A uniformly charged disk of radius $a$ lies in the $xy$-plane with surface charge density $\sigma$. A test charge $q$ is on the $z$-axis at height $z$. Which setup is the most appropriate first step for an integral solution using concentric rings?

• A. Use $dQ=\rho dV$ because every disk is fundamentally a three-dimensional volume distribution
• B. Treat the disk as rings with $dQ=\sigma \left(2\pi rdr\right)$ and integrate the surviving axial contributions
• C. Treat the disk as a single point charge located at its center for all values of $z$
• D. Integrate the tangential components around the disk because symmetry makes the axial component cancel

Answer: (B)