Question 2
- State two properties of a parallel RLC circuit at resonance.
- .
In Figure 1, calculate the value of
- Impedance;
- Total current;
- Voltage drop across the capacitor.
Observation
The expected answers were:
(a) Two properties:
(i) Impedance is very high.
(ii) Current is minimum.
(iii) XC = XL
(b) (i)
= 11.18 Ω
(ii)
= 1.073A
(iii)
= 1.073 x 20
= 21.46V
This is one that the examiner believes should have been differently posed; the first part
was on ‘parallel resonance’, whereas the second on series resonance. The parallel
resonance is being featured for the first time and important properties derived from the
V-I relationship given below as follows:
; from which we find that at resonance;
- Capacitive reactance = inductive reactance
- Impedance is maximum (or admittance is minimum) and is a pure resistance
- Current is minimum or voltage is maximum.
Not many candidates could give the correct responses.
The second part is well-known series RLC circuit (though not at resonance). One common mistake by the candidates is that quite a few expressed impedance as
instead of