Question

If an electron enters into a space between the plates of a parallel plate capacitor at an angle $\alpha$ with the plates and leaves at an angle $\beta$ to the plates. The ratio of it's kinetic energy while entering the capacitor to that leaving will be :

• A. $(\frac{cos\alpha }{cos\beta }{)}^2$
• B. $(\frac{cos\beta }{cos\alpha }{)}^2$
• C. $(\frac{sin\alpha }{sin\beta }{)}^2$
• D. $(\frac{sin\beta }{sin\alpha }{)}^2$

Answer 1

Answer: (B)

$(\frac{cos\beta }{cos\alpha }{)}^2$

Electron doesn't experience a force in x-direction thus momentum in x-direction is conserved.
Therefore,
$\Rightarrow vcos\beta =ucos\alpha$
$\therefore \frac{K_{in}}{K_{out}}=\frac{\frac{1}{2}m{u}^2}{\frac{1}{2}m{v}^2}=\frac{co{s}^2\beta }{co{s}^2\alpha }$