Three charges$q_1= 3 \mu C$, $q_2 = 4 \mu C$ and $q_3 = 7 \mu C$ are placed on the circumference of a circle of radius $1.0 \mathrm{m}$ as shown. What is the value of charge $q_4$ placed on the same circle, if the potential at the centre,$V_0 = 0$?

        

At a point $P$ situated at a certain distance from a point charge $+ q$, the electric field is $E$ and the electric potential is$V$. If another charge $-q$ be placed at the same distance on the other side of $P$, then what will be resultant field and potential at point $P$ ?