Electromagnetic induction is also the principle behind transformers. An alternating current in the primary coil creates a changing magnetic field in the iron core, which induces an alternating voltage in the secondary coil.
Transformer equation: Vₛ/Vₚ = Nₛ/Nₚ (voltage ratio = turns ratio)
Why the National Grid uses high voltage: Electrical power = current × voltage. To transmit the same power at a higher voltage, less current is needed. Less current means less energy wasted heating the transmission cables (power wasted = I²R). Step-up transformers near power stations increase the voltage to around 400,000 V for transmission; step-down transformers near homes and businesses reduce it to 230 V for safe use.
Why transformers only work with AC: Transformers need a constantly changing magnetic field to induce a voltage in the secondary coil. Direct current produces a constant (non-changing) field, so it cannot induce a continuous voltage in the secondary coil. Alternating current continually reverses direction, so the magnetic field keeps changing, inducing a continuous alternating voltage in the secondary coil.
This higher tier covers Higher Tier: Transformers and the National Grid within Electromagnetic Induction for GCSE Physics. Revise Electromagnetic Induction in Magnetism for GCSE Physics with 18 exam-style questions and 12 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 6 of 9 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Practice questions for Electromagnetic Induction
What is electromagnetic induction?
Explain how a potential difference (voltage) is induced in a conductor in a magnetic field.