Solution:
Velocity gained after moving a distance \(\mathrm{y}\) is \(v^{2}=u^{2}+2 a y\)
Here \(u=0\) and \(a=\frac{q E}{m}\)
\(v^{2}=2\left(\frac{q E}{m}\right) y\)
Kinetic energy, \(E=\frac{1}{2} m v^{2}\)
\(=\frac{1}{2} \times m \times\left(2 \times \frac{q E}{m} \times y\right)\) \(E=q E y\)