In an AC circuit, how does impedance affect current for a given voltage?

• A. Higher impedance increases current
• B. Higher impedance reduces current
• C. Impedance has no effect on current
• D. Impedance only affects voltage

Answer: (B)

In an AC circuit, current is determined by impedance just like voltage is by resistance in a DC circuit. For a sinusoidal source at a fixed voltage, the current’s magnitude is I = V / |Z|, where |Z| is the impedance’s magnitude. That means when the voltage stays the same and the impedance increases, the current decreases. Impedance combines resistance and reactance (from inductors and capacitors), so larger opposition to current flow—whether from pure resistance or from reactive effects—shrinks the current. The reactive parts can also tilt the current relative to the voltage (phase shift), but the amount of current drops as impedance grows. For example, with 120 V and 20 Ω of impedance, the current is 6 A; increasing impedance to 60 Ω reduces the current to 2 A.