Electric motors and loudspeakers (HT; loudspeakers physics only)

SPEC 4.7.2.3 + 4.7.2.4 • MOTOR EFFECT APPLICATIONS

🔵 HIGHER TIER ONLY

⚡ LOUDSPEAKERS PHYSICS-ONLY

Electric Motors & Loudspeakers

Take the motor effect from 7.03 and put it to work. A coil in a field rotates — that's a motor. A coil in a field vibrates — that's a loudspeaker.

Two Applications, One Principle HT

Both devices in this lesson are direct consequences of Fleming's left-hand rule: current + field → force. Change the geometry and the same principle does two completely different jobs.

DC Motor

Coil in a field → forces produce a turning couple → continuous rotation. Powers fans, drills, EVs, washing machines.

Loudspeaker PO

Coil attached to a cone, in a field → alternating current → cone vibrates → sound waves. Same principle in headphones.

The Synoptic Link

Everything in this lesson rests on the motor effect equation F = BIl and Fleming's left-hand rule (current → force). If those aren't solid, revisit 7.03 before continuing.

Inside a DC Motor HT

A DC motor has five essential parts. Each one earns its keep.

The Five Parts

Rectangular coil of wire — carries the current; the part that rotates

Magnet (permanent or electromagnet) — provides the magnetic field

Axle — allows the coil to rotate freely

Split-ring commutator — reverses current direction every half-turn (the clever part)

Brushes — sliding contacts that connect the battery to the spinning commutator

How the Motor Turns HT

The two long sides of the coil carry current in opposite directions — because the wire loops back. That's the whole trick.

Applying Fleming's LH Rule To Both Sides

In the diagram on the previous slide:

Top wire: current comes out of the page (⊙). Field points right. By LH rule → force upwards.

Bottom wire: current goes into the page (⊗). Field points right. By LH rule → force downwards.

The Turning Couple

Two equal and opposite forces acting on parallel sides of the coil at a distance apart produce a turning effect (moment). The coil rotates about the axle. This pair of forces is called a couple.

Why The Two Sides Always Disagree

Because the coil is one continuous loop, current flowing "up the left side" must come "down the right side." If both sides had current in the same direction, the forces would just push the coil sideways with no rotation. The opposing directions are what create the couple.

The Half-Turn Problem HT

The motor would jam after half a turn — unless something clever happens.

What Goes Wrong

After the coil has rotated 180°, the two long sides have swapped places. The wire that was on top is now on the bottom — but the current direction in that wire (relative to the field) hasn't changed.

Apply Fleming's LH rule again: the force on each side is now in the opposite direction. Instead of pushing the rotation onwards, the forces now try to push the coil back the way it came.

Without a Commutator

The coil would oscillate back and forth around the vertical position — never make a full rotation, never be useful. To get continuous rotation in one direction, the current in the coil must reverse exactly when the coil reaches the half-turn point.

The Split-Ring Commutator HT

The fix is a simple mechanical device: a metal ring split into two halves, mounted on the axle.

Split-Ring Commutator

A device on a DC motor that reverses the direction of the current in the coil every half-turn, so that the rotation continues in the same direction.

The Result

Every half-turn, the current in the coil reverses. So the force on the (new) top side is still up, and the force on the (new) bottom side is still down. The rotation continues in the same direction — turn after turn.

Examiner Alert — Commutator Role HT

The role of the split-ring commutator is one of the most-asked conceptual questions in Module 7. Precision in wording is everything.

Weak (loses marks)

"It keeps the motor running"

"It makes the current flow"

"It connects the battery to the motor"

"It allows the coil to spin"

All too vague. The current would flow with or without it — the commutator's specific job is missed.

Strong (full marks)

"The split-ring commutator reverses the direction of the current in the coil every half-turn, so that the rotation continues in the same direction."

Two specific elements: (1) reverses the current direction in the coil, (2) every half-turn (the timing matters), (3) so rotation stays one-way.

Memorise This Sentence

If you can write the strong version word-for-word, you'll get full marks on any "explain the role of the split-ring commutator" question. Practise saying it aloud until it's automatic.

Four Ways to Speed Up a Motor HT

The motor spins faster when the turning couple is larger. The size of each force comes straight from F = BIl — so the factors are familiar.

↑ I

More Current

Larger I in F = BIl → larger force on each side

↑ B

Stronger Field

Use stronger magnets (or an electromagnet on full power)

↑ N

More Turns

More turns of wire in the coil → force adds up across each turn

↑ A

Larger Coil

Greater distance between forces → larger turning moment

Common Wrong Answer

"Use a stronger magnet" → some students think this makes the motor go backwards. It does not — it just makes the existing rotation faster. Direction depends only on current and field direction; magnitude only changes speed.

The Moving-Coil Loudspeaker HT PHYSICS ONLY

Loudspeakers and headphones both use the motor effect — exactly the same principle as the DC motor, but with a different geometry that produces vibration instead of rotation.

The Three Parts

Permanent magnet (often cylindrical, with an arrangement of poles that produces a radial field in a narrow gap)

Coil of wire sitting in the gap, in the magnetic field

Paper cone rigidly attached to the coil; flexible leads connect the coil to an electrical circuit

How the Loudspeaker Works HT PHYSICS ONLY

Four steps. Each one earns a mark on the standard 4-mark question.

