Transformers

Objectives

Lesson outcomes

Describe the structure of a simple transformer and explain why it works with alternating current.
Use the terms primary, secondary, step-up, and step-down correctly.
Use the turns ratio to calculate voltage changes across a transformer.
Explain why high-voltage transmission reduces power loss in cables.

Syllabus

CIE 0625 syllabus points

8 linked

4.5.6.1 Describe the construction of a simple transformer with a soft-iron core, as used for voltage transformations
4.5.6.2 Use the terms primary, secondary, step-up and step-down
4.5.6.3 Recall and use the equation Vp Np Vs = Ns where p and s refer to primary and secondary
4.5.6.4 Describe the use of transformers in high- voltage transmission of electricity
4.5.6.5 State the advantages of high-voltage transmission
4.5.6.6 Explain the principle of operation of a simple iron-cored transformer
4.5.6.7 Recall and use the equation for 100% efficiency in a transformer IpVp = IsVs where p and s refer to primary and secondary
4.5.6.8 Recall and use the equation P = I2R to explain why power losses in cables are smaller when the voltage is greater

Definitions

Required definitions

Transformer
a device with a primary coil, secondary coil, and soft-iron core that changes an alternating voltage.
Step-up transformer
a transformer with more turns on the secondary coil than the primary coil, so the output voltage is larger.
Step-down transformer
a transformer with fewer turns on the secondary coil than the primary coil, so the output voltage is smaller.

Lesson Notes

Student guidance and lesson notes

Overview

This lesson takes the induction idea from the previous lesson and applies it to transformers. You need to understand both the small-scale physics inside the transformer and the big-scale reason they matter in electricity transmission.

What You Need to Know

A simple transformer has a primary coil, a secondary coil, and a soft-iron core.
The alternating current in the primary coil produces a changing magnetic field in the core.
That changing field links with the secondary coil and induces an alternating e.m.f. in it.
A step-up transformer has more turns on the secondary than the primary, so the output voltage is larger.
A step-down transformer has fewer turns on the secondary, so the output voltage is smaller.
The voltage ratio matches the turns ratio: Vp / Vs = Np / Ns
For an ideal transformer, input power equals output power, so if voltage goes up, current goes down.
High-voltage transmission is useful because a smaller current means smaller power losses in the cables, since power loss depends on I^2R.

How to Work Through It

Start by drawing a simple labelled transformer and tracing the energy transfer from primary to secondary.
Sort examples into step-up and step-down transformers by comparing the number of turns.
Practise turns-ratio calculations until you can move between turns and voltage confidently.
Finish by linking high voltage, low current, and lower cable losses in transmission questions.

Check Your Understanding

Why does a transformer need alternating current rather than direct current?
How can you tell from the turns which transformer is step-up and which is step-down?
If the voltage increases in an ideal transformer, what must happen to the current?
Why does using a high voltage reduce energy losses in transmission cables?

Common Mistakes

Reversing the turns ratio. Keep primary and secondary values in the same order throughout the calculation.
Thinking a transformer creates energy. It transfers energy from one coil to another.
Forgetting that high voltage is useful in transmission because it allows a lower current for the same power.

Next Steps

Practise both turns-ratio and ideal-power questions until the relationship between voltage and current feels secure.
Keep the direction ideas from earlier lessons in mind because they reappear in the charged-particle lesson.