Question

An electron is projected with an initial speed $v_0=1.60\times {10}^{8}m{s}^{-1}$ into the uniform field between the parallel plates as shown in figure. Assume that the field between the plates is uniform and directed vertically downward, and that the field outside the plates is zero. The electron enters the field at a point midway between the plates. Mass of electron is $9.1\times {10}^{-31}kg$. If the electron just misses the upper plate, the time of flight of the electron up to this instant is :

• A. $1.25\times {10}^{-9}s$
• B. $32.5\times {10}^{-6}s$
• C. $1.25\times {10}^{-10}s$
• D. $32.5\times {10}^{-8}s$

Answer 1

The correct option is C $1.25\times {10}^{-10}s$

Passing between the charged plates, the electron feels a force upward and just misses the top plate.

The distance it travels in the y-direction is $0.005m$.
In $x$ direction electron is traveling 2 cm distance with velocity $v_0=1.60\times {10}^{8}ms$
Time of flight is

$t=\frac{2cm}{1.60\times {10}^{8}m{s}^{-1}}$ $=1.25\times {10}^{-10}s$