Halliday, Resnick & Walker — Electric Fields: Problem 18

18 A certain electric dipole is placed in a uniform electric field \(\vec{E}\) of magnitude \(50 \text{ N/C}\). Figure 22-32 gives the potential energy \(U\) of the dipole versus the angle \(\theta\) between \(\vec{E}\) and the dipole moment \(\vec{p}\). The vertical axis scale is set by \(U_s = 100 \times 10^{-28} \text{ J}\). What is the magnitude of \(\vec{p}\)?