EField4 Physics Example: Problem 2

A fixed particle is shown in the animation along with a drag-able particle having a charge of 1 uC. The vector represents the force on the drag-able particle and the message box displays the magnitude of this force as you drag the particle. The force on the fixed particle is not shown but you know it exists from Newton's third law. Is the force acting between the two particles a Coulomb force?  If so, what is the charge on the fixed particle?   Start