# How can a magnetic field exert a torque that causes a disc to rotate?

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.
$$\hspace{175px}$$ 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?

• Please give me suggestion what should I do to improve my question. Any specific reason to down vote this question? – jayant98 Feb 3 at 12:43
• This looks very much like a "how do I solve this problem" question which is considered off-topic here, hence the downvotes. – Kyle Kanos Feb 3 at 13:34
• @Kyle But, I am only asking about which torque operates and how? Not asking people to solve the numerical for me. Just about the torque and its working. – jayant98 Feb 3 at 13:36
• If that is your interest, I think you need to edit the question to remove the "this is my homework, what do I do next" aspect and focus on the conceptual question you want to ask. – Kyle Kanos Feb 3 at 13:44
• @Kyle Please see if now it is okay to ask? – jayant98 Feb 3 at 13:52