Question

Four students are investigating electric motors. They have created an electric motor as shown in the diagram below.
They used coils of copper wire, permanent magnets, an axle, and wires attached to the power source.
Which of the following changes should the students make if the students want to decrease the strength of the motor?
A. Increase the strength of the power source being used to operate the motor.
B. Reduce the number of coils of copper wire.
C. Change the placement of the magnets in relation to the copper wire.
D. Increase the number of coils of copper wire.

Answer 1

Correct answer: B.

Reason: The motor's turning torque depends on the magnetic field produced by the coil and the current through it. The coil's magnetic moment is proportional to the number of turns N (magnetic moment ∝ N·I·area). Reducing the number of coils lowers N, so the coil produces a weaker field and less torque, making the motor weaker. (A and D would strengthen the motor; C is ambiguous.)