A non-conducting thin disc of radius R charged uniformly over one side with surface density s rotates about its axis with an angular velocity `omega`. Find
(a) the magnetic induction at the centre of the disc,
(b) the magnetic moment of the disc.

Explanation

To solve the problem, we will follow these steps:

### Step 1: Define the parameters and setup
We have a non-conducting thin disc of radius \( R \) that is uniformly charged with a surface charge density \( \sigma \) and rotates about its axis with an angular velocity \( \omega \).

### Step 2: Consider an elemental ring
Consider a small elemental ring of radius \( r \) and thickness \( dr \) on the disc. The area of this ring is given by:
\[
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