Edexcel IGCSE Physics · Spec 6.8-6.9
Transformers
How transformers change the voltage of an alternating supply.
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Transformers
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Explained
Transformers, and the journey across the National Grid
A transformer changes the size of an alternating voltage. It is two coils of insulated wire wound on the same soft iron core, and no electrical connection runs between them at all.
How it works
An alternating current in the primary coil produces a magnetic field that is constantly changing, because the current itself is constantly changing direction.
The iron core carries that changing field round to the secondary coil. A changing magnetic field through a coil induces a voltage in it, so an alternating voltage appears across the secondary.
This is why a transformer will not work on direct current. A steady current gives a steady magnetic field, a steady field induces nothing, and the secondary coil stays dead. Connect a transformer to a battery and the only moment anything happens is when you make or break the connection, because that is the only moment the field is changing.
The word induced is worth using. Nothing crosses between the coils except the magnetic field, and saying that the primary sends electricity to the secondary describes a connection that does not exist.
Step up and step down
The ratio of the voltages equals the ratio of the numbers of turns. More turns on the secondary than the primary gives a higher output voltage, which is a step up transformer. Fewer turns on the secondary gives a lower output voltage, which is a step down transformer.
The equation is the primary voltage divided by the secondary voltage equals the primary turns divided by the secondary turns. Which quantity goes on top does not matter as long as primary is over primary and secondary is over secondary on both sides.
Voltage is not created out of nothing. In an ideal transformer the power in equals the power out, so stepping the voltage up steps the current down by the same factor, and the other way round. That trade is the entire reason transformers are useful.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme awards five marks for explaining the transmission of electricity across the National Grid, and prints seven points to choose from.
A step up transformer is used before transmission. The voltage is increased before transmission. The current is reduced before transmission. There is less heating in the transmission wires. Less energy is wasted. A step down transformer is used after transmission. And the voltage is decreased after transmission for safety, allowing the point that a larger current is required by some household devices.
Its note adds that these ideas can be credited from a diagram, so a clearly labelled sketch of the grid can earn the marks.
Read the seven as a single chain in order, because that is what they are. Every point names either a transformer, a quantity that changed, or a consequence, and the middle three are the physics: voltage up, current down, less heating. An answer that jumps from step up transformer to less energy wasted has stated the beginning and the end and left out the reason.
Notice that the final point is about safety rather than efficiency. Stepping back down is not done to save energy; it is done because 400 000 volts arriving at a house would be lethal, and because appliances are built for 230 volts.
Why high voltage transmission saves energy
The energy wasted heating a cable depends on the current squared multiplied by the resistance of the cable.
Squared is what makes this worth doing. Reducing the current to a tenth reduces the heating loss to a hundredth. Transmitting at 400 000 volts instead of 230 volts allows a current so small that the losses over hundreds of miles of cable stay manageable.
The resistance of the cable is fixed by its material and its length, so the current is the only quantity that can be changed. That is why the answer to why we transmit at high voltage is always really about keeping the current low.
Why they are not perfectly efficient
Transformers are among the most efficient machines built, but some energy is still wasted.
Resistance in the coils heats the wire. The alternating field induces currents circulating within the iron core itself, which heat it, and laminating the core into thin insulated layers reduces them. And magnetising and demagnetising the core repeatedly takes energy, which is why soft iron is used rather than steel.
Efficiency is calculated as the useful output power divided by the total input power, multiplied by 100, with power found from current times voltage on each side.
Spec 6.8-6.9
What you need to know
- Describe how a transformer works
- Tell step-up and step-down transformers apart
- Explain their use in the National Grid
Active recall
Quick check
Answer each question before opening the answer.
What does a transformer do, and why must it use a.c.?
It changes the size of an alternating voltage; it needs a changing (a.c.) current so the magnetic field keeps changing and induces a voltage.
What is the difference between a step-up and a step-down transformer?
Step-up increases the voltage (more turns on the secondary); step-down decreases it (fewer turns on the secondary).
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