An electron in the beam of a typical television picture tube is accelerated through a potential difference of 20V before striking the face of the tube. What is the energy of this electron, in Joules and in electron volts, and what is its speed when it strikes the screen?
energy in = volts * charge = 20 * e = 20 * 1.6*10^-19 Joules
so
20 * 1.6*10^-19 = (1/2) m v^2 { if we do not approach the speed of light}
20 * 1.6*10^-19 = (1/2)(9.1 * 10^-31) v^2
v^2 = (40 * 1.6 /9.1) 10^12 = 7 * 10^12
v = 2.65 *10^6 m/s
ok, speed of light over 100 about