Physics (AS & A Level) · Oct / Nov 2020 · Paper 12 · Question 33

Four point charges, each of charge Q, are placed on the edge of an insulating disc of radius r. The disc rotates at a rate of n revolutions per unit time. [diagram: circle of radius r with four charges Q on its edge, arrow indicating rotation] What is the equivalent electric current at the edge of the disc?

1. A4Qn
2. B\frac{4Q}{n}
3. C8\pi rQn
4. D\frac{2Qn}{\pi r}

As printed in the paper