Four point charges are placed at the corners of a square ABCD as shown. Determine the net electrostatic force experienced by a charge located at the centre O of the square.

Zero
Along the diagonal AC
Along the diagonal BD
Perpendicular to side AB

Correct answer: Along the diagonal BD

Solution

In the standard version of this problem the corner charges are arranged so that contributions along one diagonal cancel while contributions along the other diagonal add up. The resultant therefore points along a diagonal of the square. For the conventional figure the net force on the central charge is directed along diagonal BD.

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