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Total resistance of a parallel combination

Aviation Mechanic General (AMG) practice question 21 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K11d · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONHow does the total resistance of a parallel combination compare with the individual resistors in it?

  1. AIt is always equal to the sum of the resistors
  2. BIt is always less than the smallest resistor
  3. CIt is always greater than the largest resistor in the group
Show the answer and explanation

Correct answer: B. It is always less than the smallest resistor

  • A — incorrect Summing the resistors gives the total for a series circuit, where current must pass through each resistor in turn.
  • B — correct Adding parallel paths reduces total resistance, so the total is always less than the value of the smallest resistor in the combination.
  • C — incorrect Parallel branches provide additional paths for current, which lowers rather than raises the total resistance.

BACKGROUND

In a parallel circuit two or more resistors are connected across the same voltage source, giving current several paths. Because current can pass through all the resistors at once, the total resistance of the combination is always less than the value of the smallest resistor in the circuit. The handbook illustrates this with a 100-ohm and a 5-ohm resistor in parallel: the combined resistance is less than 5 ohms. The general formula is 1/RT = 1/R1 + 1/R2 + ... , and for two resistors RT = (R1 x R2)/(R1 + R2).

STUDY TIP

Before calculating, sanity-check a parallel answer: it must be smaller than the smallest branch.

KEY POINTS

  • Parallel total resistance is always less than the smallest branch resistor.
  • Series total is the sum; parallel total uses reciprocals.
  • Two resistors in parallel: product over sum.

SOURCES

  • FAA-H-8083-30B ch12 "Total Parallel Resistance"; "Resistors in Parallel"
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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.