Ch 16 · 4.5.6 The Transformer · O Level Physics 5054

Topic 16 of 17 in Electricity and Magnetism — O Level Physics 5054 — 49 lessons.

Introduction

Power stations generate electricity at about 25 000 V, the grid carries it across the country at 400 000 V or more, and your phone charger needs just 5 V. The device that changes the voltage at each step is the transformer. (Syllabus section 4.5.6.)

Key ideas — Structure

A simple transformer has:

🧑‍🏫 Teacher's explanation

Imagine a transformer as a translator between two languages. The primary coil speaks 'input' language (alternating current), and the secondary coil speaks 'output' language. They don't touch, but they communicate through a shared medium: the magnetic field. Think of it like two people standing on opposite sides of a glass window; they can't touch hands, but they can see each other's movements through the glass.

📖 Definitions

🔍 Worked example — Easy

A step-down transformer has 2000 turns on the primary coil and 50 turns on the secondary coil. If the input voltage is 240 V, calculate the output voltage.

🔍 Worked example — Standard

A step-up transformer increases the voltage from 120 V to 2400 V. If the primary coil has 100 turns, how many turns are in the secondary coil?

🔍 Worked example — Exam-style

Power lines transmit electricity at 10,000 V to minimize energy loss. If the current is 5 A, calculate the power being transmitted. Then, explain why increasing the voltage to 20,000 V reduces power loss in the cables (assume current drops proportionally).

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — How it works

A transformer works only with a.c. A steady d.c. gives a steady field, and a field that does not change induces nothing.

🧑‍🏫 Teacher's explanation

Imagine a transformer as a bridge that allows energy to jump from one circuit to another using magnetism. Instead of using wires to connect them, we use a magnetic field. Think of a person on one side of a glass wall waving a flashlight (the primary coil) and a person on the other side (the secondary coil) seeing the light patterns and reacting to them. The glass is the iron core, which carries the 'signal' across.

📖 Definitions

🔍 Worked example — Easy

A step-down transformer has 500 turns on the primary coil and 50 turns on the secondary coil. If the input voltage is 230 V, calculate the output voltage.

🔍 Worked example — Standard

A step-up transformer increases the voltage from 12 V to 240 V. If the primary coil has 100 turns, how many turns are in the secondary coil?

🔍 Worked example — Exam-style

A power station transmits electricity at a voltage of 400,000 V. The voltage is stepped down to 230 V for domestic use. If the step-down transformer has 2000 turns on the primary coil, calculate the number of turns on the secondary coil. (Assume 100\% efficiency).

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Step-up and step-down

V_p/V_s = N_p/N_s

🧑‍🏫 Teacher's explanation

Imagine a transformer as a translator between two different languages. The primary coil receives an alternating current (AC) and creates a changing magnetic field. This changing field travels through the iron core and 'translates' that energy into the secondary coil. The iron core acts like a highway, ensuring the magnetic flux moves efficiently from one coil to the other.

📖 Definitions

🔍 Worked example — Easy

A step-down transformer has 1000 turns on the primary coil and 200 turns on the secondary coil. If the primary voltage V_p is 240 V, calculate the secondary voltage V_s.

🔍 Worked example — Standard

A step-up transformer increases the voltage from 230 V to 11,500 V. If the primary coil has 500 turns, how many turns are in the secondary coil?

🔍 Worked example — Exam-style

An electric kettle is connected to a 230 V supply. It uses a step-down transformer to operate at 11.5 V. If the primary coil has 1000 turns, calculate the number of turns in the secondary coil.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — High-voltage transmission

Electricity is sent across the country through the National Grid at a very high voltage:

🧑‍🏫 Teacher's explanation

Imagine you are trying to move a large amount of water through a long, narrow garden hose. If you try to push a huge volume of water through very quickly (high flow rate/current), the friction against the hose walls becomes massive, and you lose a lot of water to splashing and heat. However, if you use a much higher pressure (high voltage) to push the water, you can move that same amount of water using a much slower flow rate (low current). Because the flow is slower, there is much less friction and wasted energy.

📖 Definitions

🔍 Worked example — Easy

A step-down transformer has 2000 turns on the primary coil and 50 turns on the secondary coil. If the input voltage is 230 V, calculate the output voltage.

🔍 Worked example — Standard

A transformer increases the voltage from 12 V to 240 V. If the primary coil has 10 turns, how many turns are in the secondary coil?

🔍 Worked example — Exam-style

A power station transmits power at 10,000 V through a cable with resistance R = 2.0Omega. The current is 50 A. Calculate the power loss in the cable. Then, calculate the power loss if the voltage is stepped up to 50,000 V for the same power transmission (assume P_transmitted is constant, so I changes).

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Example 1. A transformer has 200 turns on its primary coil and 4000 on its secondary. The primary is connected to a 230 V a.c. supply. Find the secondary voltage, and say what type of transformer it is.

🔍 Worked example — Easy

A transformer has 200 turns on its primary coil and 4000 turns on its secondary coil. The primary is connected to a 230 V a.c. supply. Calculate the secondary voltage and state if it is a step-up or step-down transformer.

🔍 Worked example — Standard

A phone charger acts as a step-down transformer. It converts 230 V to 5.75 V. If the primary coil has 2000 turns, how many turns are on the secondary coil?

🔍 Worked example — Exam-style

A power station transmits 1.0 MW of power. Compare the current at 10,000 V and 400,000 V. How many times smaller is the current at the higher voltage?

⚠️ Common errors

Common mistakes

Practice

Answers: (1) Soft iron is easily magnetised and demagnetised, so it carries the changing magnetic field from the primary to the secondary efficiently. (2) A d.c. current produces a steady magnetic field; a steady field does not induce an e.m.f. in the secondary coil. (3) N_p = 60 × 240/12 = 1200 turns. (4) For the same power, a higher voltage means a smaller current; a smaller current wastes much less energy as heat in the resistance of the cables, and thinner, cheaper cables can be used.

Ch 16 · 4.5.6 The Transformer · O Level Physics 5054

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