Transformers structure and operation (HT, physics only)

Module 7 · Lesson 7.07 · Spec 4.7.3.4

Transformers · Structure and Operation

Higher Tier only

Physics only

The generator effect with no moving parts. A changing current in one coil induces a changing pd in another — linked through an iron core. The basis of every power supply, every National Grid pylon, every laptop charger.

From Moving Coils to Static Transformers

In Lessons 7.05–7.06 the generator effect needed a moving conductor (a rotating coil or a vibrating diaphragm) to produce a changing magnetic field. A transformer cuts out the moving parts.

The core idea

In a transformer, the changing magnetic field is supplied by an alternating current in a fixed coil. No spinning, no vibrating — just AC in one coil creating a changing flux that induces a pd in a second coil. Generator effect, no moving parts.

By end of lesson

Describe the structure of a basic transformer · explain the 4-step operating mechanism · distinguish step-up from step-down · explain why iron is used for the core and why AC (not DC) is essential · know the mark-scheme phrasings examiners reward.

Transformer Structure

A basic transformer has just three components: a primary coil, a secondary coil, and a soft iron core linking them. There is no direct electrical connection between the two coils — they are linked only through the magnetic flux in the core.

Primary and secondary coils wound on a soft iron core. AC supply on primary; alternating pd induced on secondary. The two coils are magnetically linked through the core, not electrically.

Components · Stock 2-mark Label Question

Jun 2022 Q7.1 is a classic stock question: you're given a transformer diagram with three labelled arrows (A, B, C) and asked to name them. The correct labels are fixed.

LabelMark-scheme answerRole

Aprimary coilThe input coil — connected to the AC supply. Carries an alternating current (the cause of everything that follows).

Bsecondary coilThe output coil — connected to the load (e.g. a lamp). An alternating pd is induced here by the changing flux in the core.

Ciron coreThe magnetic link between primary and secondary. Carries the alternating magnetic flux from primary to secondary. Magnetic , not electrical — no current flows in the core itself.

Stock 2-mark question · Jun 2022 Q7.1

Two marks are split: 1 for naming the two coils correctly (primary + secondary), 1 for naming the iron core. Just "core" alone may not be accepted — specify "iron".

How a Transformer Works · 4-Step Mechanism

The chain is fixed. Each step is a mark in the explain question. Memorise the order, and the language.

AC in primary

An alternating current flows in the primary coil.

Changing flux

The AC produces a changing (alternating) magnetic field around the primary, in the iron core.

Flux through secondary

The iron core channels the changing flux through the secondary coil.

Induced AC out

The changing flux induces an alternating pd across the secondary (generator effect).

Mark-scheme phrasing (Jun 2022 Q7.3, 3 marks)

"(the alternating current causes) a changing magnetic field around the primary (coil)"

"creates magnetic field that changes direction in the core" (or "creates a changing magnetic field in the core")

"this induces an alternating potential difference across the secondary (coil causing an alternating current)"

Output frequency = input frequency

The induced secondary pd alternates at the same frequency as the primary current. The transformer changes the size of the alternating pd (and current), but it does not change the AC frequency. UK mains is 50 Hz on both sides of every transformer in the National Grid.

Step-Up vs Step-Down

What decides whether the output pd is bigger or smaller than the input? The ratio of turns. Specifically: how many turns on the secondary compared with the primary.

Step-up Transformer

More turns on secondary than primary (N_s > N_p)

Output pd is greater than input pd: V_s > V_p

Used at: power stations — to send electricity through transmission cables at very high voltage (so the current is low → low I²R losses).

Step-down Transformer

Fewer turns on secondary than primary (N_s < N_p)

Output pd is smaller than input pd: V_s < V_p

Used at: substations near homes — to reduce transmission voltage back to a safe domestic level (e.g. 230 V mains in the UK).

The turns ratio decides the pd ratio. Same iron core, same mechanism — just different numbers of turns.

Why Iron? · Why Soft Iron?

