Question

The electron gun in a TV picture tube accelerates electrons between two parallel plates 1.20 cm apart with a 25.0 kV potential difference between them. The electrons enter through a small hole in the negative plate, accelerate, and then exit through a small hole in the positive plate. Assume that the holes are small enough not to affect the electric field or the potential. (a) What is the electric field between the plates? (b) What is the exit speed of an electron if its entry speed is close to zero?

Answer 1

a)

E = V / d = 25*10^3 / 1.2*10^–2 = 20.83* 10^5 N/C

b)

Exit energy = V . e = 20.83* 10^5* 1.6* 10^–19 = 33.33*10^-14 J

v = √ 2 * energy / mass = √ 2 * 33.33*10^-14 / 9.1 * 10^–31 = 8.55* 10^8 m/s