Halliday, Resnick & Walker — Electric Fields: Problem 46

46 A certain electric dipole is placed in a uniform electric field \(\vec{E}\) of magnitude \(40 \text{ N/C}\). Figure 22-41 gives the magnitude \(\tau\) of the torque on the dipole versus the angle \(\theta\) between field \(\vec{E}\) and the dipole moment \(\vec{p}\). The vertical axis scale is set by \(\tau_s = 80 \times 10^{-28} \text{ N}\cdot\text{m}\). What is the magnitude of \(\vec{p}\)?