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 given charge situated at a distance $r$ from an electric dipole on it axis experiences a force $F$. If the distance of the charge from the dipole is doubled, the force acting on the charge will be

Five balls marked a to $e$ are suspended using separate threads. Pairs $(b, c)$ and $(d, e)$ show electrostatic repulsion while pairs $(a, b),(c, e)$ and $(a, e)$ show electrostatic attraction. The ball marked a must be

In the given figure, three capacitors $C_1, C_2$ and $C_3$ are joined to a battery, with symbols having their usual meanings, the correct conditions will be

The plates of a parallel plate capacitor are charged up to $100\,volt$. A $2\,mm$ thick plate is inserted between the plates, then to maintain the same potential difference, the distance between the capacitor plates is increased by $1.6\,mm$. The dielectric constant of the plate is 

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,