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Electronics Paper 2, Nov/Dec. 2008  
Questions: 1 2 3 4 5 6 7 Main
General Comments

Question 2

(a)State Kirchhoff’s laws





Fig. 6 is a two-source closed loop circuit.  Calculate the voltage VB.





(c) Calculate the current I5 in figure 7. 



The expected answers are as stated below:

(a) The Kirchhoff’s Laws are as follows:
- Voltage Law, which states that in any closed loop of an electric circuit or network, net emf equals sum of voltage drops.
- Current Law, which states that algebraic sum of current entering (or leaving) any node of an electric circuit or network equals zero.

(b) Applying the KVL to the circuit, we find that

Net emf  =  12 + VB
Sum of voltage drops = 18v
12 + VB  =  18
VB = (18 – 12)v = 6v

            (c)        Using KCL
                        I1 = I2 + I3 + I4+I5
                        I5 = I1 – I2 – I3 – I4
      = 8 – 13

        I5 = - 5A
This question was on circuit analysis.  It was quite popular among the candidates.

In part (a), many candidates stated KVL and KCL well.  Most of the candidates failed to recognize the fact that KVL is an energy balance law which stands as “in any closed loop of an electric circuit, net emf equals total voltage drop and not as many candidates suggested that “algebraic sum of voltages equals zero.”
The candidates had no difficulty in answering part (c) of the question on KCL.

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