The opposing force to a flow of alternating current is called

• A. Capacitance
• B. Inductance
• C. Impedance
• D. Resistors

Answer: (D)

In alternating current circuits, the total opposition to current is called impedance. It combines resistance (the real part that dissipates power as heat) with reactance (the imaginary part that comes from energy storage in magnetic and electric fields). Reactance depends on frequency: inductive reactance rises with frequency, capacitive reactance falls with frequency. The impedance is a complex quantity, Z = R + jX, whose magnitude |Z| determines how much current a given AC voltage will drive, and whose phase angle shows how far the current lags or leads the voltage. Capacitance and inductance describe how a circuit can oppose current through energy storage, but the complete description of opposition in AC is impedance. Resistors are the real, frequency-independent part of that opposition.