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Thumb in the right-hand motor rule

Aviation Mechanic General (AMG) practice question 32 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K8 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONWhen the right-hand motor rule is applied, what does the thumb indicate?

  1. AThe direction of the magnetic field between the poles
  2. BThe direction of current flow through the conductor
  3. CThe direction the current-carrying wire moves
Show the answer and explanation

Correct answer: C. The direction the current-carrying wire moves

  • A — incorrect The index finger, not the thumb, is pointed in the direction of the magnetic field.
  • B — incorrect The second finger, not the thumb, is pointed in the direction of current flow.
  • C — correct With the index finger pointed along the magnetic field and the second finger along the current, the thumb indicates the direction the current-carrying wire moves.

BACKGROUND

Whenever a current-carrying wire is placed in the field of a magnet, a force acts on the wire at right angles both to the wire and to the magnetic field. The right-hand motor rule is used to determine the direction the wire moves. The index finger of the right hand is pointed in the direction of the magnetic field, the second finger in the direction of current flow, and the thumb then indicates the direction the current-carrying wire moves. This is the principle that produces torque in a DC motor.

STUDY TIP

Assign the fingers in order: index = field, second finger = current, thumb = motion.

KEY POINTS

  • Right-hand motor rule gives the direction a current-carrying wire moves in a magnetic field.
  • Index finger = magnetic field; second finger = current.
  • Thumb = direction of motion of the wire.

SOURCES

  • FAA-H-8083-30B ch12 "Developing Torque" (right-hand motor rule)
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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.