Find capacitance across $AB$

The work done required to put the four charges together at the corners of a square of side $a$ , as shown in the figure is

Three identical uncharged metal spheres are at the vertices of an equilateral triangle. One at a time, a small sphere is connected by a conducting wire with a large metal sphere that is charged. The center of the large sphere is in the straight line perpendicular to the plane of equilateral triangle and passing through its centre (see figure). As a result, the first small sphere acquires charge $q_1$ and second charge $q_2 (q_2 < q_1)$ . The charge that the third sphere $q_3$ will acquire is (Assume $l >> R$ , $l >> r$ , $d >> R$ , $d >> r$ )

Charge of $2Q$ and $-Q$ are placed on two plates of a parallel plate capacitor if capacitance of capacitor is $C$ find potential difference between the plates

A charge $q$ is placed at the centre of the line joining two equal charges $Q$. The system of the  three charges will be in equilibrium, if $q$ is equal to

Two masses $M_1$ and $M_2$ carry positive charges $Q_1$ and $Q_2$, respectively. They are dropped to the floor in a laboratory set up from the same height, where there is a constant electric field vertically upwards. $M_1$ hits the floor before $M_2$. Then,