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Electronics Paper 2, May/June 2008  
Questions: 1 2 3 4 5, 6, 7 8 9 10 Main
General Comments

Question 2

(a) Define the term resonance with reference to a series RLC circuit.
s                                                  Figure 6
In figure 6, calculate the
               (i) resonant frequency;
               (ii) current at resonance.
(b) State two devices where resonant circuits are applied.



The expected answers were:
(a)Resonance with reference to a series RLC circuit is that circuit condition for which                                             capacitive reactance equals inductive reactance, so that the circuit's impedance becomes                            purely resistive
(b)  (i) At resonance, inductive resonance equals capacitive resonance, or
                      XL = XC
                    2πfL = 1(2πfC) or
                    Fr = 1/ (2πVLC)
  When the numerical figure values of L and C are substituted into this expression, the result is
    (i)   fr = 1/( 2πѴ0.25x28.14x10-6)Hz
               = 103/(2πѴ7.035)Hz = 60.035Hz
               ≈ 60Hz

   (ii)Current flow at resonance is given by

              I = V/Z = V/R = 240/6 A = 40A
This question was on the application of RLC circuits.
A surprising number of candidates could not correctly define resonance as the condition for which          inductive reactance (XL) is equal to capacitive reactance (XC).
Candidates who were able to give correct definition computed resonance frequency with ease, but a    few of them lost tract of getting b (ii). Nevertheless, at resonance, impedance is purely resistive, so that current at resonance is simply a ratio of voltage to resistance.


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