Transformers and the National Grid
A transformer uses a changing magnetic field to raise or lower an alternating voltage, in the ratio of the turns on its two coils. The grid steps voltage up for transmission because a smaller current wastes less energy heating the cables.
What a learner can do afterwards
- Uses the turns ratio to find an output voltage, and states why a transformer does nothing with direct current
- Applies power in equals power out for an ideal transformer to find the secondary current
- Explains why transmitting at high voltage and low current cuts the energy wasted in the cables
1 · Read
A transformer is two coils sharing one iron core, with no wires joining them. Alternating current in the first coil makes a changing magnetic field, and that change induces a voltage in the second coil. The voltages follow the turns: twice the turns means twice the voltage.
A transformer with 100 turns on the primary and 200 on the secondary doubles the voltage. Feed in 230 V and you get 460 V out. Direct current cannot do this trick, because a steady current makes a steady field and only a changing field induces voltage.
An ideal transformer moves power without loss, so power in equals power out. Stepping voltage up forces current down by the same factor: doubling voltage halves the current. Cables waste energy as heat in proportion to current squared, so high voltage with low current slashes the waste.
That trade is why the grid steps up to around 400 kV for long lines, then steps back down near towns and homes. Generators make AC, motors use it to spin, and transformers shift its voltage. Every step keeps waste low and users safe.
Turns set the voltage, power stays constant, and high voltage carries power with less waste.
More to read
23.7 Transformers
A transformer is two coils sharing one iron core, with no wires joining them. Alternating current in the first coil makes a changing magnetic field, and that change induces a voltage in the second coil. The voltages follow the turns ratio: twice the turns means twice the voltage.
Direct current cannot do this trick, because a steady current makes a steady field and only a changing field induces voltage. That is why transformers work on mains AC but sit silent on a battery. Step up transformers raise voltage for the journey, and step down transformers lower it for safe use.
15.6 Transformers
An ideal transformer moves power from coil to coil without loss, so power in equals power out. That means stepping voltage up forces the current down by the same factor. A transformer that doubles voltage halves the current, keeping V times I the same on both sides.
This trade is the whole point of the grid. Cables waste energy as heat in proportion to current squared times resistance, so pushing power at high voltage and low current slashes the waste. The grid steps up to around 400 kV for long lines, then steps back down near towns and homes.
20.2 Motors, Generators, and Transformers
Motors, generators, and transformers are one family built on changing magnetism. Generators spin coils in a field to make AC, motors feed current into a field to spin, and transformers pass AC between coils through a changing field. All three only work because motion or alternation keeps the field changing.
Follow the energy from power station to phone charger. The station generates at moderate voltage, transformers step it sky high for thin efficient lines, and substations plus chargers step it down again. Every step exists to keep waste low and users safe.
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2 · Watch
En anglaisTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.