Final answer:
Two electric charges that create a point where the net electric field is zero must have opposite signs. This is because the electric fields from unlike charges can cancel out, whereas like charges have fields that don't cancel but instead create a region of weaker field strength between them.
Step-by-step explanation:
If two electric charges produce a point where the net electric field is zero, they must have opposite signs. This occurs because the electric field vectors produced by each charge point in opposite directions when the charges are unlike, which means they can cancel each other out at some point between them.
Figures showing the electric field of two unlike charges indicate that the field is stronger in the region between the charges, due to the vector addition of the field lines.
In contrast, two like charges produce fields that exert opposing forces on any charge between them, causing the field lines to be farther apart and weaker in that region. Hence, if there is a zero electric field point between the charges, it suggests that the charges are opposite in sign.
Furthermore, considering the electrical potential energy, two point charges of the same sign have positive potential energy while those of opposite signs have negative potential energy, this is related to the work needed to bring charges together. The presence of a zero electric field point implies that work is done to bring the charges to that specific configuration.