The spec is explicit: iron is used because it is easily magnetised. The mark scheme has accepted this single phrasing across multiple years.

iron · it is easily magnetised (and demagnetised) · allow nickel / cobalt · do not allow steel · allow "it is a magnetic material" · MP 2 is dependent on MP 1 — AQA Mark Scheme, Jun 2023 Paper 2 Q8.1 (verbatim)

Why "easily magnetised" matters

Soft iron magnetises and demagnetises quickly as the primary current alternates. This allows the magnetic flux in the core to change rapidly, which is essential — because by the generator effect, only a changing flux induces a pd in the secondary. Steel resists demagnetisation (it's a "hard" magnetic material) so it would waste energy and be slow to respond → steel is explicitly not accepted.

Spec note · 4.7.3.4

"Knowledge of laminations and eddy currents in the core is not required." Don't write about them in answers — they're outside the spec. Stick to "iron is easily magnetised".

Why AC? · Why Not DC?

Transformers only work with AC. This is non-negotiable, and a recurring exam stem.

AC input → works

Alternating current → continuously changing magnetic flux in the core → flux through secondary continuously changes → an alternating pd is induced in the secondary (generator effect).

DC input → fails

Direct current → steady, unchanging magnetic flux in the core → no flux change → no induced pd in the secondary. Output is zero.

Why this is forced by the physics

From Lesson 7.05: an induced pd requires a change in magnetic field (or relative motion). A steady DC produces a steady field — no change → no induced pd. The whole transformer rests on the generator effect, and the generator effect rests on change.

Where Transformers Are Used · The National Grid

Most transformers students see in exam questions sit somewhere on the National Grid. Two are essential:

Step-up at the power station → high-voltage low-current transmission → step-down near homes back to safe 230 V mains.

Why this layout works

Stepping up the pd steps down the current (assuming 100% efficient: V_p × I_p = V_s × I_s). Low current in the transmission cables means low I²R heating losses over hundreds of kilometres of cable. Detailed calculations come in Lesson 7.08.

Worked Example · Iron Core Question

Question2 marks

A transformer is used to convert 230 V AC into a lower potential difference to power a lamp. State the material used for the core of the transformer, and give the reason for using this material.

Mark 1

Material: iron (also accepted: nickel or cobalt — but iron is the standard answer; "steel" is not accepted)

Mark 2

Reason: it is easily magnetised (and demagnetised) (also accepted: "it is a magnetic material")

⚠ MS rule

Mark 2 is dependent on Mark 1. If you state the wrong material (e.g. "copper", "steel"), you can't get Mark 2 even with a correct-sounding reason. Lock in "iron" first, then add the reason.

Worked Example · How Does AC Cross the Transformer?

Question3 marks

The primary coil of a transformer is connected to an alternating current supply. The secondary coil is connected to a circuit containing a lamp. Explain how the alternating current in the primary coil produces an alternating current in the secondary circuit.

Mark 1

(The alternating current causes) a changing magnetic field around the primary (coil).

Mark 2

This creates a magnetic field that changes direction in the (iron) core. (Also accepted: "creates a changing magnetic field in the core".)

Mark 3

This induces an alternating potential difference across the secondary (coil), causing an alternating current in the circuit.

⚠ Common loss

Many students jump from "AC in primary" straight to "AC in secondary" without mentioning the changing flux in the core as the linking mechanism. That middle step is worth a mark on its own.

Examiner Alert · Iron's Role Is Magnetic, Not Electrical

The most common transformer misconception examiners record: students explain iron's use by appealing to its electrical conductivity. This is wrong — and reveals a deeper misunderstanding of how transformers work.

Most students knew that iron is used for the core of a transformer and most of those were able to give an explanation. A significant number of students thought that iron was used because it is a good conductor of electricity, which demonstrates a misunderstanding of how transformers work. — AQA Examiner Report, Jun 2023 Paper 2 Q8.1 (verbatim)

Why this misconception fails

The iron core does not carry electric current — the primary and secondary coils are not electrically connected. The core carries magnetic flux, not current. Iron is used because of its magnetic properties (easily magnetised), not electrical ones.

