Four charges are arranged at the corners of a square $ABCD$,as shown. The force on a $+ve$ charge kept at the centre of the square is

Two equal charges $q$ are placed at a distance of $2a$ on the $x$-axis. A third charge $q$ of mass $m$ is placed at the midpoint between them. If this charge is displaced by a small distance $x$ from the equilibrium position, the particle will:

Two protons are separated by a distance of $10^{-10} \ m$. If they are released,what will be their total kinetic energy at an infinite distance?

Six charges,three positive and three negative of equal magnitude,are to be placed at the vertices of a regular hexagon such that the electric field at $O$ is double the electric field when only one positive charge of the same magnitude is placed at $R$. Which of the following arrangements of charges is possible for $P, Q, R, S, T,$ and $U$ respectively?

$A$ pendulum bob of mass $30.7 \times 10^{-6} \, kg$ and carrying a charge $2 \times 10^{-8} \, C$ is at rest in a horizontal uniform electric field of $20000 \, V/m$. The tension in the thread of the pendulum is $(g = 9.8 \, m/s^2)$.

Two point charges $+q$ and $-q$ are held fixed at $(-d, 0)$ and $(d, 0)$ respectively of a $(X, Y)$ coordinate system. Then: