Charges of +2q, -q and -q sit at the three corners of an equilateral triangle. Consider the point at the centroid (centre) of the triangle. There,

the electric field is zero but the potential is non-zero
the electric field is non-zero but the potential is zero
both the field and the potential are zero
both the field and the potential are non-zero

Correct answer: the electric field is non-zero but the potential is zero

Solution

All vertices are at the same distance R from the centre, so V = k(2q - q - q)/R = 0. The field is the vector sum of three contributions; since +2q is unequal to the two -q charges, the vectors do not cancel and the net field is non-zero.

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