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Four point charges q_A = 2 μC, q_B = –5 μC, q_C = 2 μC, and q_D = –5 μC are located at the corners of a square ABCD of side 10 cm. What is the force on a charge of 1 μC placed at the centre of the square?

Q 1.6: Four point charges q_{A}= 2 \mu C, q_{B}= -5 \mu C, q_{C}= 2 \mu C, and q_{D}= -5 \mu C are located at the corners of a square ABCD of side 10 cm. What is the force on a charge of 1 μC placed at the centre of the square?

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Charges at (A, C) and (B, D) are pairwise diametrically opposite and also equal.

Therefore, their force on a point charge at the centre of the square will be equal but opposite in directions.

Now, AC = BD = \sqrt{2}\times(10\times10^{-2} m) = \sqrt{2}\times0.1 m

\therefore AO = BO = CO = DO = r = Half of diagonal = \sqrt{2}\times0.05 m

Force on point charge at centre due to charges at A and C = F_{A} = -F_{C} = \frac{k(2\mu C)(1\mu C)}{(r)^2}

Similarly, Force on point charge at centre due to charges at B and D = F_{B} = -F_{D} = \frac{k(-5\mu C)(1\mu C)}{(r)^2} 

\therefore Net force on point charge =  \\F_{A} + F_{B} + F_{C} + F_{D} \\

                                               = -F_{B} +F_{B} - F_{D} + F_{D} = 0.

Hence, the charge at the centre experiences no force.

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