$A$ point charge of $40 \ statC$ is placed $2 \ cm$ in front of an earthed metallic plane plate of large size. The force of attraction on the point charge is.....$dynes$.

Three charges,each $+Q$,are placed at the vertices of an equilateral triangle of side $a$. What is the net electrostatic force on any one charge? $\left( k = \frac{1}{4\pi \varepsilon _0} \right)$

Five charges, each $+Q$, are placed at five vertices of a regular hexagon of side length $L$. What is the magnitude of the force on a charge $-Q$ placed at the center of the hexagon?

Three charges $+Q, q, +Q$ are placed respectively at distances $0, \frac{d}{2},$ and $d$ from the origin on the $x$-axis. If the net force experienced by the charge $+Q$ placed at $x = 0$ is zero,then the value of $q$ is:

$A$ particle of mass $M$ and charge $q$ is placed at the midpoint between two fixed charges each of magnitude $Q$,separated by a distance $2d$. The system is collinear as shown in the figure. If the particle is displaced by a small distance $x$ $(x \ll d)$ along the line joining the two charges and released,it will oscillate about the mean position with a time period $T$. ($\varepsilon_{0}$ is the permittivity of free space)

Two point charges $Q$ each are placed at a distance $d$ apart. $A$ third point charge $q$ is placed at a distance $x$ from the mid-point on the perpendicular bisector. The value of $x$ at which charge $q$ will experience the maximum Coulomb force is ...............