Transformers — Physics with Kate

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4.7.3.4

AQA GCSE Physics · Topic 4.7 Magnetism and Electromagnetism · the equations: physics (triple) only

6.17P – 6.20P

Edexcel International GCSE Physics · Section 6 Magnetism and electromagnetism · 6 (d) Electromagnetic induction · physics only

Transformers: Step-Up, Step-Down and the Transformer Equations

In this lesson you'll learn to: describe the structure of a transformer, explain how it works and why it only works on a.c., say what makes a transformer step-up or step-down, use V_p ÷ V_s = n_p ÷ n_s and V_p × I_p = V_s × I_s, and explain why the National Grid uses both kinds.

▶ Prefer to watch? Scroll to the bottom for the video on this topic ▶ A video walkthrough is coming soon — it will appear at the bottom of this page

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Start here — have a play

Don't read anything yet. On tab 1, look at the primary circuit, then the secondary circuit, and watch the lamp. Then swap the a.c. supply for a cell and switch it on and off: when does the lamp light? On tab 2, make a step-up and a step-down transformer and watch the numbers in both equations.

Supply

* Simplified magnetic field: the field lines round the iron core.

Look at

Primary circuit

Secondary circuit

Slowed right down so you can see it. Mains a.c. changes direction 100 times a second.

▲ Two tabs: how a transformer works (a.c. or d.c.), then step-up and step-down with the two equations.

Seen the lamp go out on d.c.? Now learn why.

The notes below have the structure, the five-step explanation, step-up and step-down, both equations and the National Grid.

Read the notes ↓

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Revision notes

The structure of a transformer

A primary coil · B iron core · C secondary coil · D a.c. supply

⭐ Describe the structure (2 marks)

Two coils of insulated wire, the primary and the secondary, wound on the same iron core. The coils have different numbers of turns. The supply joins to the primary coil; the output comes from the secondary coil.

How a transformer works

primary circuitAn alternating current in the primary coil makes it an electromagnet
primary circuitIt creates a magnetic field that keeps changing direction
primary circuitThe iron core becomes an induced magnet and creates a magnetic field
secondary circuitThe secondary coil cuts the changing magnetic field
secondary circuitSo an alternating p.d. is induced across the secondary coil

⭐ Why a transformer does not work on d.c.

A direct current gives a magnetic field in one direction that does not change. The secondary coil only cuts it for a moment, as the field grows (switched on) or collapses (switched off). While the current flows steadily there is no cutting, so no p.d. is induced.

Step-up and step-down

	Step-up	Step-down
Turns	more turns on the secondary than the primary	fewer turns on the secondary than the primary
Voltage (p.d.)	goes up: V_s > V_p	goes down: V_s < V_p
Current	goes down	goes up
Used	at the power station, before the cables	near homes, down to 230 V

The two transformer equations AQA: physics (triple) only

Voltages and turns

V_p ÷ V_s = n_p ÷ n_s

input (primary) voltage ÷ output (secondary) voltage = primary turns ÷ secondary turns

Use it when the question gives the number of turns.

Power in = power out (100% efficient)

V_p × I_p = V_s × I_s

input power = output power

Use it when the question gives a current.

⭐ Units

For the turns equation the two voltages just need the same unit (both in kV is fine). For the power equation, use volts: 25 kV = 25 000 V.

Transformers in the National Grid

Power station 25 kV → step-up → cables 400 kV → step-down → homes 230 V

step-upThe step-up transformer increases the voltage and decreases the current
step-upPower in = power out, so the same power is transferred with a smaller current
step-upA smaller current means less heating of the cables
step-upSo less energy is wasted to the surroundings: it is more efficient
step-downThe step-down transformer decreases the voltage to 230 V, which is safer in homes

More on this in the National Grid lesson.

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How to write it in the exam

Every "how does a transformer work" answer has the same links

An alternating current in the primary coil → a changing magnetic field (the iron core becomes an induced magnet) → the secondary coil cuts it, so a p.d. is induced.

the a.c.the changing fieldthe induced p.d.

Workbook page 14Workbook page 13 · explain how a step-down transformer works · 4 marks

An alternating current in the primary coil creates a changing magnetic field (1)
The iron core becomes an induced magnet and creates a magnetic field (1)
The changing magnetic field induces an alternating voltage across the secondary coil (1)
Fewer turns on the secondary coil, so a lower voltage: step-down (1)

Workbook page 15Workbook page 14 · worked example · a laptop charger

The mains input is 230 V and the output is 20 V. The primary coil has 1150 turns.

(a) Calculate the number of turns on the secondary coil. [3]

Write the equation: V_p ÷ V_s = n_p ÷ n_s
Substitute: 230 ÷ 20 = 1150 ÷ n_s
Rearrange: n_s = 1150 × 20 ÷ 230
Answer: 100 turns

(b) The laptop draws 3.0 A from the secondary coil. Calculate the current in the primary coil. [3]

Write the equation: V_p × I_p = V_s × I_s
Substitute: 230 × I_p = 20 × 3.0
Rearrange: I_p = 60 ÷ 230
Answer: 0.26 A

Workbook page 15Workbook page 14 · your turn · a power station

A power station generates electricity at 25 kV. A transformer increases this to 400 kV for the cables. The primary coil has 800 turns.

