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

A $2\,\mu F$ capacitor is charged to a potential $=10\ V$ . Another $4\,\mu F$ capacitor is charged to a potential $= 20\ V$ . The two capacitors are then connected in a single loop, with the positive plate of one connected with negative plate of the other. What heat is evolved in the circuit ?.........$\mu J$

Two thin wire rings each having a radius $R$ are placed at a distance $d$ apart with their axes coinciding. The charges on the two rings are $+ q$ and $-q$. The potential difference between the centres of the two rings is

A point charge $+Q$ is positioned at the centren of the base of a square pyramid as shown. The flux through one of the four identical upper faces of the pyramid is 

Five point charges each having magnitude $'q'$ are placed at the corners of regular hexagon as shown in figure. Net electric field at the centre $'O'$ is $\vec E$ . To get net electric field at $'O'$ to be $6\vec E$ , charge placed on the remaining sixth corner should be

The equivalent capacitance between $A$ and $B$ is (in $\mu\, F$)