20.5 AC Generators and Alternating Current

An AC generator rotates a coil in a magnetic field so the angle between coil normal and field changes continuously. That makes flux through each turn vary sinusoidally in time, so by Faraday's law induced emf is also sinusoidal.

Because angular speed is omega = 2pi f, doubling rotation frequency doubles peak voltage in the same machine. This direct scaling appears constantly in exam data questions where frequency changes but coil geometry stays fixed.

For a purely resistive load, i(t) is in phase with v(t): current reaches maxima and zero crossings at the same times as voltage. Instantaneous power is therefore p(t) = v(t)i(t), always non-negative for this ideal resistive case, with frequency double that of voltage waveform peaks.

This chapter keeps phase treatment at resistive-load level. More advanced phase-lag and reactance effects appear when full AC circuit analysis includes capacitors and inductors together. The generator relationships here remain the starting point for those extensions.

Alternating current means charge carriers oscillate back and forth around local positions while energy transfer still proceeds through fields and circuit interaction. You do not need net one-way electron travel around an entire grid to deliver useful power to loads.