(a) Calculate the number of turns on the secondary coil. [3]: 25 ÷ 400 = 800 ÷ n_s, so n_s = 800 × 400 ÷ 25 = 12 800 turns
(b) The current in the primary coil is 2000 A. Calculate the current in the cables. [3]: 25 000 × 2000 = 400 000 × I_s, so I_s = 50 000 000 ÷ 400 000 = 125 A (volts, not kV, in the power equation)

Workbook page 15Workbook page 14 · quick fire: the missing values

	V_p	V_s	n_p	n_s	I_p	I_s	Type
1	230 V	12 V	2300	120	—	—	step-down
2	12 V	240 V	50	1000	—	—	step-up
3	120 V	6.0 V	400	20	—	—	step-down
4	230 V	46 V	—	—	0.50 A	2.5 A	step-down

Workbook page 14Workbook page 13 · exam-style question · 6 marks

Electricity is generated at 25 kV. It is sent across the country through the National Grid at 400 kV, and used in homes at 230 V. Explain why step-up and step-down transformers are used in the National Grid. Any six from:

a step-up transformer increases the voltage (1)
so it decreases the current in the cables (1)
power in = power out (V_pI_p = V_sI_s, assuming 100% efficient) (1)
a current heats the transmission cables (1)
a smaller current means less energy is wasted as heat (1)
so transmission is more efficient (1)
a step-down transformer reduces the voltage to 230 V, a safer level for homes (1)

Workbook page 16Exam practice · Question 1 · how a transformer works · 6 marks

(a) Explain how the transformer produces a potential difference across the secondary coil. [4]: an alternating current in the primary coil (1); creates a changing magnetic field in the iron core (1); the iron core carries the changing field to the secondary coil (1); so a p.d. is induced across the ends of the secondary coil (1).
(b) Explain why a transformer does not work with a direct current (d.c.) supply. [2]: d.c. gives a constant (unchanging) magnetic field (1); so no p.d. is induced across the secondary coil (1).

Workbook page 16Exam practice · Question 2 · a phone charger · 7 marks

The input p.d. is 230 V. The primary coil has 4600 turns and the secondary coil has 100 turns.

(a) Calculate the output p.d. [3]: V_s = V_p × n_s ÷ n_p = 230 × 100 ÷ 4600 = 5.0 V
(b) Step-up or step-down? Give a reason. [1]: step-down: fewer turns on the secondary coil (both needed)
(c) The output current is 2.0 A and the transformer is 100% efficient. Calculate the current in the primary coil. [3]: I_p = V_s × I_s ÷ V_p = 5.0 × 2.0 ÷ 230 = 0.043 A

Workbook page 16Exam practice · Question 3 · the National Grid · 10 marks

A power station generates electricity at 25 000 V. A step-up transformer increases this to 400 000 V for the transmission cables.

(a) The primary coil has 2000 turns. Calculate the number of turns on the secondary coil. [3]: n_s = n_p × V_s ÷ V_p = 2000 × 400 000 ÷ 25 000 = 32 000 turns
(b) The power transmitted is 100 MW. Calculate the current in the cables. Use P = V I. [3]: 100 MW = 100 000 000 W; I = P ÷ V = 100 000 000 ÷ 400 000 = 250 A (it would be 4000 A at 25 kV)
(c) Explain why transmitting electricity at a high p.d. makes the National Grid more efficient. [4]: for the same power, a high p.d. means a low current (1); a low current means less heating of the cables (1); so less energy is wasted, transferred to the thermal store of the surroundings (1); so more of the energy reaches homes: the grid is more efficient (1).

📘 Now do it in your workbook

Page 14: label the transformer, explain it, and the National Grid 6-marker. Page 15: the two equations, a worked example, your turn and the quick-fire table. Page 16: three exam questions.

Page 13: label the transformer, explain it, and the National Grid 6-marker. Page 14: the two equations, a worked example, your turn and the quick-fire table.

AQA GCSE Physics Workbook · MAGNETISM AND ELECTROMAGNETISM · pages 14, 15 and 16 Edexcel iGCSE Physics Workbook · MAGNETISM & ELECTROMAGNETISM · pages 13 and 14

⬇ Free: pages 14, 15 and 16 (PDF) 📄 Full workbook PDF · £2.99 Paperback on Amazon ⬇ Free: pages 13 and 14 (PDF) 📄 Full workbook PDF · £2.99 Paperback on Amazon

The full workbook covers the whole Magnetism and Electromagnetism topic, with exam-style questions and worked answers.

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Prefer to watch? Here's the whole thing

Video walkthrough — coming soon

Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.

Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.

🎬

Video coming soon

The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.

Now put it into practice

Pages 14 to 16 of the workbook have the transformer to label, the National Grid 6-marker, both equations with a worked example, and three exam questions, with full answers at the back.

Pages 13 and 14 of the workbook have the transformer to label, the National Grid 6-marker and both equations with a worked example, with full answers at the back.

⬇ Free: pages 14, 15 and 16 (PDF) 📄 Full workbook PDF · £2.99 Paperback on Amazon ⬇ Free: pages 13 and 14 (PDF) 📄 Full workbook PDF · £2.99 Paperback on Amazon