SkyDrill

Current through a capacitor at 60 cps

Aviation Mechanic General (AMG) practice question 27 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K3 · 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 circuit is supplied with 110 volts at 60 cycles per second and contains an 80-microfarad capacitor. Using Xc = 1/(2 pi f C), approximately how much current flows?

  1. A33.2 amperes, the same figure as the reactance
  2. B0.30 ampere
  3. C3.31 amperes
Show the answer and explanation

Correct answer: C. 3.31 amperes

  • A — incorrect 33.2 is the capacitive reactance in ohms, not the current; the current is found by dividing the voltage by that reactance.
  • B — incorrect This results from dividing the reactance by the voltage instead of the voltage by the reactance.
  • C — correct 80 microfarads is 0.000080 farad, so Xc = 1/(6.28 x 60 x 0.000080) = 33.2 ohms, and I = E/Xc = 110/33.2 = 3.31 amperes.

BACKGROUND

Capacitive reactance (Xc) is the opposition a capacitor offers to alternating current and is measured in ohms. It is found from Xc = 1/(2 pi f C), where f is the frequency in cycles per second, C is the capacitance in farads, and 2 pi is taken as 6.28. Once the reactance is known, Ohm's Law is used in the same way as in a DC circuit: I = E/Xc. In the handbook example, 80 microfarads is converted to 0.000080 farad, giving a reactance of 33.2 ohms and a current of 3.31 amperes at 110 volts.

STUDY TIP

Convert microfarads to farads before using the formula, then treat the reactance exactly like a resistance in Ohm's Law.

KEY POINTS

  • Xc = 1/(2 pi f C), measured in ohms.
  • 1,000,000 microfarads equal 1 farad.
  • Current in a capacitive circuit is I = E/Xc.

SOURCES

  • FAA-H-8083-30B ch12 "Capacitive Reactance" sample problem (Xc = 33.2 ohms, I = 3.31 amperes)
Practice the Aviation Mechanic General (AMG) test →Timed 60-question exam, by ACS area, or review your misses. Free.
MORE ON FUNDAMENTALS OF ELECTRICITY AND ELECTRONICS

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.