The expected answers are:

(a)Thermionic emission is the release/escape of electrons from the surface of a metal sufficiently heated (to a high temperature). Any additional information e.g.

-It is a surface phenomenon
-Number/rate of electrons emitted increases sharply with increase in temperature.

(b)Any two correct applications e.g.

-In advertising industry/Neon signs
-In lighting/fluorescent tubes
-Identification of gases.

This question on thermionic emission was not popular among the candidates. The performance of the candidates was fairly satisfactory. In part (a) most of the candidates were able to define thermionic emission correctly. A few of them could not give any additional information to earn the full marks for this question. Also most candidates could not give more than one correct application of electrical conduction in gases.