An electron is released from rest at a point $A.$ Then it accelerates toward a negatively charged plate and leave the plate through a small hole at point $D.$ The potential at point $A$ is $1800$ V greater than that of the point $D.$

i$.$ Find the speed of the electron when it leaves the negatively charged plate.

ii$.$ If the electron enters a constant magnetic field of $200$ Gauss and the speed $(v)$ is perpendicular to the field $(B),$ find the magnitude of the radius of the circular trajectory.

$(20$ points$)$