Question

There are a large number of free electrons moving randomly in a metal. At room temperature, why do they not come out of the surface of metal?

Answer 1

In a metals like copper or tungsten, there are plenty of free electrons. At room temperature these electrons wander randomly in the atomic structure, but they cannot leave the metallic surface. At room temperature ordinary metals do not loose their electrons. This means that a force must exist, which prevents electrons form leaving the metallic surface permanently. To understand what this force is and how it is created, let us assume that due to its random motion, an electron leaves the surface. Immediately after it leaves the surface, the metal gains a positive charge (losing a negatively charged electron is equivalent to gaining a positive charge). This positive charge exerts a force of attraction on the emitted electron. This force pulls the electron back to the metal. For an electron to escape from the metal surface, it must have sufficient kinetic energy to overcome this force. This force is described as surface barrier.