An electron is released with a velocity of $5 \times {10}^6 {\mathrm{ms}}^{-1}$ in an electric field of ${10}^3 \mathrm{N} {\mathrm{C}}^{-1}$ which has been applied so as to oppose its motion. How much time could it take before it is brought to rest?

Since electric field is applied so as to oppose the motion of electron,

$\begin{array}{l}\mathrm{a}=-\frac{\mathrm{eE}}{\mathrm{m}}\left(\mathrm{retardation}\right)=-\frac{1.6\times {10}^{-19}\times {10}^3}{9.1\times {10}^{-31}}=-1.758\times {10}^{14}{\mathrm{ms}}^{-2}\\ \mathrm{Now},\mathrm{u}=5\times {10}^6{\mathrm{ms}}^{-1},\mathrm{v}=0\\ \mathrm{Using}\mathrm{the}\mathrm{relation}:{\mathrm{v}}^2-{\mathrm{u}}^2=2\mathrm{as},\mathrm{we}\mathrm{get}\\ \mathrm{s}=7.11\times {10}^{-2}\mathrm{m}\\ \mathrm{Using}\mathrm{the}\mathrm{relation}:\mathrm{v}=\mathrm{u}+\mathrm{at},\mathrm{we}\mathrm{get}\\ \mathrm{t}=2.844\times {10}^{-8}\mathrm{s}\end{array}$