Walker, Halliday & Resnick — Electric Potential: Problem 38

38 Particle 1 (with a charge of +5.0 $\mu$C) and particle 2 (with a charge of -3.0 $\mu$C) are fixed in place with separation $d = 5.0$ cm on the $x$ axis shown in Fig. 24-40a. Particle 3 can be moved along the $x$ axis to the right of particle 2. Figure 24-40b gives the electric potential energy $U$ of the three-particle system as a function of the $x$ coordinate of particle 3. The scale of the vertical axis is set by $U_s = 5.0$ J. What is the charge of particle 3?