Uses of the generator effect; microphones (HT, physics only)

Module 7 · Lesson 7.06 · Spec 4.7.3.2 + 4.7.3.3

Uses of the Generator Effect · Microphones

Higher Tier only

Physics only

Three devices, one principle. Alternator → AC. Dynamo → DC. Microphone → electrical signal from sound. All four use the generator effect from Lesson 7.05.

From Principle to Devices

In Lesson 7.05 you saw the generator effect: motion through a magnetic field → induced pd. This lesson applies that to three real devices.

Alternator

In: rotation (motion)

Out: alternating current (AC)

Key part: slip rings

Dynamo

In: rotation (motion)

Out: direct current (DC)

Key part: split-ring commutator

Microphone

In: sound waves

Out: alternating electrical signal

Key part: diaphragm + coil

By end of lesson

Explain how an alternator generates AC and a dynamo generates DC · sketch and interpret pd–time graphs for both · explain how a moving-coil microphone works · apply the generator-effect principle (including "easier to turn when disconnected").

Alternator · Apparatus

A coil rotates between the poles of a magnet. The coil's two ends are each connected to a slip ring — a continuous metal ring that rotates with the coil. Two carbon brushes press against the rings, transferring current to the external circuit.

Coil rotates between magnet poles. Each end is connected to its own continuous slip ring. Brushes press onto the rings → AC flows to the external circuit unchanged.

Alternator · How It Produces AC

The mechanism, step by step:

1

The coil rotates between the magnet poles — its sides move through the magnetic field.

2

As the coil rotates, the sides cut magnetic field lines → a pd is induced in the coil (generator effect).

3

Each half-turn , the same side of the coil moves in the opposite direction relative to the field → the induced pd reverses direction .

4

The slip rings stay in continuous contact with the brushes throughout the rotation, so the alternating pd is passed directly to the external circuit.

5

Result: an alternating current (AC) flows through the external circuit.

Role of slip rings

Slip rings are continuous (not split). Each ring stays connected to one end of the coil all the time. This means the natural alternating pd from the rotating coil passes unchanged to the external circuit — AC out.

Exam alert — Nov 2020 Q7.4

Only 9% of students correctly stated the function of slip rings. Many gave vague answers like "to connect the coil" or "to keep it spinning". The accepted answer: slip rings provide a continuous (moveable) electrical contact between the rotating coil and the external circuit / brushes, or alternatively, they stop the wires from twisting together as the coil rotates.

Alternator Output · Sinusoidal AC

For an alternator, the pd–time graph is a smooth sine wave, alternating between positive and negative.

Alternator output: smooth, continuous sine wave alternating between positive and negative each half-turn.

Dynamo · Apparatus

Same rotating coil. Same magnet. Different output device. The coil's two ends are connected to a split-ring commutator — a single metal ring with a gap (split) on each side, dividing it into two half-rings. Each brush touches one half at a time.

Same coil and magnet as the alternator — but a single split-ring (cut into two halves) replaces the slip rings. Each brush touches one half-ring at a time → DC output.

Dynamo · How the Commutator Gives DC

The trick is the split-ring: every half-turn, the two halves of the commutator swap which brush they touch.

1

The coil rotates and cuts field lines → an alternating pd is induced in the coil itself.

2

But every half-turn , the commutator halves swap brushes. So the brush that was getting positive pd now gets the other end of the coil — which is also positive (because the pd in the coil has just reversed).

3

Result: the pd in the external circuit is always in the same direction — direct current (DC).

4

The current is not steady — it pulses up and down between zero and a maximum — but it never goes negative.

Mark-scheme phrasing (Jun 2023 Q9.1)

"Each half-revolution, the two halves of the commutator switch brushes / contacts · so the direction of the (induced) current does not reverse every half rotation · because the half of the coil connected to each brush always moves in the same direction."

Dynamo Output · Bumpy DC

For a dynamo, the pd–time graph is the absolute value of a sine wave. The negative half of each cycle is "flipped up" into positive territory by the commutator. The result: a series of positive humps, all on one side of the time axis.

Dynamo output: four positive half-cycles per two complete revolutions. The pd never goes negative — always one direction. Same frequency of humps, but no sign change.

