Separation between the plates, $d=1.5\, \mathrm{cm}$ Force of attraction between the plates, $F=$ $25 \times 10^{-6}\, \mathrm{N}$
$F=Q E$
$F=\frac{Q^{2}}{2 A \in_{0}} \quad$ ($E$ due to parallel plate
$=\frac{\sigma}{2 \in_{0}}=\frac{Q}{A 2 \in_{0}})$
But $Q=C V=\frac{\in_{0} A(V)}{d}$
$\therefore \,F = \,\frac{{({\in_0}A{V^2})}}{{{{\text{d}}^2} \times 2{\text{A}}{\in_0}}}$
$ = \frac{{{{\left( {{\in_0}A} \right)}^2} \times {V^2}}}{{{d^2} \times 2 \times \left( {A{\in_0}} \right)}} = \frac{{\left( {{\in_0}A} \right) \times {V^2}}}{{{d^2} \times 2}}$
or, $25 \times {10^{ - 6}} = \frac{{\left( {8.85 \times {{10}^{ - 12}}} \right) \times \left( {200 \times {{10}^{ - 4}}} \right) \times {V^2}}}{{2.25 \times {{10}^{ - 4}} \times 2}}$
$ \Rightarrow V\, = \frac{{25 \times {{10}^{ - 6}} \times 2.25 \times {{10}^{ - 4}} \times 2}}{{8.85 \times {{10}^{ - 12}} \times 200 \times {{10}^{ - 4}}}}$ $ \approx 250\,V$
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$(A)$ A zener diode is connected in reverse bias, when used as a voltage regulator.
$(B)$ The potential barrier of $\mathrm{p}-\mathrm{n}$ junction lies between $0.1\, \mathrm{~V}$ to $0.3 \,\mathrm{~V}$.