Phrases that earn 0 marks

"Iron is a good conductor" · "iron conducts the current" · "iron conducts the electricity" · "iron carries the charge". All of these are about electrical conduction and are explicitly rejected.

Weak vs Strong · Transformer Explain

Weak · 0/3

"The current goes into the primary coil. It crosses through the iron core to the secondary coil. The iron is a good conductor of electricity so the current passes through. The lamp lights up."

Loses every mark: says the iron conducts the current (electrical, rejected). No mention of changing magnetic field. No mention of induction. Mistakes magnetic linking for electrical linking.

Strong · 3/3

"The alternating current in the primary coil creates a changing magnetic field around the primary. The iron core carries a magnetic field that changes direction. The changing flux in the secondary coil induces an alternating potential difference across it, driving an alternating current through the lamp."

Earns every mark: "changing magnetic field around the primary" (MP1) · "changes direction in the core" (MP2) · "induces an alternating potential difference across the secondary" (MP3). Every phrase from the mark scheme.

Common Errors

Students often writeCorrect versionWhy it matters

"Iron is used because it conducts electricity"Iron is used because it is easily magnetised . The core carries magnetic flux, not currentJun 2023 examiner: this demonstrates a fundamental misunderstanding of transformers

"Transformers work with DC"Transformers only work with AC . DC produces a steady flux → no change → no induced pd → output is zeroFoundational requirement

"A step-up transformer increases the current"Step-up increases the pd . The current decreases (assuming 100% efficient — Vp·Ip = Vs·Is)Inverse relationship — covered in 7.08

"The primary current flows through the iron core into the secondary"No current flows between the coils. The link is magnetic flux in the core, not electric currentMechanism is induction, not conduction

"The core is steel because it's stronger"The core is iron , and steel is not accepted . Steel is a "hard" magnetic material — resists demagnetising, so wouldn't work in a transformerJun 2023 MS explicitly rejects "steel"

Mentioning laminations or eddy currents in an answerThese are outside the GCSE spec — knowledge of them is explicitly not required. Stick to "easily magnetised"Wastes time and shows you're trying to use A-level content unnecessarily

What's Next · Transformer Calculations

You now understand how a transformer works. Lesson 7.08 turns this into numbers — using two equations that appear on the Physics Equations Sheet.

Equation 1 · Turns ratio (Spec 4.7.3.4)

V_p / V_s = N_p / N_s

The ratio of pd across the coils equals the ratio of turns. V_p, V_s in volts. N_p, N_s = number of turns (no units).

Equation 2 · Power conservation (Spec 4.7.3.4)

V_s × I_s = V_p × I_p

Assumes the transformer is 100% efficient: electrical power output = electrical power input. I_p, I_s in amperes. So step up the pd and the current must step down in the same ratio.

Coming in 7.08

How to orient the turns ratio so you don't invert · how to chain the two equations when only some quantities are given · how to use the inverse V–I relationship to explain why high-voltage transmission reduces power losses · worked examples at every demand level.

✓ Checkpoint

You can now

Name the three components of a transformer: primary coil, secondary coil, iron core (Jun 2022 Q7.1 stock question)

Explain the 4-step mechanism : AC in primary → changing field around primary → changing flux in iron core → induces AC in secondary

Distinguish step-up (N_s > N_p, V_s > V_p) from step-down (N_s < N_p, V_s < V_p)

Explain why iron is used : it is easily magnetised (Jun 2023 MS Q8.1 verbatim) — and avoid the "good conductor" trap

Explain why AC is essential : DC produces no flux change → no induced pd

Describe how transformers fit into the National Grid : step-up at the power station, step-down near homes

Use the mark-scheme phrasings examiners reward: "changing magnetic field around the primary", "changes direction in the core", "induces an alternating potential difference across the secondary"

Next · Lesson 7.08 Transformer calculations

Two equations from the Physics Equations Sheet — turns ratio and power conservation — applied to step-up, step-down, and chained-equation problems. Including the highest-marked transformer pattern across AQA exam history.