Realise that potential due to system of charges is the sum of potential due to individual charges and is independent of presence of other charges. Hence it follows superposition principle.
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Two electric charges q and -2q are placed at a distance 3a apart. The locus of points in the plane of the charges where the field potential is zero is:
Four charges 10−8;−2×10−8;+3×10−8 and 2×10−8 coulombs are placed at the four corners of a square of side 1m. The potential at the centre of the square is