Explain how a dynamo generates an e.m.f.

Relative motion between a conductor and a magnetic field changes the flux linked with the conductor, and that changing flux linkage induces an e.m.f.

Why does it not matter whether the coil moves or the magnet moves in a dynamo?

Only the relative motion of the flux and the conductor matters, because it is the rate of change of flux linkage that sets the induced e.m.f.

In a rotating-coil generator, in which position is the induced e.m.f. greatest?

The e.m.f. is greatest when the plane of the coil lies along the field lines, because the sides of the coil are then cutting flux lines at the greatest rate.

In a rotating-coil generator, in which position is the induced e.m.f. zero?

The e.m.f. is zero when the plane of the coil is at right angles to the field, because the flux linkage is then at a maximum and momentarily not changing.

OCR A-Level Physics B H557

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Electromagnetism

The dynamo

3 things to know
Components 01, 02 and 03

Overview

Recall questions on how a dynamo induces an e.m.f. through relative motion, the alternating-current generator, coil positions of greatest and zero e.m.f., output frequency, ε = BLv for a sliding rod, faster rotation, soft-iron armatures, why a loaded generator is harder to turn, bicycle dynamos, commutators in direct-current dynamos, and the e.m.f. across an aircraft wing.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe the construction of a simple alternating-current generator.

Key term

How is the frequency of the output of a simple generator related to its rotation?

Numbers

A rod 0.40 m long slides at 2.5 m s⁻¹ along rails in a field of flux density 0.20 T. Calculate the e.m.f. generated.

Questions from this subtopic

Each one with the model answer underneath.

1
Explain how a dynamo generates an e.m.f.
Model answer
Relative motion between a conductor and a magnetic field changes the flux linked with the conductor, and that changing flux linkage induces an e.m.f.
2
Why does it not matter whether the coil moves or the magnet moves in a dynamo?
Model answer
Only the relative motion of the flux and the conductor matters, because it is the rate of change of flux linkage that sets the induced e.m.f.
3
Describe the construction of a simple alternating-current generator.
Model answer
A coil of many turns on a soft-iron armature is rotated between the poles of a magnet, and the ends of the coil are connected through slip rings and brushes to the external circuit.
4
In a rotating-coil generator, in which position is the induced e.m.f. greatest?
Model answer
The e.m.f. is greatest when the plane of the coil lies along the field lines, because the sides of the coil are then cutting flux lines at the greatest rate.
5
In a rotating-coil generator, in which position is the induced e.m.f. zero?
Model answer
The e.m.f. is zero when the plane of the coil is at right angles to the field, because the flux linkage is then at a maximum and momentarily not changing.
6
How is the frequency of the output of a simple generator related to its rotation?
Model answer
The output completes one full cycle for each revolution of the coil, so the frequency in hertz equals the number of revolutions per second.

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