C] Key Idea: Kinetic energy obtained by the PARTICLE is equal to the work DONE in moving a distance y. Electric force on charged particle \[F=qE\] Kinetic energy attained by particle = work done = force \[\times \] DISPLACEMENT \[=qE\times y\] Alternative: Force on charged particle in a uniform electric field is \[F=ma=Eq\] or \[a=\frac{Eq}{m}\] (i) From the equation of motion, we have \[{{v}^{2}}={{u}^{2}}+2ay\] \[=0+2\times \frac{Eq}{m}\times y\] \[=\frac{2Eqy}{m}\] Now kinetic energy of the particle \[K=\frac{1}{2}m{{v}^{2}}\] \[=\frac{m}{2}\times \frac{2\,E\,qy}{m}=qEy\]