OCR GCSE Physics A: Electromagnetic induction, alternators and dynamos (J249)

This guide is aligned to the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), for first assessment in 2018. It covers statements P4.2e and P4.2f from topic P4.2, Uses of magnetism. Both statements are in bold in the specification, so they are tested in the Higher Tier papers only. Use it with the revision notes and the practice questions. The course hub is at OCR GCSE Physics, the printable checklist lists every statement, and the free diagnostics show you where to start. For the field patterns this page relies on, see magnets, magnetic fields and electromagnets.

What this unit covers

Statement	What you must be able to do	Tier
P4.2e	Recall that a change in the magnetic field around a conductor can give rise to an induced potential difference across its ends, which could drive a current, generating a magnetic field that would oppose the original change	Higher Tier only
P4.2f	Explain how this effect is used in an alternator to generate a.c., and in a dynamo to generate d.c.	Higher Tier only

Motors and the force on a current-carrying wire, and transformers, microphones and loudspeakers, are separate statements in the same topic. They are not repeated here.

Induced potential difference (P4.2e, Higher Tier only)

A conductor sitting in a magnetic field does nothing by itself. A potential difference appears across its ends only when the magnetic field around it changes. This is called electromagnetic induction, and the p.d. is called an induced potential difference.

The field around a conductor can change in two common ways.

• The conductor moves across the field lines of a magnet, so that it cuts through them.
• The magnet moves, or the field is switched on, off or reversed, so that the field through a stationary coil changes.

If the conductor is part of a complete circuit, the induced p.d. drives a current. If the circuit is open, you get the p.d. but no current.

What you can observe

Connect a coil to the galvanometer and push a bar magnet into it.

Action	Reading on the galvanometer
Magnet held still inside the coil	Zero
Magnet pushed in	Deflection one way
Magnet pulled out	Deflection the other way
Magnet pushed in faster	Bigger deflection
Magnet turned round (other pole leads) and pushed in	Deflection reversed
More turns in the coil, or a stronger magnet	Bigger deflection

The key idea is that the reading depends on how fast the field is changing, not on how strong the field is at that moment. A magnet held still inside the coil is a strong field, but it is not changing, so there is no induced p.d.

The induced current opposes the change

Now the part that is easy to forget. When an induced current flows, it generates its own magnetic field. The direction of that field is always such that it opposes the change that caused it. This is the content of Lenz’s law, although the specification asks you to recall the effect rather than to name the law.

Take a coil connected into a complete circuit, and push the north pole of a magnet towards it.

1. The field through the coil is increasing.
2. The induced current flows in the direction that makes the nearest end of the coil a north pole.
3. Two north poles repel, so the coil pushes back against the magnet.
4. You have to do work to keep pushing, and that work is the energy that ends up in the circuit. If the induced field helped the change instead, energy would appear from nothing.

Now pull the magnet back out. The field through the coil is decreasing, so the current reverses. The end of the coil nearest the magnet becomes a south pole, which attracts the departing north pole. Again the effect resists what you are doing.

Worked example 1: direction of the opposing field

A south pole of a bar magnet is pushed towards the left-hand end of a closed coil. State the polarity that the induced current gives the left-hand end of the coil, and explain what happens when the magnet is pulled away again.

1. Pushing the south pole closer increases the field through the coil.
2. The induced current opposes this by making the left-hand end a south pole, which repels the approaching south pole.
3. Pulling the magnet away decreases the field. The induced current reverses and makes the left-hand end a north pole, which attracts the south pole and resists its removal.

The polarity is south while approaching, north while leaving.

Worked example 2: comparing readings

A coil is connected to a voltmeter. A magnet is pushed into it in 2.0 s, then pushed in again from the same start in 0.5 s. Compare the two readings.

The total change in field is the same each time, but in the second push it happens four times faster (2.0 ÷ 0.5 = 4). The change is faster, so the induced p.d. is bigger. It lasts for a shorter time. The reading is in the same direction both times because the magnet moves the same way.

The alternator (P4.2f, Higher Tier only)

An alternator uses induction to turn rotation into an alternating current. A simple model has a rectangular coil that spins between the opposite poles of a magnet. A turbine, or in a small demonstration a hand crank, supplies the rotation.

The coil is connected to the outside circuit through slip rings, two complete metal rings fixed to the shaft, one connected to each end of the coil. Fixed carbon brushes press against the rings and carry the current to the circuit. Because each ring is unbroken, each brush always stays connected to the same end of the coil.

Why the output alternates

As the coil turns, its sides cut through the field lines of the magnet. The side moving up through the field has an induced p.d. in one direction. Half a turn later that same side is moving down through the field, so the p.d. across the coil is reversed. The slip rings do nothing to change this, so the output reverses direction twice for every complete turn. That is alternating current, a.c.

