Question

The negatively and uniformly charged nonconducting disc as shown, is rotated clockwise. The direction of the magnetic field at point A in the plane of the disc is :

• A. into the page
• B. out of the page
• C. up
• D. down
• E. zero

Answer 1

The correct option is A into the page

The disc behaves like made up of coils arranged in a plane in which current is flowing in anti-clockwise direction since negative charge is rotating in clock-wise direction.
Magnetic field at point A due to small elements of a ring or coil will be into the plane of the page using Biot Savart law.$\overrightarrow{B}=\frac{{\mu }_0}{4\pi }i(\overrightarrow{dl}\times \overrightarrow{r}{r}^3)$
$\overrightarrow{dl}\times \overrightarrow{r}$ is into the page.