Question

$P$ and $Q$ are oppositely charged metal plates, held parallel to each other and a thermion beam is passed in the space between $P$ and $Q$. If $P$ is positively charged and $Q$ is negatively charged, then how does the thermion beam deflect and Why?

Why is a tungsten filament coated with alkali metal oxides considered a good thermion emitter?

Answer 1

Thermionic emission is the emission of elections from a heated source. Electrons are negatively charged. So it has a tendency to deflect towards the positive charged, $P$ metal plate. A good thermionic emitter has a high melting point and low work function. Tungsten has high melting point of 3665K. The work function of alkali metals is much less than other thermionic emitters. It is less than one electron volt and corresponds to temperature of about 1000K. Thus tungsten filament coated with alkali metal oxides is an efficient thermionic emitter.