D.4.6 (HL)—Rotation frequency and emf

Read the frequency effect

For a rotating coil with fixed $N$, $B$ and $A$, increasing rotation frequency increases the rate of change of magnetic flux. The induced-emf amplitude therefore increases with frequency, while the emf waveform oscillates more rapidly.

Transform the graph

If the rotation frequency doubles, the emf period halves and the number of cycles per unit time doubles. For the same coil and field, the peak emf also doubles. If frequency is halved, period doubles and peak emf halves.

Hold variables fixed

These comparisons assume the coil area, number of turns and magnetic-field strength stay unchanged. Changing those quantities also changes the emf amplitude, so identify which parameter the question varies.

Common trap

Do not change only the period when frequency changes. For a fixed rotating coil, frequency affects both waveform frequency and peak emf through the rate of flux change.