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Effect of doubling resistance in a DC circuit

Aviation Mechanic General (AMG) practice question 18 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K6 · 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 series DC circuit has a 12-volt source and a total resistance of 40 ohms. If the total resistance is doubled to 80 ohms while the source voltage stays the same, what happens to the circuit current?

  1. AIt is reduced to half its original value, 0.15 A
  2. BIt doubles to 0.6 A because more resistance draws more current
  3. CIt stays at 0.3 A because the source voltage is unchanged
Show the answer and explanation

Correct answer: A. It is reduced to half its original value, 0.15 A

  • A — correct By Ohm's Law, I = E/R. With 12 V and 40 ohms the current is 0.3 A; with 80 ohms it is 12/80 = 0.15 A, half the original value.
  • B — incorrect Current varies inversely with resistance; adding resistance reduces current. A current of 0.6 A would result from halving the resistance to 20 ohms.
  • C — incorrect Current depends on both voltage and resistance. With the voltage fixed, changing the resistance changes the current.

BACKGROUND

The series DC circuit is the most basic electrical circuit: components are connected end to end so that the same current flows through each of them. The handbook works an example with a 12-volt source and two series resistors of 10 ohms and 30 ohms, giving a total resistance of 40 ohms and a current of 12 V / 40 ohms = 0.3 A. It then doubles the total resistance to 80 ohms, which gives 12 V / 80 ohms = 0.15 A, and halves it to 20 ohms, which gives 0.6 A. The relationship between voltage, current, and resistance described by Ohm's Law is linear: with voltage constant, doubling resistance halves current.

STUDY TIP

Practice re-solving I = E/R after a change in one variable; the test often asks for the direction and size of the change rather than a raw number.

KEY POINTS

  • In a series circuit total resistance is the sum of the individual resistances.
  • With the source voltage fixed, doubling the resistance halves the current.
  • The handbook example: 12 V across 40 ohms gives 0.3 A; across 80 ohms gives 0.15 A.

SOURCES

  • FAA-H-8083-30B ch12 "Series DC Circuits" (Figure 12-82 example: doubling and halving RT)
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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.