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Why in eV? why not V?1eV = 1.6 x 10^-19 J
so it is basically another way of expressing energy ... hence kinetic energy of an electron...
the reason why we use eV is for the stopping voltage experiment (JJ thomson experiment i believe). in this experiment a voltage is applied to oppose the photoelectric current, and the voltage when the net current reaches zero is the stopping voltage (in eV). Since current is just a measure of the kinetic energy of an electron, the stopping voltage is the maximum kinetic energy of the electron.