In alternating current (AC) circuit theory, the combined opposition to the flow of AC known as impedance is measured in which SI unit?

Introduction / Context:
This question checks your knowledge of electrical units in AC circuit analysis. In direct current (DC) circuits we talk about resistance, but in AC circuits the opposition to current flow is more general and includes resistance, inductive reactance and capacitive reactance together. This combined effect is called impedance. Knowing the correct SI unit for impedance is essential for understanding AC circuit calculations and reading circuit diagrams and specifications.

Given Data / Assumptions:

• We are dealing with AC circuits where impedance is defined.
• Impedance is the generalised opposition to alternating current.
• Options for the unit include ohm, henry, tesla and hertz.
• We assume standard SI units are used with no special derived units beyond the usual ones.

Concept / Approach:
Impedance is usually denoted by Z and is a complex quantity that combines resistance R and reactance X. However, its unit is the same as that of resistance because both represent an opposition to current. Resistance is measured in ohm, symbol Ω. Therefore impedance is also measured in ohm. The henry is the unit of inductance, tesla is the unit of magnetic flux density and hertz is the unit of frequency. They do not measure impedance directly. So the correct unit is ohm.

Step-by-Step Solution:
Step 1: Recall that in DC circuits, resistance R is the opposition to current and is measured in ohm. Step 2: In AC circuits, the total opposition to current includes resistance and reactance and is called impedance Z. Step 3: Even though impedance can be represented as a complex number (R + jX), the magnitude of impedance still has the same unit as resistance. Step 4: Therefore, the SI unit of impedance is the ohm (Ω). Step 5: Henry (H) measures inductance, which describes how a coil opposes changes in current, not the overall impedance itself. Step 6: Tesla (T) measures magnetic flux density in electromagnetism, not opposition to current. Step 7: Hertz (Hz) measures frequency and does not represent impedance.

Verification / Alternative check:
In practical AC circuit problems, you often use formulas such as Z = V / I, where V is voltage in volt and I is current in ampere. This is similar to Ohm law R = V / I. Since volt per ampere defines the ohm, and Z is computed by the same ratio, impedance must also be expressed in ohm. Component datasheets for resistors, inductors and capacitors in AC circuits usually specify impedance magnitude in ohm at a given frequency, further confirming this unit choice.

Why Other Options Are Wrong:
Henry is the unit of inductance, which is a property of coils and describes induced emf, not the overall impedance. Tesla is used for magnetic flux density and is unrelated to direct measurement of circuit opposition to current. Hertz is the unit of frequency, describing how many cycles occur per second, not the opposition to current.

Common Pitfalls:
Some students may associate impedance mainly with inductors and capacitors and mistakenly choose henry as the unit. It is important to remember that while inductance and capacitance contribute to impedance, their own units are henry and farad, not ohm. Impedance itself, as the combined effect of resistance and reactance, has the same unit as resistance. Keeping the distinction clear between physical quantity and its contribution to impedance prevents confusion.

Final Answer:
Impedance in AC circuits is measured in the SI unit called the ohm.