Question 4
(a) Define
(i) thermionic emission;
(ii) photoelectric emission.
(b) State one application each of the emissions defined in 4 (a).
(c) Complete Table 1.
Table `1
TRANSDUCER TYPE |
CONVERSION |
APPLICATION |
FROM |
TO |
Loudspeaker |
Electrical energy |
Sound energy |
Public address system |
Microphone |
|
|
|
|
|
|
Waveform display |
|
Moving picture |
|
|
The expected answers were:
(a)(i) Thermionic emission is the process of liberating electrons from the surface of a metal when the metal is heated.
(ii) Photoelectric emission is the emission of electrons from a metallic
surface when light of certain frequency is incident on it.
(b)(i) Any one application of thermionic emission
- Thermionic converter
- Electron tube or valves
(ii) Any one application of thermionic emission
- Solar cell
- Photoconductive devices
- Photo cells
- Security doors
- Alarm circuit
- Security light
- Charging system
(c)
TRANSDUCER TYPE |
CONVERSION |
APPLICATION |
FROM |
TO |
Loudspeaker |
Electrical energy |
Sound energy |
Public address system |
Microphone |
Sound energy |
Electrical energy |
Public address system |
Cathode Ray Tube (CRT) |
Electrical energy |
Light |
Waveform display |
Digital camera |
Moving picture |
Electrical signal |
Video recording or CCTV |
The question was on Electronic devices and circuits. It was unpopular with the candidates. It showed that many candidates that attempted it had little depth of knowledge. The performance of candidates was reported to be average.
|
