change in magnetic flux. But, how will this emf help this disc to rotate?
Question: How does the magnetic field perpendicular to the disc cause it to rotate?