Electron Emission and Work Function

Identify the energy source, decide the type of emission, compare supplied energy per electron with phi0, and state whether emission is possible. If units are mixed, convert joule to eV or eV to joule before comparison.

A useful sketch may show a metal surface, bound electrons inside it, and an escaping electron after receiving energy. Label metal surface, incident energy source, emitted electron, and work function barrier.

Wrong: Work function is the energy needed to remove all electrons from a metal. Correct: It is the minimum energy needed by one surface electron to escape from the metal.

Electron emission is the liberation of electrons from the surface of a metal when they receive enough energy to overcome the surface binding. The minimum energy needed by an electron to just escape from a metal surface is called the work function, usually denoted by phi0.

Electron emission is the liberation of electrons from the surface of a metal when they receive enough energy to overcome the surface binding. The minimum energy needed by an electron to just escape from a metal surface is called the work function, usually denoted by phi0. Key rule: emission occurs only when supplied energy per electron is at least phi0. Work function in joule is related to electron volt by 1 eV = 1.6 x 10^-19 J. For photoelectric emission, threshold condition is h nu0 = phi0, where h is Planck's constant in J s and nu0 is threshold frequency in Hz. If a metal has work function 2.0 eV, an electron must receive at least 2.0 eV to escape with zero kinetic energy. If it receives more than this, the extra energy appears as kinetic energy of the emitted electron.

Electrons in a metal are not free to leave the surface without an energy supply because the metal surface holds them back. Energy can be supplied by heating, strong electric field, or incident light. Thermionic emission occurs due to heat, field emission occurs due to a strong electric field, and photoelectric emission occurs when suitable radiation falls on the surface. Work function is a property of the metal surface and is commonly expressed in electron volt for atomic-scale energy comparison. Key rule: emission occurs only when supplied energy per electron is at least phi0. Work function in joule is related to electron volt by 1 eV = 1.6 x 10^-19 J. For photoelectric emission, threshold condition is h nu0 = phi0, where h is Planck's constant in J s and nu0 is threshold frequency in Hz. A metal has work function 3.2 x 10^-19 J. In eV, phi0 = (3.2 x 10^-19)/(1.6 x 10^-19) = 2.0 eV. This means each surface electron needs at least 2.0 eV to escape from that metal surface. Usually asked as a definition, distinction between types of emission, or a threshold condition linked with photoelectric effect. Assertion-reason questions often test whether high intensity can replace threshold frequency. Wrong: Work function is the energy needed to remove all electrons from a metal. Correct: It is the minimum energy needed by one surface electron to escape from the metal.