Question

A negatively charged plate has charge density of $2\times {10}^{-6}C/m^2$. The initial distance of an electron which is moving toward plate, cannot strike the plate, if it is having energy of 200 eV

• A. 1.77 mm
• B. 3.51 mm
• C. 1.77 cm
• D. 3.51 cm

Answer 1

The correct option is A 1.77 mm

Let an electron is projected towards the plate from the r distance as shown in fig.

It will not strike the plate if and only if $KE\le e(E.r)$ (where E = Electric field due to charge plate $=\frac{\sigma }{2{ϵ}_0}$)
$\Rightarrow r\ge \frac{KE}{eE}=\frac{200eV}{e\times \frac{\sigma }{2{ϵ}_0}}=\frac{400\times 8.86\times {10}^{-12}}{2\times {10}^{-6}}=1.77mm$