Radio,Television And Electronic Paper 3, WASSCE (SC), 2016

Question 3

  • AIM: To verify Kirchhoff’s Laws.

    (a)  Connect the circuit as shown in Fig. 1.
    (b)  Ask the supervisor to check the circuit connection.
    (c)  Copy Table 3 into your answer booklet.


    Table 3

    I1(mA)

    V1(V)

    I2(mA)

    I3(mA)

    V2(V)

    I1R1

    I2 R2

    I3 R3

    50

     

     

     

     

     

     

     

    100

     

     

     

     

     

     

     

     

    (d)  Close switch S.
    (e)  Vary the battery voltage to set current I1 to 50mA.
    (f)  Read and record in Table 3 the values of I2, I3, V1 and V2.
    (g)Repeat steps (e) and (f) for I1 set to 100mA.
    (h)  Using the readings recorded in Table 3, calculate the potential difference across R1,
    R2 and R3 and record as I1R1,I2R2 and I3R3 respectively.
    (i)   Compare I2R2and I3R3 with V2.
    (j)   From the results recorded in Table 3, show whether or not the following laws are 
    justified:

    1. Kirchhoff’s Voltage Law;
    2. Kirchhoff’s Current Law.

    (k)  State two precautions taken when carrying out this experiment.

Observation

The candidates were expected to connect the circuit as shown in Fig.1, measure and record in Table 3. And the expected answers were:

 

  1. (a)Correct circuit connection
  2. (c) Table 3 copied correctly


       Table 3

I1(mA)

V1(V)

I2(mA)

I3(mA)

V2(V)

I1R1

I2 R2

I3 R3

50

5.5

30.0

20.0

3.0

2.5

3.0

3.0

100

11.0

60.0

40.0

6.0

5.0

6.0

6.0

 

  • I2R2 and I3R3 are equal/approximately equal to V2.

 I2R2 ≈  I3R3 ≈ V2.
            (j)       Kirchhoff’s Laws Justification
                      (i) Kirchhoff’s Voltage Law
                           V1 = I1R1 +I2R2
                          5.5V = 2.5V + 3.0V        OR            11.0V = 6.0V + 5.0V
                           5.5V = 5.5V                                     11.0V = 11.0V

 

  • (ii) Kirchhoff’s Current Law

I1 = I2 + I3
50 = 30 + 20                         OR       100 = 60 +40
50 = 50                                              100 = 100

 

 

     (k)  Precautions taken

  • I ensured tight connections to avoid sudden disconnection in the course of experiment.
  • I ensured correct polarity in the meter connections.
  • I avoided dry joints.

Many candidates found it difficult to take correct readings with simple electronic measuring instruments.