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Capacitor charge after the source is removed

Aviation Mechanic General (AMG) practice question 15 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.R3 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA capacitor in a circuit has been charged from a DC source, and the switch is then opened, removing the source. According to the handbook, what is the condition of the capacitor plates?

  1. AThey discharge immediately once the switch is opened
  2. BThey discharge slowly through the open switch contacts
  3. CThey remain charged until the capacitor is shorted
Show the answer and explanation

Correct answer: C. They remain charged until the capacitor is shorted

  • A — incorrect Opening the switch removes the charging path but does not discharge the capacitor; the plates keep their charge.
  • B — incorrect An open switch provides no path for current; discharge occurs only when a path such as a short is provided across the capacitor.
  • C — correct The handbook states that if the switch is opened the plates remain charged, and once the capacitor is shorted it discharges quickly.

BACKGROUND

A capacitor consists of two plates separated by a dielectric and serves as a reservoir for electricity. When it is connected across a DC source and the switch is closed, current flows until the voltage between the plates equals the source voltage, then stops. If the switch is then opened, the plates remain charged; once the capacitor is shorted, it discharges quickly. This is why a charged capacitor in a de-energized circuit can still be hazardous, and why the handbook's procedure for testing a capacitor with an ohmmeter begins by shorting the two leads of the capacitor to ensure it is entirely discharged. The voltage rating of a capacitor specifies the maximum DC voltage that can be applied without risk of damage.

STUDY TIP

Treat any capacitor in a circuit as charged until you have deliberately discharged it, even after power is removed.

KEY POINTS

  • Opening the switch leaves the capacitor plates charged.
  • The capacitor discharges quickly only when it is shorted.
  • Before testing a capacitor with an ohmmeter, short its leads to discharge it.

SOURCES

  • FAA-H-8083-30B ch12 "Capacitors in Direct Current" (Figure 12-114 B and C)
  • FAA-H-8083-30B ch12 "Capacitance Measurement"
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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.