A point charge Q is “nailed down” on a table. Around it, at radius R, is a frictionless circular track on which a dipole p rides, constrained always to point tangent to the circle. Use Eq. 4.5 to show that the electric force on the dipole is

Notice that this force is always in the “forward” direction (you can easily confirm this by drawing a diagram showing the forces on the two ends of the dipole). Why isn’t this a perpetual motion machine?

F - 0 P 470 R3