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RL circuit current after one time constant

Aviation Mechanic General (AMG) practice question 17 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K4 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a series resistor-inductor (RL) circuit, to what percentage of its full value does the current rise in one time constant after the circuit is closed?

  1. A63 percent
  2. B50 percent, one-half of full value
  3. C99 percent
Show the answer and explanation

Correct answer: A. 63 percent

  • A — correct The handbook states that in a series RL circuit the current increases to 63 percent of its full value in one time constant after the circuit is closed.
  • B — incorrect The current buildup follows an exponential curve, and the value reached after one time constant is 63 percent, not one-half.
  • C — incorrect About 99 percent of full value is reached after the fifth time constant, not the first.

BACKGROUND

An inductor opposes any change in the current through it, so current in an inductive circuit cannot change instantaneously. The rate of change is set by the time constant, represented by the Greek letter tau, which for an RL circuit equals inductance in henries divided by resistance in ohms (tau = L/R). In a series RL circuit the current rises to 63 percent of its full value in one time constant after the circuit is closed. The buildup follows an exponential curve, similar to the voltage buildup in a charging RC circuit, and reaches about 99 percent after the fifth time constant.

STUDY TIP

Remember the pair 63 percent at one time constant and 99 percent at five time constants; it applies to both RL and RC circuits.

KEY POINTS

  • The RL time constant is tau = L/R, in seconds.
  • Current reaches 63 percent of its final value after one time constant.
  • Current reaches about 99 percent after five time constants.

SOURCES

  • FAA-H-8083-30B ch12 "Inductance" > "The RL Time Constant"
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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.