In a simple a.c. generator, what is the orientation of the rotating coil relative to the magnetic field?

The coil rotates between the poles of a magnet so that it cuts through the magnetic field lines.

How does the flux linkage through the coil of an a.c. generator vary with time?

It varies sinusoidally: NΦ = NBA cos ωt, where ω is the angular velocity.

Why is the e.m.f. produced by a simple a.c. generator sinusoidal?

Because the rate of change of the sinusoidally varying flux linkage produces a sinusoidal e.m.f.

How does the changing current in the primary coil of a transformer induce an e.m.f. in the secondary?

The changing current produces a changing magnetic flux in the core, which links the secondary coil and induces an e.m.f. by electromagnetic induction.

What is the purpose of the soft iron core in a transformer?

It links both coils magnetically, providing a low-reluctance path for the changing magnetic flux.

What is the function of slip rings and brushes in an a.c. generator?

They transfer the alternating current from the rotating coil to the external circuit while allowing continuous rotation.

OCR A-Level Physics A H556

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Electromagnetism

Transformers and the a.c. generator

3 things to know
Papers 1, 2 and 3

Overview

Recall questions on how an a.c. generator produces a sinusoidal e.m.f., slip rings and brushes, how a transformer works, its core, the turns-ratio equations with calculations, and investigating transformers.

What you need to know

Answer each one before you reveal it.

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Key term

In a simple transformer, what is connected to the primary coil?

Numbers

For an ideal transformer, state the relationship between input and output power.

Key term

State the transformer equation relating voltages and turns for an ideal transformer.

Questions from this subtopic

Each one with the model answer underneath.

1
In a simple a.c. generator, what is the orientation of the rotating coil relative to the magnetic field?
Model answer
The coil rotates between the poles of a magnet so that it cuts through the magnetic field lines.
2
How does the flux linkage through the coil of an a.c. generator vary with time?
Model answer
It varies sinusoidally: NΦ = NBA cos ωt, where ω is the angular velocity.
3
Why is the e.m.f. produced by a simple a.c. generator sinusoidal?
Model answer
Because the rate of change of the sinusoidally varying flux linkage produces a sinusoidal e.m.f.
4
How does the changing current in the primary coil of a transformer induce an e.m.f. in the secondary?
Model answer
The changing current produces a changing magnetic flux in the core, which links the secondary coil and induces an e.m.f. by electromagnetic induction.
5
What is the purpose of the soft iron core in a transformer?
Model answer
It links both coils magnetically, providing a low-reluctance path for the changing magnetic flux.
6
What is the function of slip rings and brushes in an a.c. generator?
Model answer
They transfer the alternating current from the rotating coil to the external circuit while allowing continuous rotation.

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