What term describes the emission of electrons from a heated tungsten filament?

• A. Thermionic emission
• B. Sublimation
• C. Photodisintegration
• D. Attenuation

Answer: (A)

Emission of electrons from a heated tungsten filament is thermionic emission. When the filament is heated to sufficiently high temperature, electrons inside the metal gain enough energy to overcome the surface work function—the barrier at the metal’s surface—and escape into vacuum, forming a cloud of electrons that can be accelerated toward an anode in devices like X-ray tubes. This relies on thermal energy rather than photons or nuclear processes. Sublimation would be solid turning into gas, not electron emission; photodisintegration is a nuclear reaction driven by high-energy photons; attenuation describes the reduction of radiation intensity as it passes through material, not the emission of electrons. The essential point is that heat supplies the energy to overcome the surface barrier and release electrons.