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A thin disk of radius $a$ has a surface charge density of $\sigma$.

What is the potential at a point $x$ along the axis of the disk?

$\cfrac{\sigma \bigg(\sqrt{x^2 + a^2}-x \bigg)}{2 \pi \epsilon_o}$

$\cfrac{\sigma }{2 \pi \epsilon_o x}$

$\cfrac{\sigma \bigg(\sqrt{x^2 + a^2}\bigg)}{2 \pi \epsilon_o}$

$\cfrac{\sigma x }{2 \pi \epsilon_o \sqrt{x^2 + a^2}}$