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\sect{Thermionic emission of electrons from a metal}

A piece of metal ("Cathode") is placed inside a vacuum metal tube ("Anode"). 
The cathode has a work function $W$ and surface $A$ and is held at temperature $T$ .

\Dn

Calculate the saturation current $I_s$ 
that woulkd be measured if the bias voltage is very large. 
Distinguish between two case: 
high temperature  (${ T \gg W }$) 
and low  temperature(${ T \ll W }$).

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