Impedo

Meet Impedo, a circuit block who holds a resistor, a coil and a capacitor and treats them as one.

In an alternating current circuit, a resistor, an inductor and a capacitor all oppose the current, but in different ways. The resistor simply turns energy into heat. The coil and the capacitor store energy and hand it back, and their opposition is called reactance.

The inductive reactance is X_L = ωL and grows with frequency. The capacitive reactance is X_C = 1/(ωC) and shrinks with frequency. Because they push in opposite directions, they partly cancel. The total opposition is the impedance Z = √(R² + (X_L − X_C)²).

Take R = 30 Ω, X_L = 80 Ω and X_C = 40 Ω. The reactances leave 40 Ω, so Z = √(30² + 40²) = 50 Ω. A 100 V supply then drives a current of 2 A.

When X_L equals X_C the reactances cancel completely, Z drops to just R, and the current peaks. This is resonance, the trick a radio uses to tune in one station.