The particle is at rest initially.
So, the initial velocity u=0ms−1.
The force on the charged particle in the uniform electric field,
F=qE
⇒ma=qE
⇒a=mqE
Applying the velocity-displacement equation under constant acceleration,
v2=u2+2ax
⇒v2=02+2(mqE)x
⇒v2=m2qEx



