The unit of impedance is

Analyze the question: You are asked for the SI unit of impedance, which is the opposition a circuit offers to alternating current (AC).

Explain the concept: Impedance (Z) combines resistance (R) and reactance (X) in an AC circuit:

Z = √(R² + X²)

Since Z is made up of resistance and reactance, it is measured in the same unit as resistance. Also, from the AC form of Ohm's law:

Z = V / I

The unit of Z is volts per ampere, which is the ohm (Ω).

Check each option:

• A. Ω (ohm): The unit of resistance, reactance and impedance. ✔
• B. T (tesla): The unit of magnetic flux density. ✘
• C. H (henry): The unit of inductance (L). It is not the unit of impedance, though inductive reactance X_L = 2πfL is measured in ohms. ✘
• D. W (watt): The unit of power. ✘

Answer: The unit of impedance is the ohm, so the correct option is A.

Tip: Remember the rule "R, X, Z are all Ω." Resistance, reactance (inductive and capacitive) and impedance are all measured in ohms. Do not confuse the henry (inductance) with the ohm (impedance), because JAMB often places both among the options.