Electrostatic Forces of Two Charges on a Third
Drag charges to move them around.
Drag black dots on horiztonal lines (sliders) to change charge values.
Drag forceScale slider to rescale all forces so they are a "nice" size relative to the screen.
In addition to the menu item, you should be able to scroll around the picture by shift-dragging and to zoom in and out by shift-rightclick-dragdown and shift-rightclick-dragup. I have my laptop set so 2-finger-tap is right-click so for me these are shift-twofingertap-drag.