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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,

1. the electric field is zero but the potential is non-zero
2. the electric field is non-zero but the potential is zero
3. both the field and the potential are zero
4. 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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