Positive point charges are placed at the vertices of a star shape as shown in the figure. Direction of the electrostatic force on a negative point charge at the centre $O$ of the star is

A simple pendulum of period $T$ has a metal bob which is negatively charged. If it is allowed to oscillate above a positively charged metal plate, its period will

A proton is fired at an initial velocity of $150 \,m/s$ at an angle of $60^o $ above the horizontal into a uniform electric field of $2 \times 10^{-4} \,N/C$ between two charged parallel plates as shown in figure. Then the total time the particle is in motion is :-

In a medium, the force of attraction between two point charges, distance $d$ apart, is $F$. What distance apart should these point charges be kept in the same medium, so that the force between them becomes $16\, F$ ?

Force between two point charges $q_1$ and $q_2$ placed in vacuum at ' $r$ ' $\mathrm{cm}$ apart is $F$. Force between them when placed in a medium having dielectric $\mathrm{K}=5$ at $\mathrm{r} / 5$ $\mathrm{cm}$ apart will be:

The ratio of the forces between two small spheres with constant charge $(a)$ in air $(b)$ in a medium of dielectric constant $K$ is