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.
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?

  • $\begingroup$ Please give me suggestion what should I do to improve my question. Any specific reason to down vote this question? $\endgroup$ – jayant98 Feb 3 '19 at 12:43
  • 2
    $\begingroup$ This looks very much like a "how do I solve this problem" question which is considered off-topic here, hence the downvotes. $\endgroup$ – Kyle Kanos Feb 3 '19 at 13:34
  • 1
    $\begingroup$ @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. $\endgroup$ – jayant98 Feb 3 '19 at 13:36
  • 4
    $\begingroup$ 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. $\endgroup$ – Kyle Kanos Feb 3 '19 at 13:44
  • $\begingroup$ @Kyle Please see if now it is okay to ask? $\endgroup$ – jayant98 Feb 3 '19 at 13:52

In principle, electromagnetic induction is about the existence of an electric field with non-conservative circulation.

This electric field can generate a current in a closed circuit, that is to say to make charges turn !

In your case, you must first look for the electric field associated with the variable magnetic field and then look at how this electric field acts on the charged disk.


Your knowledge of e-m induction will have enabled you to find the emf around a circle of radius r on the disc. That means you know know much work per coulomb would be done on a test charge taken around that circle. But you know what work means, so you can calculate the force per unit charge on your test charge. What do we call this?


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.