Question

What voltage on $y$-plate will give such a deflection.

• A. $0.11V$
• B. $1.1V$
• C. $11V$
• D. none

Answer 1

Answer: (A)

Let the velocity of electron is v , so when there is no deflection

$evB=eE\to ev\times 2\times {10}^{-3}=e\times 2\times {10}^3$

$v={10}^6$ , distance to screen to 20 cm or 0.2m

so time taken to strike the screen t= $\frac{0.2}{{10}^6}sec$

let the electric field on y plate be E

so acceleration on electron $a=\frac{eE}{m_e}$

now using S=$a{t}^2/2$, S= 2cm

we get E=5.68V/m

$V=.02\times 5.68=0.11V$

So, the answer is option (A).