12 min readPathway sheetSection A passage (5 marks) and MCQ (1 mark) on slip rings and the commutator; Section B long answer (5 marks), often with a choice: the construction and working of an AC generator with a labelled diagram

AC and DC generators; alternating and direct current compared

A generator turns a coil in a magnetic field to change mechanical energy into electrical energy. Slip rings give AC; a split ring (commutator) gives DC.

PrincipleElectromagnetic induction

Energy changeMechanical energy into electrical energy

AC or DCTwo slip rings give AC; a split ring (commutator) gives DC

AC supply in India50 Hz: 50 cycles a second, so the current changes direction 100 times a second

Parts of an AC generator

Part	What it does
Strong magnet (N and S poles)	Gives the magnetic field in which the coil turns
Armature coil ABCD	A rectangular coil of many turns of insulated copper wire on a frame. It is turned by a turbine, an engine or by hand
Two slip rings (E and F)	Full rings fixed to the axle; each end of the coil is joined to one ring, and they turn with it
Two carbon brushes (G and H)	Press lightly on the slip rings and carry the current out to the outer circuit
Outer circuit	A galvanometer, bulb or appliance that uses the current

How an AC generator works

1
Mechanical energy turns the coil ABCD between the poles of the magnet.
2
As it turns, side AB moves down through the field while side CD moves up. The field through the coil keeps changing, so a current is induced.
3
The currents in AB and CD flow the same way round the coil, so they add up.
4
After half a turn, AB and CD swap places. Each side now moves the opposite way through the field, so the current reverses.
5
The current is largest when the plane of the coil is along the field lines, and zero when it is at right angles to them.
6
Each slip ring always touches the same brush, so the reversing current reaches the outer circuit unchanged.
7★
So the current changes direction twice in every turn: it is alternating current (AC). Its direction can be found with Fleming's right-hand rule.

★ key step

AC generator vs DC generator

AC generatorThe coil ends are joined to two full slip rings. Each ring keeps touching the same brush, so the current in the outer circuit reverses every half turn: AC.

DC generatorThe coil ends are joined to one split ring made of two half rings, called a commutator. Every half turn the halves swap brushes, just as the current in the coil reverses. So the current in the outer circuit always flows one way: DC.

The size of the voltage from either generator rises with more turns in the coil, a stronger field and faster turning. The frequency of AC depends on how fast the coil turns.

Alternating current vs direct current

Point	AC	DC
Direction	Changes direction again and again, in a regular pattern	Flows in one direction only
Source	Power station generators; the sockets at home	Cells and batteries; DC generators
Changing the voltage	Easily stepped up or down with transformers	Transformers do not work with DC
Sending over long distances	Sent at very high voltage with little loss of energy	Large loss of energy over long lines
Typical uses	Homes, factories, motors of fans and pumps	Electrolysis and electroplating, charging batteries, electromagnets
Changing one into the other	A rectifier changes AC into DC	An inverter changes DC into AC

How often does AC change direction?

Problem

The AC supply in India has a frequency of 50 Hz. How many times does the current change direction in one second, and how long does one cycle take?

Working

50 Hz means 50 complete cycles in 1 second.
In each cycle the current flows one way and then the other way, so it changes direction 2 times.
Changes of direction per second = 50 × 2 = 100.
Time for one cycle = 1 ÷ 50 s = 0.02 s.

Answer

100 times a second; one cycle takes 0.02 s

In real life

Hydro, thermal and nuclear power stations spin huge AC generators with turbines. A phone charger contains a rectifier that turns the AC from the socket into the DC the battery needs.

Exam angle

1 marks: “Name the part of a DC generator that makes the current flow in one direction in the outer circuit.” The split ring, called the commutator.

2 marks: “Why is electricity sent from power stations as AC and not as DC? Give two reasons.” AC voltage can be stepped up and down easily with transformers, which do not work with DC. So AC can be sent at very high voltage over long distances with little loss of energy.

5 marks: “Explain, with a labelled diagram, how an AC generator produces a current that reverses every half turn. How is a DC generator different?” Principle: electromagnetic induction. Parts: magnet, armature coil ABCD, slip rings, brushes. Turning coil cuts the field; induced current; after half a turn the sides swap and the current reverses; slip rings pass it out as AC. A DC generator uses a split ring commutator, so the outer current flows one way.

Remember it

Slip rings: swinging current (AC). Split ring: single direction (DC). India's AC: 50 cycles, 100 reversals a second.