Four capacitors with capacitances $C_1 = 1\,μF, C_2 = 1.5\, μF, C_3 = 2.5\, μF$ and $C_4 = 0.5\, μF$ are connected as shown and are connected to a $30\, volt$ source. The potential difference between points $B$ and $A$ is....$V$

A charge $q$ is placed at the centre of cubical box of side a with top open. The flux of the electric field through one of the surface of the cubical box is 

As shown in the fig. charges $+\,q$ and $-\,q$ are placed at the vertices $B$ and $C$ of an isosceles triangle. The potential at the vertex $A$ is

Four charges equal to $-Q$ are placed at the four corners of a square and a charge $q$ is at its centre. If the system is in equilibrium, the value of $q$ is

A spherical shell with an inner radius $'a'$ and an outer radius $'b'$ is made of conducting material. A point charge $+Q$ is placed at the centre of the spherical shell and a total charge $-q$ is placed on the shell. Final charge distribution on the surfaces as

A charge $Q$ is divided into two parts of $q$ and $Q - q$. If the coulomb repulsion between them when they are separated is to be maximum, the ratio of $\frac{Q}{q}$ should be