How does an AC generator work?

• A. A simple ac generator consists of a coil rotated in a magnetic field. As the coil rotates the magnetic flux through it changes continuously, inducing a voltage that reverses sign every half rotation, producing AC.
• B. A stationary coil with a rotating capacitor causing alternating current.
• C. A coil rotated in a static electric field yields DC.
• D. A magnet moves around a stationary coil producing alternating current via rectification.

Answer: (A)

Rotating a coil inside a magnetic field causes the magnetic flux linking the coil to change over time. By electromagnetic induction, a voltage is induced in a conductor when the magnetic flux through it varies. As the rotor turns, the angle between the coil and the field changes smoothly, so the flux follows a cyclic pattern (roughly cosine in time). The rate of flux change is highest as the coil moves away from alignment and zero when it passes through the field’s axis, so the induced voltage swings in polarity and magnitude in a sinusoidal way. Because the polarity reverses every half turn, the current in the external circuit becomes alternating current.

The output amplitude depends on how many turns the coil has, the magnetic field strength, the coil area, and how fast the rotor spins. In short, changing magnetic flux in a rotating coil is the mechanism that creates an alternating voltage and thus an AC supply.

Other ideas don’t fit because they misrepresent how induction works: a rotating capacitor doesn’t drive a steady, looping current through a coil, a coil in a static field would not produce a changing flux and thus not an AC, and moving a magnet with rectification would mix generation with a conversion step that isn’t how AC is produced.