Question

Four point charges are placed at the corners of a square $ABCD$ of side $10cm$, as shown in figure. The force on a charge of $1\mu C$ placed at the centre of square is

• A. $7N$
• B. $8N$
• C. $2N$
• D. zero

Answer 1

Answer: (D)

Forces of repulsion on $1\mu C$ charge at $O$ due to $3\mu C$ charge, at $A$ and $C$ are equal and opposite. So they cancel each other.
Similarly, forces of attraction of $1\mu C$ charge at $O$ due $-4\mu C$ charges at $B$ and $D$ are also equal and opposite. So they also cancel each other.
Hence the net force on the charge of $1\mu C$ at $O$ is zero.