Basic Electronics Paper 2, WASSCE (PC 2ND), 2018

Question 3

       

  1.  State two applications of:
  2. (i)         thermionic emission;
    (ii)        photo emission.

     

  3. Fig. 2 is a schematic diagram of a tetrode valve.
  4.  

    Name the parts labelled X and Y.

     

  5. State two advantages of semiconductor devices over vacuum tubes
  6.  
 

 

The expected responses were:

 

(a) (i)   Applications of thermionic emission

  • Vacuum tubes
  • CRT (in television sets and oscilloscope)
  • X – rays
  • Fluorescent tubes

    (ii)    Applications of photo emission

  • Photocells
  • Solar cells
  • Burglar alarms
  • Television cameras
  • Automatic door switching
  • Automatic light switching devices
  • Fibre optic communication

 (b)       X – screen grid                       
          Y – control grid                                                            

 

(c)        Advantages of semiconductor devices over vacuum tubes

  • Much smaller size
  • No/less heat production
  • Longer life span
  • Faster response time
  • Lower power consumption

It was reported that quite a few candidates had problems recalling the applications of thermionic emission and photo emission in part (a). Part (b)is essentially a ‘recall’ type question, for the question on pentode, so that X represented the screen grid, and Y, the control grid. The Chief Examiner reported that many candidates listed these correctly, quite a few mixed them (one for the other) and still many could not correctly identify the grids. Performance in the question (c) was rated rather poor.