Exam alert — Jun 2023 Q9.2

Only 17% of students drew this graph correctly. Most drew a full sine wave (alternator output) or wrong number of half-cycles. For 2 complete revolutions you need 4 half-cycle humps, all positive.

Alternator vs Dynamo · Side by Side

AlternatorDynamo

Coil + magnetIdenticalIdentical

Output deviceTwo slip rings (continuous)One split-ring commutator (two halves)

Brushes do whatStay in contact with the same ring throughoutSwap which half-ring they touch every half-turn

Output typeAC — alternating currentDC — direct current (pulsing, one direction)

Output graphSmooth sine wave (positive AND negative)Series of positive humps only

Memory cue

The pd in the coil itself is always alternating (because the coil rotates). The output device decides whether the external circuit sees AC or DC. Slip rings → AC out. Commutator → DC out.

Microphone · Apparatus

A microphone converts sound waves into an electrical signal using the generator effect.

Sound waves vibrate the diaphragm → attached coil oscillates in the magnet's field → an alternating pd is induced in the coil (generator effect).

Microphone · How It Converts Sound to Electricity

Mark-scheme answer in three steps (Jun 2024 Q8.4 phrasing):

1

The pressure variations in the sound waves cause the diaphragm to vibrate . (Don't just say "moves the diaphragm" — that's insufficient per the mark scheme.)

2

The vibrating diaphragm causes the coil (or wire) to vibrate too — the coil is attached to the diaphragm.

3

The coil moves back and forth in the magnetic field of the permanent magnet → it cuts magnetic field lines → an alternating pd (and current, in a circuit) is induced in the coil.

Key feature · frequency matching

The frequency and amplitude of the induced electrical signal match those of the sound wave. The electrical signal is a faithful reproduction of the original sound — it can then be amplified, recorded, or transmitted.

Microphone vs Loudspeaker

A microphone uses the generator effect: sound → electrical signal. A loudspeaker uses the motor effect: electrical signal → sound. Same components (coil + magnet + diaphragm/cone), opposite direction of conversion.

Stock 1-mark fill-in (Jun 2023 Q4.1 pattern)

"The microphone is used to convert the pressure variations in sound waves into variations in ___ ." Mark scheme accepts current, potential difference, or charge flow. The spec wording is "variations in current" — but the MS allows the other two. Don't write "sound", "energy", or "vibrations".

Worked Example · Sketch the Dynamo Output

Question1 mark

Sketch a graph on the axes provided to show how the potential difference generated across the lamp varies for two complete revolutions of the dynamo coil.

Step 1

The output is DC (positive only) — never goes below the time axis.

Step 2

Each complete revolution produces two half-cycles (two humps), because the pd reaches zero and rebuilds twice per turn.

Step 3

Two complete revolutions therefore give 4 positive half-cycles — four humps, all on the positive side of the time axis.

⚠ Common error

Drawing a full sine wave (alternator output) instead of half-cycles → 0 marks. Drawing 2 humps instead of 4 → 0 marks. Drawing humps that go below zero → 0 marks. 4 humps, all positive — exactly.

Worked Example · 4-Mark Microphone Explain

Question4 marks

A moving-coil microphone has a diaphragm attached to a coil that sits inside the field of a permanent magnet. Explain how the microphone converts sound waves into an alternating electrical signal.

Mark 1

The pressure variations in the sound waves cause the diaphragm to vibrate.

Mark 2

The vibrating diaphragm causes the coil to vibrate too (the coil is attached to the diaphragm).

Mark 3

The coil moves through the magnetic field, cutting magnetic field lines → an induced pd is produced (generator effect).

Mark 4

The coil repeatedly changes direction (vibrating), so the induced pd alternates → an alternating current is induced in the circuit.

⚠ MS rules

"Diaphragm moves" alone is insufficient. "Moves the diaphragm up and down" / "moves the coil up and down" — both do not accept. Use vibrate, and link sound waves to diaphragm to coil to induced pd, in that order.

Examiner Alert · "Why Easier to Turn?"

The hardest application question on the generator effect at GCSE. Asked twice (Jun 2020, Jun 2023). Both times, performance was extremely low.

