Basic Electricity 3, WASSCE (PC), 2017

Question 1

 

AIM: To determine the phase angle in an RL a.c. circuit


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  1. Connect the circuit as shown in Fig.1.
  2. Ask the supervisor to check the circuit connection.
  3. Fill in Table 1 as appropriate in this booklet as presented below.

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  1. Connect the variac to the mains supply and switch on.
  2. Adjust the variac to read 0V on voltage VS.
  3. Repeat and record the corresponding values of voltages VR and VL.
  4. Repeat steps (e) and (f) for the other values of voltages VS in Table 1.
  5. Plot a graph of voltages VR on the horizontal axis and VL on the vertical axis
  6. From the graph, determine the phase angle.
  7. At VS = 60V, calculate:
    1. VR + VL;

 

    1. )
  1. Briefly comment on your results in step (j)

Observation

       The expected answers were;

 

TABLE 1

          VS(V)

VR(V)

VL(V)

0

0

0

20

10

12

40

24

28

60

37

47

80

47

55

100

59

70

120

72

85

 

Correct readings for VR 
Correct readings for VL
Correct libeling of axes  
Reasonable scale 
Plotting of points        
Line of best fit   
Expression Correct substitution into expression
Correct answer                                                                                             
Phase angle  

 

 

(xi)    At Vs = 60V,
(A) VR + VL = 37 + 47                                                
= 84V.                                                                                           

       (B)  =                                
=                            
=
= 59.82V.                                                                                   

(xii) Reasonable comment on (xi)
e.g. The arithmetic sum is different from the algebraic sum.                                

The question required candidates to use the provided apparatus to connect a circuit, take readings, plot a graph, perform calculations and make comment. The circuit was connected but inaccurate readings were recorded in most cases.
Candidates were unable to calculate the required angle were blank in commenting on the results they obtained. Performance was generally poor.