--- title: "Transformers and the National Grid" description: "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 " canonical: https://lightmysky.com/learn/science/transformers-and-the-national-grid-mt_ExV4Kc_7_P source: https://lightmysky.com/learn/science/transformers-and-the-national-grid-mt_ExV4Kc_7_P.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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. Subject: Science · Area: Energy · Ages 14 to 16 Page: https://lightmysky.com/learn/science/transformers-and-the-national-grid-mt_ExV4Kc_7_P ## Ready when they can - 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 ## Lesson: Shifting voltage for the long journey 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. **Example.** 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. **Tip.** 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. **Recap.** Turns set the voltage, power stays constant, and high voltage carries power with less waste. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Electromagnetic Induction and Generators](https://lightmysky.com/learn/science/electromagnetic-induction-and-generators-mt_e0jgUKANT-) - [Mains Electricity: a.c., Earthing and Fuses](https://lightmysky.com/learn/science/mains-electricity-a-c-earthing-and-fuses-mt_jKTotBL78P) ## Opens up - [Lenz's Law and the Alternating-Current Generator](https://lightmysky.com/learn/science/lenzs-law-and-the-alternating-current-generator-mt_69u-fTOSku) - [Static Charge and Electric Fields](https://lightmysky.com/learn/science/static-charge-and-electric-fields-mt_6ii6CVOTI5)