Four electric charges $2 \mu \mathrm{C}, Q, 4 \mu \mathrm{C}$ and $12 \mu \mathrm{C}$ are placed on $X$-axis at distance $x=0,1 \mathrm{~cm}, 2 \mathrm{~cm}$ and 4 cm respectively. If the net force acting on the charge at origin is zero, then $Q=$

If a particle of mass 10 mg and charge $2 \mu \mathrm{C}$ at rest is subjected to a uniform electric field of potential difference 160 V , then the velocity acquired by the particle is

An electron and a positron enter a uniform electric field $E$ perpendicular to it with equal speeds at the same time. The distance of separation between them in the direction of the field after a time ' $t$ ' is

( $\frac{e}{m}$ is specific charge of electron)

A charge $q$ is placed at the centre ' $O$ ' of a circle of radius $R$ and two other charges $q$ and $q$ are placed at the ends of the diameter $A B$ of the circle. The work done to move the charge at point $B$ along the circumference of the circle to a point $C$ as shown in the figure is