The size of the p.d. also changes through each turn.

Coil position	Sides cutting field lines?	Induced p.d.
Plane of the coil along the field lines	Yes, at the fastest rate	Maximum
Plane of the coil at right angles to the field lines	No, the sides move along the lines	Zero
Past the zero position	Yes, in the opposite sense	Maximum in the opposite direction

On an oscilloscope the trace rises to a peak, falls through zero, reaches a negative peak and returns. One complete up-and-down pattern is one cycle, which matches one complete rotation of the coil.

Frequency and speed

For the simple model above, one rotation of the coil gives one cycle. So the frequency of the output equals the number of rotations per second. The period is the time for one cycle, and frequency = 1 ÷ period.

Two changes follow when you spin the coil faster.

• The sides cut field lines faster, so the peak p.d. increases. In the simple model it is proportional to the rotation speed.
• Each cycle takes less time, so the frequency increases.

A stronger magnet or more turns on the coil also increases the peak p.d. but does not change the frequency.

Worked example 3: reading a trace

An oscilloscope shows the output of an alternator. The time-base is set to 4.0 ms per division and the trace covers 10 divisions horizontally, showing exactly 5 complete cycles. The vertical scale is 2.0 V per division and the peak is 2.5 divisions above the centre line.

1. Total time shown = 10 × 4.0 ms = 40 ms.
2. Period = 40 ms ÷ 5 = 8.0 ms = 0.0080 s.
3. Frequency = 1 ÷ 0.0080 = 125 Hz.
4. Peak p.d. = 2.5 × 2.0 V = 5.0 V.

Worked example 4: changing the speed

A small alternator turns at 1500 revolutions per minute and produces a peak p.d. of 6.0 V. Find the frequency and period. Then predict both quantities when it is turned at 3000 revolutions per minute.

1. 1500 revolutions per minute = 1500 ÷ 60 = 25 revolutions per second, so the frequency is 25 Hz.
2. Period = 1 ÷ 25 = 0.040 s.
3. At 3000 revolutions per minute the speed has doubled, giving 50 Hz and a period of 0.020 s.
4. The peak p.d. also doubles in the simple model, to 12 V.

The trace would show twice as many cycles in the same time, each twice as tall.

The dynamo (P4.2f, Higher Tier only)

A dynamo has the same basic parts as the alternator: a coil, a magnetic field and rotation. The difference is how the coil is connected to the outside circuit. Instead of two slip rings it has a split-ring commutator, one ring cut into two halves with a gap between them, each half joined to one end of the coil. Brushes touch the halves.

Inside the coil the induced p.d. still reverses every half turn, exactly as in the alternator. At the moment the p.d. would reverse, the gaps in the ring pass under the brushes. Each brush swaps to the other half-ring, which is now connected to the other end of the coil. The swap cancels the reversal, so the current in the external circuit always flows the same way. That is direct current, d.c.

The output is not steady. It rises from zero to a maximum, falls back to zero and then repeats in the same direction. Each half turn of the coil produces one pulse, so there are two pulses for every rotation.

The wind-up radios and torches suggested in the specification contain a generator of this kind: cranking the handle turns a coil in a magnetic field, and the induced p.d. lights the bulb or powers the radio.

Alternator and dynamo compared

Feature	Alternator	Dynamo
Output	a.c., reverses direction each half turn	d.c., always the same direction
Connection to the coil	Two slip rings	Split-ring commutator
Pulses or cycles per rotation	One full cycle	Two pulses

Do not confuse a dynamo with a motor. A motor is supplied with a current and produces rotation. A dynamo is supplied with rotation and produces a potential difference. Both have a coil in a magnetic field, so the layouts look similar.

Worked example 5: pulses from a dynamo

A dynamo turns at 12 revolutions per second. How many pulses of p.d. does it produce each second, and how long is there between the start of one pulse and the start of the next?

1. Two pulses per rotation, so 2 × 12 = 24 pulses per second.
2. Time between pulses = 1 ÷ 24 = 0.042 s (to 2 significant figures).

Common errors

• Writing that a magnet inside a coil induces a p.d. because the field is strong. Only a changing field counts.
• Saying the induced field is in the same direction as the magnet’s field. It opposes the change, and that means a south pole opposite the approaching south pole and so on.
• Saying the coil in an alternator “changes direction” because the brushes switch. The brushes do not switch in an alternator. The reversal comes from the sides of the coil moving through the field in opposite directions.
• Writing that the commutator “makes the current direct” without saying it swaps the connections every half turn.
• Mixing up the zero positions. The p.d. is zero when the sides of the coil are moving along the field lines, not when the coil is moving fastest.

Official syllabus

This page is based on the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), for first assessment in 2018, published by OCR, topic P4.2 Uses of magnetism, statements P4.2e and P4.2f.