# Equation of infinitesimal ring when finding $\vec{E}$ of a disc?

When trying to find the electric field created by a uniformly charged disc at a point P on axis of the disc, it can be done by integration.

We start by finding the electric field dE created by each infinitesimal disc of thickness da, as shown in the picture.

The area of each infinitesimal ring is said to be 2pia*da.

Shouldn't it be pi*(a+da)^2 -pi*a^2? Because it should be the area of the large ring of radius (a+da) minus the smaller ring inside of radius a to find the area of just the ring thickness da?

I suspect it's something to do with the expansion of what I believe it should be, because I'm not exactly sure what (da)^2 would equal? Is this the error?

• Please change your post to use mathjax to display math Sep 23, 2021 at 8:35
• Imagine using $\Delta a$ instead for a moment and comparing it to $(\Delta a)^2$ as $\Delta a \to 0$. For example what if $\Delta a \sim 10^{-10}$? Then remember that differentials are infinitesimally small in comparison. Sep 23, 2021 at 8:39

• da or $\Delta$a is a very small value , suppose something like $10^{-10}$, square of which is even small => $10^{-20}$ which you can assume 0. Sep 24, 2021 at 7:34