OCR GCSE Physics A: Transformers, microphones and loudspeakers (J249) – Revision Notes

These notes condense statements P4.2g to P4.2j of the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026). All four statements are in bold in the specification, so they are Higher Tier only. For full explanations and worked examples, read the study guide. When you are ready, move to the practice questions.

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Key facts at a glance

Statement	Remember	Tier
P4.2g	Alternating current in the primary gives a changing field, which induces an alternating potential difference in the secondary	Higher Tier only
P4.2h	Vp / Vs depends on Np / Ns: more secondary turns, bigger output	Higher Tier only
P4.2i	Vp / Vs = Np / Ns, with Vp Ip = Vs Is for an ideal transformer	Higher Tier only
P4.2j	Microphone: sound to current. Loudspeaker and headphones: current to sound	Higher Tier only

The power equation Vp Ip = Vs Is is listed with topic P8.2 in the specification. It is included here because it goes with transformer calculations.

P4.2g: how a transformer works

• Two coils, primary (input) and secondary (output), on the same core. No electrical connection between them.
• Alternating current in the primary gives a changing magnetic field in the core.
• The secondary is in this changing field, so a potential difference is induced across it.
• A complete secondary circuit carries an alternating current.
• A battery (direct current) gives a steady field, so nothing is induced. Transformers need a.c.

Method in steps: explain a transformer

1. Alternating current in the primary coil.
2. It produces a changing magnetic field in the core.
3. The changing field passes through the secondary coil.
4. A potential difference is induced across the secondary.
5. It drives an alternating current if the circuit is complete.

Use the words alternating, changing and induced in every explanation.

P4.2h: turns and potential difference

Type	Turns	Potential difference	Example
Step-up	Ns > Np	Vs > Vp	Secondary has more turns
Step-down	Ns < Np	Vs < Vp	Secondary has fewer turns
Equal	Ns = Np	Vs = Vp	Isolates two circuits

Explaining the dependence: each turn of the secondary is in the same changing field, so each adds to the induced potential difference. More turns therefore means a bigger total. Doubling Ns doubles Vs. Doubling Np (with Vp and Ns unchanged) halves Vs.

P4.2i: equations

Equation	Use
Vp / Vs = Np / Ns	Potential differences and turns
Vp × Ip = Vs × Is	Power in equals power out, ideal transformer

Rearrangements:

Find	Formula
Vs	Vp × Ns / Np
Vp	Vs × Np / Ns
Ns	Np × Vs / Vp
Np	Ns × Vp / Vs
Ip	Vs × Is / Vp
Is	Vp × Ip / Vs

Method in steps: transformer calculation

1. Write down what you know: Vp, Vs, Np, Ns (and Ip or Is).
2. Choose the equation: turns and potential difference, or power.
3. Rearrange for the unknown before you substitute.
4. Substitute with units.
5. Check: bigger potential difference with bigger turns; bigger potential difference with smaller current.

Worked reminder

A transformer has 500 turns on the primary and 125 turns on the secondary. The primary potential difference is 80 V.

Vs = Vp × Ns / Np = 80 × 125 / 500 = 20 V

Fewer secondary turns, so this is step-down.

If the secondary current is 1.5 A and the transformer is ideal:

Vp × Ip = Vs × Is
80 × Ip = 20 × 1.5 = 30
Ip = 30 / 80 = 0.375 A

The current in the primary is smaller than in the secondary, as it must be when the potential difference is larger.

Second worked reminder: find the turns

A transformer must change 160 V to 20 V. The primary has 960 turns.

Ns = Np × Vs / Vp = 960 × 20 / 160 = 120 turns

The ratio of turns is 960 : 120, which is 8 : 1, the same as the ratio of the potential differences, 160 : 20. A quick check like this catches inverted fractions.

Model answer: explain why Vs is greater than Vp

Use this sentence pattern for any “explain” question on a step-up transformer:

1. The secondary coil has more turns than the primary coil.
2. Each turn is in the same changing magnetic field, so each turn has a potential difference induced across it.
3. The potential differences add along the coil, so the total across the secondary is greater than across the primary.
4. The ratio Vs / Vp is the same as Ns / Np.

P4.2j: microphone, loudspeaker and headphones

Microphone (sound to current)

1. Sound wave: pressure variations push the diaphragm.
2. The diaphragm and its attached coil vibrate.
3. The coil moves in the field of a permanent magnet.
4. A potential difference is induced (electromagnetic induction).
5. An alternating current flows, with the same frequency as the sound.

