The distance between two fixed positive charges $4e$ and $e$ is $l.$ How should a third charge $q$ be arranged for it to be in equilibrium? Under what condition will equilibrium of the charge $q$ be stable (for displacement on the line joining $4e$ and $e$) or will it be unstable? 

Three charges, each of value $q$, are placed at the corners of an equilateral triangle. A fourth charge $Q$ is placed at the centre $O$ of the triangle. 

a) If $Q=–q$, will the charges at corners start to move towards the center or away from it. 

b) For what value of $Q$ at $O$ will the charges remain stationary?

A uniform electric field $E=91\times {10}^{-6} \mathrm{V} {\mathrm{m}}^{-1}$ is created between two parallel charged plates as shown in the figure. An electron enters the field symmetrically between the plates with a speed $v_0=4\times {10}^3 \mathrm{m} {\mathrm{s}}^{-1}$. The length of each plate is $l=1 \mathrm{m}$. Find the angle of deviation of the path of the electron as it comes out of the field. (Mass of the electron is $m=9.1\times {10}^{–31} \mathrm{kg}$ and its charge is $e=–1.6\times {10}^{–19} \mathrm{C}).$ 

Two point charges $3\mathrm{\mu C} \mathrm{and} 2.5\mathrm{\mu C}$ are placed at point $\mathrm{A} (1,1,2)\mathrm{m} \mathrm{and} \mathrm{B} (0,3,–1)\mathrm{m}$ respectively. Find out electric field intensity at point $\mathrm{C}(3, 3, 3)\mathrm{m}.$