Question

As the frequency of an $AC$ circuit increases, the current first increases and then decreases. What combination of circuit elements is most likely to comprise the circuit?

• A. Inductor and capacitor
• B. Resistor and inductor
• C. Resistor, inductor and capacitor
• D. (a) and (c) above

Answer 1

The correct option is D (a) and (c) above

Impedance is given by,
$Z=\sqrt{R^2+{\left(X_L-X_C\right)}^2}=\sqrt{R^2+{\left(2\pi fL-\frac{1}{2\pi fC}\right)}^2}$


At resonance frequency, $Z$ is minimum and current is maximum. For any other value of frequency, the value of current will decrease (when compared to current at resonance).
Clearly, the circuit must have inductor and capacitor. However, it may be possible that the circuit also has a resistor besides inductor & capacitor.