Question

Two plates are kept at a distance of 0.1m and their potential difference is 20V. An electron is kept at rest on the surface of the plate with lower potential. What will be the velocity of the electron when it strikes another plate?

(a) 1.87*10^6 m/s

(b) 2.65*10^6 m/s

(c) 7.02*10^12 m/s

(d) 32*10^-19 m/s

The question was posed to me in examination.

The question is from Electrostatic Potential due to a Point Charge topic in portion Electrostatic Potential and Capacitance of Physics – Class 12

Answer 1

Right option is (b) 2.65*10^6 m/s

To elaborate: Increase in potential energy of the electron when it strikes another plate = Potential difference*charge of electron=20*1.6*10^-19 J=3.201*10^-18J and as the electron was at rest initially, this energy will be converted to kinetic energy completely. Therefore, 0.5*mass of electron*(velocity)^2=3.201*10^-18J. Substituting m=9.11*10^-31kg, we get v=2.65*10^6 m/s.