Q.What is thermionic emission? State two practical ways of increasing the rate of thermionic emission of electrons from a metal filament.

Thermionic emission is the release of electrons from a metal surface once thermal energy exceeds the metal's work function $\phi$. Two practical ways to increase its rate: (1) raise the filament's operating temperature, since emission rises very steeply with temperature; (2) use, or coat the filament with, a material of LOWER work function (e.g. thoriated tungsten or an oxide coating), which needs less thermal energy per electron to escape, so the same temperature releases more electrons.

Define thermionic emission, then give the two governing factors as the two practical levers.

• Suggesting only temperature can be changed -- the emitting material's work function is equally important in practice.
• Overstating how much a small temperature rise helps without noting the relationship is very steep (not linear).