Question

Alice builds an electric motor by positioning an electromagnet between the poles of a permanent magnet. When the electromagnet turns, the current through it switches direction because its connections to the voltage source are reversed. The repeated switching of the electromagnet's poles causes it to spin. This particular motor is used to turn the wheels of a toy car.



Which of the following actions would increase the amount of force applied to the wheels of the toy car?
A.
decreasing the electric current running through the electromagnet
B.
replacing the magnet with a stronger magnet
C.
increasing the distance between the magnet's poles and the electromagnet
D.
decreasing the number of loops in the electromagnet's coil

Answer 1

B

Explanation: The magnetic force (and torque) on the electromagnet increases with the magnetic field strength. Replacing the permanent magnet with a stronger one increases the field B and thus increases the force on the rotor. The other choices (decreasing current, increasing distance, or decreasing coil loops) each reduce the electromagnetic field or its coupling and would lower the force.