Series RC under AC — name of total opposition to current In an AC-driven series R–C circuit, what is the proper term for the total opposition to current considering both resistance and capacitive effects?

Introduction / Context:
AC circuit analysis uses a generalized concept of opposition to current that combines resistive and reactive effects. Correct terminology helps when reading datasheets and designing filters or timing networks.

Given Data / Assumptions:

• Series combination of a resistor and a capacitor.
• Sinusoidal steady-state operation.
• Ideal, linear components.

Concept / Approach:
Resistance (R) opposes current independent of frequency. Reactance (X) is the frequency-dependent opposition from energy storage (capacitors and inductors). The complex quantity that combines them is impedance Z, where Z = R - jXc for an RC series circuit, and |Z| = sqrt(R^2 + Xc^2).

Step-by-Step Solution:
Define capacitive reactance: Xc = 1 / (2 * π * f * C).Form the complex impedance: Z = R - jXc.Magnitude of total opposition: |Z| = sqrt(R^2 + Xc^2).Therefore, the correct term for the total opposition is impedance.

Verification / Alternative check:
Power factor and phase angle are derived from impedance: cos(φ) = R / |Z|. These quantities are only meaningful using impedance, not resistance or reactance alone.

Why Other Options Are Wrong:

• Inductance: property of inductors (henry), not total opposition.
• Reactance: only the reactive part; it excludes the resistive part.
• Resistance: excludes capacitive effects and phase shift.

Common Pitfalls:

• Using “reactance” when “impedance” is intended, leading to design errors.
• Ignoring frequency dependence of Xc when sizing components.

Final Answer:
impedance