Final answer:
When the electric field is zero on the line between two charges, it means that the charges have opposite signs and the magnitude of their electric fields at that point are equal and cancel each other out.
Step-by-step explanation:
When an electric field is zero at a point on the line between two charges, it implies that the electric forces due to the charges are exactly balanced at that point. For this to happen, the charges must have opposite signs and the magnitude of the electric field they produce must be equal and opposite at the point of interest.
Let's consider two charges q1 and q2 that are separated by a distance d. If they are of opposite signs and the electric field is zero at a point between them, then the magnitude of the charges and the distances from this point to each charge must be such that the electric fields they produce are equal in magnitude and opposite in direction. Only then will they cancel each other out.
Remember that the number of electric field lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge. The strength of the field is directly related to the closeness of the field lines. Therefore, if there are two charges with opposite signs and different magnitudes, the one with fewer field lines and thus weaker electric field strength must have a smaller magnitude of charge.