Thermionic emission releases electrons from a heated filament

Turning Points in Physics | AQA A-Level Physics

Key Definition

Thermionic emission: The release of electrons from a metal surface when it is heated. The thermal energy gives electrons enough kinetic energy to escape the surface.

The cathode ray tube

A cathode ray tubeAn evacuated glass tube containing a heated cathode (electron gun) and an anode with a hole in it, used to produce a narrow beam of fast-moving electrons. (CRT) uses thermionic emission to produce a controllable beam of electrons.
A filament heats the cathode. The thermal energy causes free electrons in the metal to gain enough energy to leave the surface.
A high potential difference between the cathode and a cylindrical or perforated anode accelerates the emitted electrons into a narrow beam.
The whole tube is evacuated so that electrons do not collide with air molecules.

Work done on the electron

When an electron is accelerated from rest through a potential differenceThe work done per unit charge in moving a charge between two points. Measured in volts (V). $V$, the work done on it by the electric field equals the kinetic energy it gains:

$$eV = \tfrac{1}{2}m_e v^2$$

Rearranging for speed: $v = \sqrt{\dfrac{2eV}{m_e}}$
Crucially, this assumes the electron starts from rest and that no energy is lost to collisions (a valid assumption in an evacuated tube).
This relationship is used throughout the Turning Points topic whenever electrons are accelerated through a known p.d.

Common Mistake

Do not confuse thermionic emission with the photoelectric effect. In thermionic emission, thermal energy provides the energy for electrons to escape. In the photoelectric effect, it is the energy of individual photons. The mechanism is different, even though both result in electron emission.