Question

An electromagnet is a device that uses an electric current to produce a magnetic field.

Electromagnets are typically composed of a piece of metal wrapped with wire and connected to a battery. The battery provides the electric current, and the loops or coils of wire enhance the magnetic field. The more loops there are, the stronger the magnetic field.

When electric current flows through the electromagnet, the metal core becomes magnetized, so it possesses a north pole and a south pole.

Motors place electromagnets inside of permanent magnets as shown below.



However, motors are designed so that the polarity of the electromagnet constantly flips. That is, the north pole becomes the south pole, and the south pole becomes the north pole, every time the electromagnet turns 180 degrees.

Why is it designed this way?
A.
to constantly polarize the field magnet
B.
so the magnetic field will get stronger
C.
so the electromagnet will continuously turn
D.
to conserve the energy in the battery

Answer 1

C. so the electromagnet will continuously turn.

Explanation: As the coil turns in the fixed magnetic field, without flipping the coil's polarity the forces would reverse or cancel every half turn and the rotor would stop (or oscillate). Reversing the current (via a commutator or electronic switching) flips the rotor poles at 180°, so the magnetic attraction/repulsion with the stator always produces torque in the same rotational direction, giving continuous rotation.