C
CraftExam
medium difficulty10 min

Electron Emission and Work Function

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.

In one line: Work function is the exit fee for one electron at the metal surface.

Concept Practice Coming Soon

Understand it

  • 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.

Example

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.

Formula and rule

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 × 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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A metal has work function 3.2 × 10−19 J.

  1. 1In eV, phi0 = (3.2 × 10−19)/(1.6 × 10−19) = 2.0 eV.
  2. 2This means each surface electron needs at least 2.0 eV to escape from that metal surface.

Method you can reuse

  1. 1Identify the energy source, decide the type of emission, compare supplied energy per electron with phi0, and state whether emission is possible.
  2. 2If units are mixed, convert joule to eV or eV to joule before comparison.

Visual tip

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is work function called a minimum energy?

Think first, then show answer
Work function is called a minimum energy because it is the least energy required for an electron at the metal surface to escape just outside the metal with no extra kinetic energy. Any additional absorbed energy becomes kinetic energy of the emitted electron.

Common wrong answer

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.

Don't mix this up

Students often write that any light can eject electrons if its intensity is high enough. In photoelectric emission, the photon frequency must be at least the threshold frequency; intensity alone cannot overcome an insufficient photon energy.

Exam tip

Mention both the minimum energy condition and the surface nature of the work function. For comparison questions, separate thermionic, field, and photoelectric emission by the energy source.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

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.
  1. 1

    Enough for 1 mark

    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.

  2. 2

    Add for 2 marks

    Example: 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.

  3. 3

    Add for 3 marks

    Electrons in a metal are not free to leave the surface without an energy supply because the metal surface holds them back.

Practice

Concept practice is coming soon

Login to access concept-level MCQs and weak-concept practice as they become available.

10 MCQs5 MinutesInstant Results
Login

Help improve this page

Found something confusing, incorrect, or missing?

Open Learning Map