The expected answers were:
(a) Thermionic emission is the emission of electrons from the surface of metals when sufficiently supplied with heat energy from external sources.
Electron emission will occur when heat energy transferred to the electron exceeds the work function of the metal.
(b) The pentode tube
This question was on electron emission and thermionic devices. Most of the candidates that responded to this question gave correct answers. However, very few indeed could identify the grids, though majority knew the anode (plate) and cathode.
(c) Advantages of indirectly heated cathode:
- Eliminates the unwanted a.c. noise associated with the directly heated cathode on account of varying voltage drop along filament length.
- Better electron emission efficiency due to improved surface area characteristics.