Question stem

"The lamp is disconnected from the dynamo. Explain why the dynamo handle becomes much easier to turn."3 marks

15% scored ≥ 1 mark (Jun 2023 Q9.3)

3%scored full marks (Jun 2023 Q9.3)

Mark-scheme answer (3 marks)

After disconnection, there is no (induced) current in the coil.

So no magnetic field is produced around the coil.

So there is no force opposing the movement of the coil (Lenz's law force is absent).

A small number of students were able to write that without a current in the coil, there would be no magnetic field in the coil; fewer were able to state that this would remove the opposing force. — AQA Examiner Report, Jun 2023 Paper 2 Q9.3

Examiner Alert · Dynamo ≠ Motor

The most-failed extended-answer topic on Paper 2 across multiple years. Students consistently confuse the dynamo with the motor — describing one when asked about the other.

2%

scored 5/5 on dynamo explain (Jun 2023 Q9.1)

75%

scored ≥ 1 mark

10%

any marks on Jun 2022 Q7.3 (alternator)

Most started the explanation by stating that a current flows through the coil, going on to explain how an electric motor works… The vast majority did not show any knowledge of how an alternator works. — AQA Examiner Report, Nov 2020 Paper 2 Q7.3

Pattern-match rule

Read the stem carefully for the direction of conversion:

Motor: "a current flows…", "explain how it rotates" → electricity → motion

Dynamo / alternator: "the coil is turned…", "explain how a pd is generated" → motion → electricity

Weak vs Strong · Dynamo Explain

Weak · 1/5

"A current flows through the coil, and the coil spins between the magnets, and the commutator keeps the motor running. The lamp lights up."

Loses marks: describes a motor (current in → motion out), not a dynamo (motion in → current out). The vague "commutator keeps the motor running" doesn't explain the half-turn switching. No mention of cutting field lines or induced pd.

Strong · 5/5

"The coil rotates and cuts the magnetic field lines, inducing a pd across the coil. The complete circuit means an induced current flows. Each half-revolution, the two halves of the split-ring commutator switch brushes, so the current to the lamp is only ever in one direction."

Earns full marks: uses every mark-scheme phrase: "cuts field lines", "induces a pd", "complete circuit", "commutator halves switch brushes every half-turn", "current in one direction".

Common Errors

Students often writeCorrect versionWhy it matters

"An alternator outputs DC"Alternator outputs AC (slip rings preserve the alternating pd). Dynamo outputs DC (commutator flips the negative halves up)Defining feature of each device

"The dynamo is the same as a motor"Opposite direction of conversion: motor = current in → motion out ; dynamo = motion in → current outMost common Paper 2 confusion

Dynamo output graph drawn as a full sine waveDynamo output is 4 positive half-cycles per 2 revolutions — humps only, never below zeroJun 2023 Q9.2: only 17% drew it correctly

"The microphone amplifies sound"The microphone converts sound to an electrical signal. Amplification is a separate downstream stageWrong function

"Sound waves move the diaphragm up and down"Sound waves cause the diaphragm to vibrate . "Up and down" is explicitly not accepted (Jun 2024 MS Q8.4)Wrong direction; "vibrate" is the spec verb

"Disconnecting the lamp means less friction at the brushes"The handle becomes easier because no induced current → no opposing field → no opposing forceWrong mechanism (Lenz's law removed)

✓ Checkpoint & Where Next

You can now

Explain how an alternator generates AC (slip rings preserve the alternation)

Explain how a dynamo generates DC (commutator flips connections every half-turn)

Draw and interpret pd–time graphs for both: smooth sine for alternator, four positive half-cycles per two revs for dynamo

Explain how a moving-coil microphone converts sound to an alternating electrical signal (sound → diaphragm → coil → induced pd)

Apply Lenz's law to explain why disconnecting a load makes a dynamo easier to turn (no current → no opposing field → no opposing force)

Distinguish four devices by input/output: motor, dynamo/alternator, microphone, loudspeaker

Next lesson · 7.07 Transformers — structure & operation

The generator effect powering a different device — but instead of a moving coil, it's a changing current in a primary coil that produces a changing magnetic flux in a secondary coil. AC only — DC doesn't work. The basis of the National Grid.