D.4.5 (HL)—Rotating-coil emf

Track one rotation

A coil rotating at constant angular speed in a uniform magnetic field has changing flux linkage. The rate of change is zero at the orientations where flux is maximum or minimum, and greatest when the flux passes through zero.

Read the emf waveform

Because the flux varies sinusoidally with rotation angle, the induced emf is sinusoidal. It changes sign every half-turn as the polarity reverses. The emf is zero when the rate of flux change is zero and has maximum magnitude when the rate is greatest.

Connect positions and graph

Match coil orientation to the graph before choosing a phase. A half-turn reverses the emf; a full turn repeats the waveform. The exact phase depends on the stated initial orientation and rotation axis.

Common trap

Do not put the emf maximum where flux is maximum. Faraday’s law depends on the rate of change of flux, not its value.

How it is assessed

Questions match a rotation axis/orientation to an emf-time graph or identify the phase of the generated emf.

Command terms

What rotation / Draw

What earns marks

Relate coil orientation to flux and its rate of change, then identify the sinusoidal emf phase, sign reversal and period.

Watch for

Putting emf peaks at maximum flux, or changing phase without tracking the stated initial orientation.

Representative question

[Maximum number: 1]

A rectangular coil rotates at a constant angular velocity. At the instant shown, the plane of the coil is at right angles to the line $Z Z^{\prime}$ . A uniform magnetic field acts in the direction $Y Y^{\prime}$ . The variation of emf with time t is shown.

What coil rotation about the axis specified produces this graph?

Through $\frac{\pi}{2}$ about $X X^{\prime}$

Through $\pi$ about $X X^{\prime}$

Through $\frac{\pi}{2}$ about $Y Y^{\prime}$

Through $\pi$ about $Y Y^{\prime}$

D.4.5 (HL)—Rotating-coil emf

Model Rotating-Coil Emf

D.4.5 (HL) Exam Analysis

Assessment in practice

Question 1