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An electron is released from the bottom plate $A$ as shown in the figure $(E = 10^4\, N/C)$. The velocity of the electron when it reaches plate $B$ will be nearly equal to

$0.85\times10^7\, m/s$
$1.0\times10^7\, m/s$
$1.25\times10^7\, m/s$
$1.65\times10^7\, m/s$
Solution

The force acting on the electron $=\mathrm{eE.}$
Acceleration of the electron $=\frac{\mathrm{eE}}{\mathrm{m}}$
$\mathrm{u}=\mathrm{0}, \mathrm{v}=? \mathrm{v}^{2}-\mathrm{u}^{2}=2 \mathrm{\,aS}$
$S=2 \times 10^{-2} \mathrm{m}$
$\therefore $ $v^{2}=2\left(\frac{e}{m}\right) E \times 2 \times 10^{-2} \,m$
$\frac{\mathrm{e}}{\mathrm{m}}=1.76 \times 10^{11}$ $\mathrm{coulomb/kg}$
$\therefore v^{2}=2 \times 1.76 \times 10^{11} \times 10^{4} \times 2 \times 10^{-2}$
$=7.04 \times 10^{13}=70.4 \times 10^{12}$
$\therefore \mathrm{v} \approx 0.85 \times 10^{7} \mathrm{\,m} / \mathrm{s}$