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Impedance of a series RL circuit

Aviation Mechanic General (AMG) practice question 35 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K11b · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA series AC circuit contains 6 ohms of resistance and 8 ohms of inductive reactance. What is the impedance of the circuit?

  1. A14 ohms, the sum of resistance and reactance
  2. B10 ohms
  3. C2 ohms
Show the answer and explanation

Correct answer: B. 10 ohms

  • A — incorrect Resistance and reactance cannot be added directly; they are combined as the sides of a right triangle.
  • B — correct Resistance and reactance act at right angles, so Z = square root of (6 squared + 8 squared) = square root of (36 + 64) = square root of 100 = 10 ohms.
  • C — incorrect Subtracting resistance from reactance is not how impedance is found; the Pythagorean combination gives 10 ohms.

BACKGROUND

The combined effects of resistance, inductive reactance, and capacitive reactance make up the total opposition to current flow in an AC circuit. This total opposition is called impedance, symbol Z, measured in ohms. Resistance and reactance cannot be added directly but can be considered as two forces acting at right angles, so the Pythagorean theorem applies: Z equals the square root of R squared plus X squared. In the handbook example, a 6-ohm lamp in series with a coil of 8 ohms inductive reactance has an impedance of 10 ohms, and 110 volts drives 11 amperes.

STUDY TIP

Picture the impedance triangle: resistance and reactance are the legs and impedance is the hypotenuse.

KEY POINTS

  • Impedance Z is total AC opposition, in ohms.
  • Z = square root of (R squared + X squared).
  • 6 ohms and 8 ohms combine to 10 ohms, not 14.

SOURCES

  • FAA-H-8083-30B ch12 "Impedance"; "Series AC Circuits" (series RL example)
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.