Verify that the potentials obtained in Examples 25.6 and 25.7 have the correct sign.

Data from Example 25.6

A thin rod of length \(\ell\) carries a uniformly distributed charge \(q\). What is the potential \(V_{\mathrm{P}}\) at point \(\mathrm{P}\) a distance \(d\) from the rod along a line that runs perpendicular to the long axis of the rod and passes through one end of the rod?

Data from Example 25.7

A thin disk of radius \(R\) carries a uniformly distributed charge. The surface charge density on the disk is \(\sigma\). What is the electrostatic potential due to the disk at point \(\mathrm{P}\) that lies a distance \(z\) from the plane of the disk along an axis that runs through the disk center and is perpendicular to the plane of the disk?