Define impedance in an AC circuit and explain its components.

• A. Impedance Z equals the sum of resistance and reactance: Z = R + X.
• B. Impedance Z combines resistance R and reactance X as Z = √(R^2 + X^2).
• C. Impedance is equal to current.
• D. Impedance is the same as voltage.

Answer: (B)

Impedance in an AC circuit is the total opposition to current flow, made up of two parts: resistance, which dissipates power (real part), and reactance, which stores energy and causes a phase shift (imaginary part). These two parts combine as a complex quantity Z = R + jX, where X is the net reactance (X = X_L − X_C, with X_L = ωL and X_C = 1/(ωC) in magnitude but with a sign for capacitive reactance).

The magnitude of impedance, which tells you how large the voltage and current are relative to each other, is |Z| = √(R^2 + X^2). This is the form that matches how resistance and reactance contribute perpendicularly in the complex plane, giving you the total opposition to current without regard to phase. The relation V = IZ means the volt­age magnitude is the product of the current magnitude and this impedance magnitude, so knowing |Z| is key for sizing circuits.

So the square-root expression correctly represents the impedance magnitude, while impedance itself is the complex combination Z = R + jX. The other statements don’t capture this relationship, since impedance is not simply current or voltage, and it’s not a plain real sum of R and X.