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Unknown voltage drop by Kirchhoff's Voltage Law

Aviation Mechanic General (AMG) practice question 30 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K7c · 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 has a 24-volt source and three resistors. Two of the voltage drops are 12 volts and 10 volts. Using Kirchhoff's Voltage Law, what is the third voltage drop?

  1. A22 volts
  2. B2 volts
  3. C46 volts, the source plus the two known drops
Show the answer and explanation

Correct answer: B. 2 volts

  • A — incorrect 22 volts is the sum of the two known drops, not the remaining drop; the remaining drop is what is left of the 24-volt source.
  • B — correct The source voltage equals the sum of the drops: 24 - 12 - 10 - E3 = 0, so E3 = 2 volts.
  • C — incorrect Adding the drops to the source is wrong; the drops are subtracted from the source because their sign is opposite to the source in the loop equation.

BACKGROUND

Kirchhoff's Voltage Law states that the algebraic sum of all voltages around a closed path or loop is zero. Another way of saying it: the sum of all the voltage drops equals the total source voltage. In the loop equation the sign of the source is opposite that of the individual voltage drops, which is why the sum comes to zero. The handbook example with a 24-volt source and known drops of 12 and 10 volts solves 24 - 12 - 10 - E3 = 0 to find E3 = 2 volts.

STUDY TIP

Write the loop equation with the source positive and each drop negative, then solve for the single unknown.

KEY POINTS

  • Algebraic sum of voltages around a closed loop is zero.
  • Sum of the drops equals the source voltage.
  • 24 V source with 12 V and 10 V drops leaves 2 V for the third resistor.

SOURCES

  • FAA-H-8083-30B ch12 "Kirchhoff's Voltage Law" (24-volt source, three-resistor worked 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.