Four point charges $Q, q, Q$ and $q$ are placed at the corners of a square of side $\text{'}a\text{'}$ as shown in the figure.

Find the resultant electric force on a charge $Q$.

Four point charges $Q, q, Q$ and $q$ are placed at the corners of a square of side $\text{'}a\text{'}$ as shown in the figure.

Find the potential energy of this system.

Three point charges $q, -4 q$ and $2 q$ are placed at the vertices of an equilateral triangle of $ABC$ of side $\text{'}l\text{'}$ as shown in the figure. Obtain the expression for the magnitude of the resultant electric force acting on the charge $q$.

Find out the amount of the work done to separate the charges at infinite distance.

An electric dipole is kept first to the left and then to the right of a negatively charged infinite plane sheet having a uniform surface charge density. The arrows $p_1$ and $p_2$ show the directions of its electric dipole moment in the two cases.

Identiy for each case, whether the dipole is in stable or unstable equilibrium. Justify each answer.