The midpoint between two charges, one +q and the other -q of equal magnitude, has what electric field value?

• A. Zero
• B. Nonzero and directed toward the positive charge
• C. Nonzero and directed toward the negative charge
• D. Undefined

Answer: (A)

This question hinges on how electric fields superpose and how symmetry can cause cancellation. For two equal-magnitude charges +q and -q, the field from the positive charge points away from it, and the field from the negative charge points toward it. At the midpoint between them, each field has the same magnitude, because the distances to both charges are equal (E = kq/r^2 for each). The directions at that point are opposite along the line connecting the charges, so the two fields cancel exactly, giving a net electric field of zero. If the charges weren’t equal, the cancellation wouldn’t be complete and you’d get a nonzero field toward the charge with greater magnitude. It’s not undefined at the midpoint, since there’s no singularity there.