Core AC/DC concepts — units check: Which SI unit is used to measure resistance, reactance, and impedance in electrical circuits?

Introduction / Context:
Knowing the correct SI units anchors understanding of circuit quantities and avoids dimensionally inconsistent calculations. Resistance (DC opposition), reactance (AC frequency-dependent opposition), and impedance (general complex opposition) are all measures of how much a circuit resists current flow.

Given Data / Assumptions:

• We must identify a single unit that applies to resistance (R), reactance (X), and impedance (Z).
• Recall common SI units: ohm (Ω), farad (F), henry (H), watt (W), siemens (S).
• Context includes both DC and AC circuits.

Concept / Approach:
Resistance, reactance, and impedance are all quantities of electrical opposition with the same dimension of volts per ampere. Volts per ampere defines the ohm. In AC, reactance X and impedance Z are often represented as frequency-dependent or complex values, but their magnitude is still in ohms. Siemens is the reciprocal unit (conductance, admittance, susceptance).

Step-by-Step Solution:

Verification / Alternative check:
Ohm’s law in DC: V = I * R (R in ohms). In AC phasors: Ṽ = Ĩ * Z̃ (Z in ohms). Reactance formulas X_L = 2 * π * f * L and X_C = 1 / (2 * π * f * C) both produce ohms, confirming consistency.

Why Other Options Are Wrong:
Farad measures capacitance, henry measures inductance, watt measures power, siemens measures conductance (the inverse of ohms).

Common Pitfalls:
Confusing admittance (S) with impedance (Ω), and forgetting that even though impedance can be complex, its unit is still the ohm.

Final Answer:
ohm