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?

Question

Philoid · Accepted Answer

Consider a general LCR circuit. The maximum current varies inversely with impedance which is given by:

where X_L is inductive reactance, X_C is capacitive reactance, R is resistance, f is frequency, L is inductance and C is capacitance.

Here current refers to current amplitude.

Now, as frequency increases, current increases and then decreases. For this to happen, we must have maximum current. Maximum current is when the value of frequency is such that impedance is minimum. For impedance to be minimum, X_L = X_C (R doesn’t depend on frequency). Hence, we must have the two components capacitor and inductor to have a maximum current (with respect to changing frequency NOT time).However, a resistor may or may not be present. Hence, the answer is (a) and (d).