An annular disc of inner radius a and outer radius '2a' is uniformly charged with uniform surface charge density `sigma`. Find the potential at a distance 'a' from the centre at a point 'P' lying on the axis.

Explanation

To find the potential at a point \( P \) located on the axis of an annular disc with inner radius \( a \) and outer radius \( 2a \), uniformly charged with surface charge density \( \sigma \), we can follow these steps:

### Step 1: Define the Geometry
Consider the annular disc with inner radius \( a \) and outer radius \( 2a \). The point \( P \) is located at a distance \( a \) from the center along the axis perpendicular to the plane of the disc.

### Step 2: Consider a Differential Element
Take a differential ring element at a distance \( r \) from the center of the disc with thickness \( dr \). The area of this ring element is given by:
\[
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