Four point charges of 1μC, −2μC, 1μC and −2μC are placed at the corners A, B, C and D respectively of a square of side 30 cm. Find the net force acting on a charge of 4μC placed at the centre of the square.
Answer: Zero.
Solution
The centre is equidistant from all four corners. The charges at opposite corners A and C are equal (1μC each), so their forces on the central charge are equal and opposite and cancel. Likewise the equal charges at B and D (−2μC each) give forces that cancel. Hence the net force on the 4μC charge at the centre is zero.
