Physics · Year 10 · Magnetism and Electromagnetism · Spring Term

Transformers and Power Transmission

Students will understand the function of transformers in changing voltage and their role in power transmission.

• National Curriculum Attainment Targets
• GCSE: Physics - Magnetism and Electromagnetism

About This Topic

Transformers play a key role in power transmission by changing voltage levels through electromagnetic induction. Year 10 students learn that a transformer consists of two coils wound around a soft iron core. Alternating current in the primary coil creates a changing magnetic field, which induces a voltage in the secondary coil. The voltage ratio depends on the number of turns in each coil: more turns in the secondary produce a step-up transformer for high-voltage transmission, while fewer turns create a step-down transformer for safe distribution.

In the context of the National Grid, step-up transformers at power stations raise voltage to around 400,000 volts to minimise energy losses, since power loss in cables follows P = I²R. Students analyse how lower current reduces heating in transmission lines and evaluate efficiency through calculations. This builds on magnetism and electromagnetism units, connecting theory to real-world systems like the UK's electricity network.

Active learning suits this topic well. Students constructing model transformers from nails, wire, and low-voltage AC supplies observe induction directly and test turns ratios with lamps or multimeters. Such practical work clarifies mutual induction and power loss, making abstract equations concrete and memorable.

Key Questions

1. Analyze how transformers efficiently change voltage levels.
2. Evaluate the importance of step-up and step-down transformers in the National Grid.
3. Design a transformer to minimize energy loss during transmission.

Learning Objectives

• Calculate the voltage and current in the secondary coil of a transformer given the primary voltage, current, and turns ratio.
• Explain the principle of electromagnetic induction as it applies to transformer operation.
• Evaluate the efficiency of a transformer by comparing input and output power, identifying sources of energy loss.
• Compare the advantages of transmitting electricity at high voltage versus low voltage for the National Grid.
• Design a basic transformer circuit to achieve a specified voltage change for a given load.

Before You Start

Electromagnetism and Magnetic Fields

Why: Students need to understand how electric currents create magnetic fields and how magnetic fields interact with conductors.

Alternating Current (AC)

Why: Transformers rely on a changing magnetic field, which is produced by alternating current, so understanding AC is crucial.

Electrical Power and Energy

Why: Students must be familiar with the concepts of power, voltage, current, and resistance, including the formula P=IV and the concept of power loss due to resistance (P=I²R).

Key Vocabulary

Electromagnetic Induction	The production of an electromotive force (voltage) across an electrical conductor in a changing magnetic field. This is the fundamental principle behind how transformers work.
Primary Coil	The coil in a transformer that is connected to the input AC voltage source. It creates the initial changing magnetic field.
Secondary Coil	The coil in a transformer where the voltage is induced by the changing magnetic field from the primary coil. This coil is connected to the output load.
Turns Ratio	The ratio of the number of turns of wire in the secondary coil to the number of turns in the primary coil. This ratio determines whether a transformer steps voltage up or down.
Step-up Transformer	A transformer that increases voltage from the primary coil to the secondary coil. It has more turns in the secondary coil than in the primary coil.
Step-down Transformer	A transformer that decreases voltage from the primary coil to the secondary coil. It has fewer turns in the secondary coil than in the primary coil.

Watch Out for These Misconceptions

Common Misconception

Transformers work with direct current (DC).

What to Teach Instead

Transformers require alternating current to produce a changing magnetic field for induction. Demonstrations comparing AC and DC inputs on model transformers show no output voltage with DC, helping students revise their ideas through direct observation and group discussion.

Common Misconception

Increasing voltage increases power losses in transmission.

What to Teach Instead

Higher voltage lowers current for the same power, reducing I²R losses. Transmission line simulations with lamps allow students to measure and compare losses firsthand, reinforcing calculations and correcting the belief via tangible evidence.

Common Misconception

Transformers create new electrical energy.

What to Teach Instead

Transformers transfer energy with some losses as heat; input power nearly equals output power. Efficiency calculations during model builds reveal small discrepancies, prompting students to explore eddy currents and resistance through hands-on testing and peer explanations.

Active Learning Ideas

Hands-On: Model Transformer Build

Provide iron nails, insulated wire, and low-voltage AC supplies. Students wind primary and secondary coils with varying turns, connect to a small bulb, and observe brightness changes. Measure input and output voltages with multimeters to verify the turns ratio formula. Groups discuss results and redesign for brighter output.

45 min·Small Groups

Simulation Game: Transmission Line Losses

Use batteries, resistors as 'cables', and lamps to model low and high voltage transmission. Connect in series for step-up simulation with thin wires over distance. Students measure current, voltage drop, and lamp brightness, then calculate power losses using P = I²R. Compare setups to see high-voltage benefits.

35 min·Pairs

Design Challenge: Efficient Grid

Challenge pairs to design a transformer system minimising loss to a distant bulb using available coils and wire lengths. Test prototypes, record data on voltage, current, and efficiency. Present best design to class with calculations explaining choices.

50 min·Pairs

Whole Class: National Grid Demo

Project a step-by-step journey of electricity from power station to home using transformer models. Students contribute predictions at each stage, then verify with live demos of step-up and step-down. Follow with quick paired calculations of losses.

30 min·Whole Class

Real-World Connections

• Electrical engineers at National Grid plc use step-up transformers at power stations like Hinkley Point C to raise voltage to 400,000 volts for efficient long-distance transmission across the UK.
• Domestic electricians install step-down transformers in consumer units to reduce mains voltage (230V) to safer levels for household appliances and lighting circuits.
• Manufacturers of electronic devices, such as mobile phone chargers and laptop power adapters, incorporate small, efficient transformers to convert mains electricity to the low DC voltages required by the devices.

Assessment Ideas

Quick Check

Present students with a diagram of a simple transformer showing the primary and secondary coils and the number of turns on each. Ask them to calculate the output voltage if the input voltage is 12V AC and state whether it is a step-up or step-down transformer.

Discussion Prompt

Pose the question: 'Why is it more efficient to transmit electricity at very high voltages over long distances, even though it requires expensive step-up and step-down transformers?' Guide students to discuss power loss (P=I²R) and the relationship between voltage, current, and power.

Exit Ticket

Ask students to write down two key differences between a step-up and a step-down transformer, and one reason why transformers are essential for the UK's National Grid.

Frequently Asked Questions

How do transformers work in the National Grid?

Transformers step up voltage at power stations to 400 kV for low-current, efficient transmission across the UK, minimising I²R losses in cables. Step-down transformers then reduce voltage progressively for homes and businesses. Students grasp this by modelling the process, seeing how the turns ratio Vp/Vs = Np/Ns enables safe, low-loss delivery of power.

Why use step-up transformers for power transmission?

Step-up transformers increase voltage and decrease current, cutting power losses since loss = I²R. For example, doubling voltage halves current, quartering losses. Practical simulations with wires and lamps let students quantify this, linking equations to real National Grid efficiency and UK energy needs.

How can active learning help students understand transformers?

Building model transformers with coils and iron cores gives direct experience of mutual induction, as students see lamps brighten with more secondary turns. Transmission simulations reveal power loss patterns through measurements, while design challenges encourage applying turns ratio and efficiency formulas. These approaches build confidence, correct misconceptions, and connect abstract GCSE concepts to observable phenomena.

What causes energy loss in transformers?

Main losses are eddy currents and hysteresis in the core, plus resistance in coils causing heat. Students calculate ideal efficiency as (Ns/Np)² but measure real values below 100% in models. Hands-on tests with thermometers detect heating, helping evaluate designs and appreciate National Grid mitigations like laminated cores.