Four-Step Mechanism

An alternating audio current from the electrical circuit flows through the coil.

The coil sits in the permanent magnet's field, so the current experiences the motor effect — a force on the coil (Fleming's LH rule).

Because the current alternates, the force alternates too. The coil — and the attached cone — moves back and forth at the same frequency as the current.

The vibrating cone pushes on air molecules, producing a sound wave at the same frequency and amplitude as the audio signal.

Headphones Work Identically

Headphones and earbuds are just miniature loudspeakers. Same components (magnet, coil, diaphragm), same motor-effect mechanism — just smaller, and aimed at one ear. The spec lists "loudspeakers and headphones" — be ready to apply the same explanation to either.

Direction Of Conversion

A loudspeaker converts variations in current → variations in air pressure (sound). Electricity in, sound out. The reverse process — sound in, electricity out — is a microphone, covered in 7.06.

Examiner Alert — Direction of Conversion HT PHYSICS ONLY

A persistent error on loudspeaker questions: getting the direction of conversion the wrong way round.

Wrong

"A loudspeaker generates electricity from sound."

This is what a microphone does (generator effect). Loudspeakers do the opposite.

Right

"A loudspeaker produces sound from an alternating current."

Current in → sound out. Motor effect, not generator effect. Use LEFT hand for the force on the coil.

The Four Sound/Electricity Devices

DeviceInputOutputEffectLoudspeakerCurrent (AC)SoundMotor effectHeadphonesCurrent (AC)SoundMotor effectMicrophone (7.06)SoundCurrent (AC)Generator effectDC motorCurrent (DC)RotationMotor effect

Practice Question 1

A simple DC motor has a rectangular coil between the poles of a permanent magnet. The N pole is on the left and the S pole is on the right. When the switch is closed, the current in the top side of the coil flows out of the page.

(a) Using Fleming's left-hand rule, state the direction of the force on the top side of the coil. 1 mark

(b) State the direction of the current in the bottom side of the coil, and the direction of the force acting there. 2 marks

a

Apply Fleming's left-hand rule: first finger points right (field N→S), second finger points out of the page (current). The thumb points upwards. Force on the top side = upwards.

b

Because the coil is a continuous loop, the current in the bottom side must flow in the opposite direction — into the page.

b

Applying LH rule on the bottom side: first finger right, second finger into the page → thumb points downwards. Force on the bottom side = downwards.

The result: top force up + bottom force down = a turning couple → the coil rotates. After half a turn, the split-ring commutator reverses the current and rotation continues.

Practice Question 2

PHYSICS ONLY

A moving-coil loudspeaker is connected to an amplifier. The amplifier delivers an alternating current to the coil — the current varies in step with the audio signal being played. The structure of the loudspeaker is shown in the diagram on slide 9.

Explain how the alternating current produces a sound wave at the loudspeaker.6 marks

Full-mark answer covers all six points:

1

An alternating current from the electrical circuit flows through the coil.

2

The coil is in the magnetic field of the permanent magnet, so the current-carrying wire experiences a force (the motor effect).

3

The direction of the force is given by Fleming's left-hand rule.

4

Because the current alternates, the force on the coil also alternates in direction.

5

The coil — and the attached paper cone — moves back and forth at the same frequency as the current.

6

The vibrating cone makes air molecules vibrate, producing a sound wave with the same frequency (and amplitude) as the original current.

Marking tip: 6 marks usually means 6 distinct points. Use a list. Naming "motor effect" and "Fleming's left-hand rule" explicitly is worth a mark each.

Common Errors to Avoid HT

Three high-frequency mistakes on motor and loudspeaker questions.

Students Often WriteCorrect VersionWhy It Matters

"The commutator keeps the motor running""The commutator reverses the current direction in the coil every half-turn so the rotation continues in the same direction"Vague answer misses the specific mechanism — examiners need the precise role

"A stronger magnet makes the motor go backwards"Stronger field → larger force → motor rotates faster in the same directionDirection depends on current/field direction ; magnitude only changes speed

"A loudspeaker generates electricity from sound"A loudspeaker produces sound from current . The reverse (sound → current) is a microphoneWrong direction of conversion — confuses motor effect with generator effect

Checkpoint HT

You should now be able to do all of the following without prompting.

You Can Now

Name the five parts of a DC motor — coil, magnet, axle, split-ring commutator, brushes

Explain how the motor turns — opposite currents in the two sides of the coil give opposite forces (LH rule); the couple turns the coil

Explain the role of the split-ring commutator precisely — reverses the current in the coil every half-turn so rotation continues in the same direction

List four ways to speed up a motor — more current, stronger field, more turns, larger coil area

Describe a moving-coil loudspeaker [Physics only] — coil in a permanent magnet's field, attached to a paper cone

Explain how a loudspeaker converts current to sound [Physics only] — alternating current → motor effect → coil vibrates → cone vibrates → sound wave

Distinguish loudspeaker from microphone — loudspeaker: current in, sound out (motor effect); microphone: opposite

Coming Next — 7.05 (HT + Physics Only)

Induced potential — the generator effect. The reverse of the motor effect: move a wire through a field and you induce a potential difference. Foundation for alternators, dynamos, microphones, and transformers.