Loudspeaker and headphones (current to sound)

1. A varying current flows in the coil.
2. The coil is in the field of a permanent magnet, so it feels a force.
3. The force changes direction with the current, so the coil and cone vibrate.
4. The cone makes pressure variations in the air.
5. These pressure variations are the sound wave.

Headphones work in the same way as a loudspeaker.

Comparing the two

	Microphone	Loudspeaker
In	Sound	Current
Out	Current	Sound
Coil	Moved by the diaphragm	Pushed by the force
Louder sound	Larger movement, larger induced potential difference	Larger current, larger force, larger movement
Higher pitch	Faster vibration, higher frequency current	Higher frequency current, faster vibration

Model answer: explain how a loudspeaker makes sound

1. A varying current from the amplifier flows in the coil.
2. The coil is in the magnetic field of the permanent magnet, so a force acts on it.
3. The current keeps changing direction, so the force on the coil keeps changing direction.
4. The coil and the cone attached to it move back and forth, at the frequency of the current.
5. The cone pushes the air, making pressure variations that travel away as a sound wave.

For a microphone, reverse the chain: sound, diaphragm, coil moving in a field, induced potential difference, varying current.

Must-know distinctions

• Primary and secondary: input and output, not first and second in size.
• Step-up and step-down describe the potential difference, not the current or the power.
• Potential difference is stepped up or down; power is conserved in an ideal transformer.
• Induced potential difference (microphone, transformer secondary) is not the same as force on a current (loudspeaker).
• Alternating supply gives a changing field; direct supply gives a steady one.

Quick self-test

All questions are Higher Tier only.

1. What is a transformer used for?
2. Why must the supply to the primary coil be alternating?
3. A transformer has 50 primary turns, 400 secondary turns and a 3.0 V a.c. input. Calculate the output.
4. A step-down transformer has Vp = 48 V, Vs = 6.0 V and 90 secondary turns. Calculate the number of primary turns.
5. An ideal transformer has Vs = 12 V, Is = 4.0 A and Vp = 240 V. Calculate Ip.
6. A transformer has 600 primary turns and Vp = 36 V. It needs an output of 9.0 V. How many secondary turns does it need?
7. A transformer has 300 primary turns, 1500 secondary turns and Vp = 20 V. Calculate Vs, and say whether it is step-up or step-down.
8. State what happens to Vs if the number of primary turns is doubled and everything else stays the same.
9. In a microphone, what moves in the magnetic field, and what is induced?
10. What effect makes the cone of a loudspeaker move?
11. Which two devices in this topic change sound into current and current into sound?

Answers

1. To change the size of an alternating potential difference.
2. A changing current gives a changing magnetic field, which induces a potential difference in the secondary. Direct current gives a steady field and no induced output.
3. Vs = 3.0 × 400 / 50 = 24 V.
4. Np = Ns × Vp / Vs = 90 × 48 / 6.0 = 720 turns.
5. Ip = Vs × Is / Vp = 12 × 4.0 / 240 = 0.20 A.
6. Ns = Np × Vs / Vp = 600 × 9.0 / 36 = 150 turns.
7. Vs = 20 × 1500 / 300 = 100 V; step-up, as Ns > Np.
8. Vs is halved.
9. The coil, which is attached to the diaphragm, moves; a potential difference is induced across it.
10. A force acts on the current-carrying coil in the permanent magnet’s field (a current in a magnetic field gives a force).
11. A microphone changes sound to current. A loudspeaker (or headphones) changes current to sound.

Where marks are usually lost

• Leaving out “alternating”, “changing” or “induced” in a transformer explanation.
• Saying the current passes through the core from one coil to the other.
• Writing Vp / Vs = Ns / Np, with the coils the wrong way round.
• Not rearranging first, then substituting numbers into the wrong place.
• Calling a transformer with fewer secondary turns step-up.
• Writing that a step-up transformer also steps up the current.
• Describing a microphone as using a force on a coil, or a loudspeaker as using induction.
• Giving no unit, or the wrong unit, on the answer.

Official syllabus

OCR GCSE (9-1) Gateway Science Physics A (J249), specification Version 5.0 (July 2026), first assessment 2018, published by OCR. These notes cover statements P4.2g to P4.2j.