### Question 2

1. State two properties of a parallel RLC circuit at resonance.
2. .

In Figure 1, calculate the value of

1. Impedance;
2. Total current;
3. Voltage drop across the capacitor.

### Observation

(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