Alternators, Dynamos & Oscilloscopes Lesson | GCSE Physics OCR 21st Century Higher Triple

Alternators, Dynamos & Oscilloscopes

This lesson covers:

The two types of generators: alternators and dynamos
How alternators generate alternating current (a.c.)
How dynamos generate direct current (d.c.)
What a.c. and d.c. look like on an oscilloscope

Alternators and dynamos

Diagram showing electromagnetic induction with a wire moving relative to a magnetic field.

If you remember from a previous lesson, electromagnetic induction (the 'generator effect') is the idea that we generate an electric current by moving a wire relative to a magnetic field.

Diagram showing differences between alternators and dynamos, including split ring commutator and slip rings with brushes.

Alternators and dynamos are generators

We use the generator effect in devices called 'generators'. These devices generate electricity from rotational motion (e.g. rotating a coil of wire).

'Alternators' and 'dynamos' are different types of generator.

Diagram illustrating the difference between alternators and dynamos, showing split ring commutator and slip rings and brushes.

A key difference

Diagram showing the difference between a dynamo and an alternator, highlighting split ring commutator in dynamo and slip rings and brushes in alternator.

Alternators and dynamos look very similar - the key difference is that dynamos have a split ring commutator, whilst alternators have slip rings and brushes.

1Due to the split ring commutator, dynamos produce direct current.

2Due to the slip rings and brushes, alternators produce alternating current.

How alternators work

Diagram showing how the coil of wire in an alternator rotates relative to the magnets, inducing a magnetic field and producing voltage and current.

1The coil of wire rotates relative to the magnets, this induces a magnetic field in the coil, which then induces a voltage and current in the coil.

Diagram showing the coil of wire rotating relative to magnets, inducing a magnetic field and voltage.

2The slip rings and brushes mean that the contacts don't swap every half turn (like they do in a motor or dynamo).

Graph showing alternating current or voltage over time with oscillations visualised.

3This means that they produce an alternating potential difference and an alternating current (a.c.). A visualisation of the current, like the image above, is produced by an oscilloscope.

Illustration showing current or voltage oscillations in an alternator with a coil rotating slowly and quickly.

4As the coil rotates faster, the peaks of the oscillations get larger, and more frequent.

How dynamos work

Illustration showing how a dynamo works with a coil of wire spinning relative to a magnet, inducing voltage and current in the coil.

1As the coil of wire spins relative to the magnet, a magnetic field, and hence a voltage and current, is induced in the coil.

Diagram showing a dynamo coil rotating within a magnetic field created by a north and south magnet, inducing voltage and current.

2The split ring commutator means that the contacts swap every half turn.

Graph showing direct current voltage over time with constant positive peaks.

3This means that they produce a direct potential difference and hence a direct current (d.c.). It's called direct current because the current is always flowing in the same direction (which is why it's always positive on the oscilloscope graph above).

Graph showing oscilloscope traces of a dynamo with coil rotating slowly and quickly, illustrating the change in current or voltage.

4As the coil rotates faster, the peaks of the oscillations get larger and more frequent.

What does a.c. stand for?

Alternating current

What type of current is generated by a dynamo?

Alternating current

Direct current

What is the difference between the design of an alternator and the design of a dynamo?

Explain how an electromagnetic induction (generator effect) is used in dynamos.

What happens to the oscillations on an oscilloscope when you increase the speed at which a coil rotates in an alternator?

The amplitude increases, but the frequency stays the same.

The amplitude and the frequency both increase.

The amplitude and the frequency both stay the same.

The frequency increases, but the amplitude stays the same.