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.
so the magnetic field will get stronger
B.
to conserve the energy in the battery
C.
so the electromagnet will continuously turn
D.
to constantly polarize the field magnet

Answer 1

C. so the electromagnet will continuously turn

Explanation: A current-carrying coil in a magnetic field feels torque that tends to align its poles with the external magnet. If the coil's polarity stayed fixed, once it rotated 180° it would reach an equilibrium and stop (or oscillate). By reversing the coil current every half turn (using a commutator or electronic switching), the forces remain in the same rotational direction, producing continuous torque and continuous rotation.

Why the other choices are wrong:
- A: Flipping polarity does not make the magnetic field stronger.
- B: The flip is about producing continuous motion, not conserving battery energy.
- D: The permanent (field) magnet is already magnetized; the flip is of the rotor electromagnet, not to "constantly polarize" the field magnet.