The principle of thermionic emission.

The idea

Thermionic emission is the release of electrons from a metal surface when it is heated to a sufficiently high temperature. In a metal, some electrons are free from individual atoms but remain bound within the metal by the surface potential barrier, often described by a work function. Heating gives a proportion of these electrons enough kinetic energy to overcome that barrier and escape into the surrounding vacuum or gas. The process is used in devices such as the electron gun in a cathode-ray tube: a filament cathode is heated by an electric current, electrons are emitted, and an applied potential difference accelerates them towards an anode. The rate of emission rises steeply with temperature because the number of electrons with sufficient energy increases rapidly. Thermionic emission is distinct from field emission, where a strong electric field pulls electrons from a surface.

What earns the marks

• Electrons in a metal are free to move within the metal but are held at the surface by a potential barrier; heating supplies energy that allows some electrons to overcome this barrier.
• The emitted particles are electrons, not atoms or ions, and emission occurs from the surface of a heated metal, commonly a filament or cathode.
• A minimum temperature is needed for significant emission; below this, very few electrons have enough energy to escape.
• The rate of emission increases with temperature because the distribution of electron kinetic energies shifts so that more electrons exceed the surface barrier.
• Thermionic emission requires a vacuum or low-pressure environment in practical devices to prevent emitted electrons from colliding with gas molecules.
• The process is used in electron guns, cathode-ray tubes and older thermionic valves to produce a beam of electrons.

Common mistakes and how to put them right

• Thinking that heating causes atoms to leave the metal.

  Put it right: Electrons are emitted.

• Believing that any temperature causes emission.

  Put it right: A sufficiently high temperature is needed for significant emission.

• Confusing thermionic emission with the photoelectric effect.

  Put it right: Thermionic emission is caused by heating the metal.

Exam tips

• State clearly that electrons are emitted from a heated metal surface and that they must overcome a surface energy barrier.
• Link the increase in emission to the increased proportion of electrons with enough kinetic energy, rather than saying that electrons simply move faster.
• When describing a device, identify the heated cathode as the source of electrons and explain that the vacuum prevents collisions.