An electron in the beam of a typical television picture tube

An electron in the beam of a typical television picture tube is accelerated through a potential different of 20 000 V before it strikes the face of the tube. What is the energy of this electron in electron volts, and what is its speed when it strikes the screen?

Solution

When an electron is accelerated by a potential of V volts, energy gained by it

= Q_eV     Joules

= V   ev - in terms of electron volts, 1ev = 1.6e-19 J

Energy of the electron after passing through 20000 V pot difference

     = 20000 ev = 20 kev

If v is the speed of the electron

KE = mv² /2 = 20e+3 * 1.6e-19

Mass of electron m = 9.0e-31 kg

v = sqrt(2*20e+3 * 1.6e-19/9.0e-31)

     = 8.43e+7 m/s