How does a changing magnetic field produce a current — and how does that principle power the National Grid?
Magnetism and electricity are two aspects of the same phenomenon. A current-carrying wire creates a magnetic field around it; a wire in a magnetic field experiences a force if a current is flowing (the motor effect). The reverse — a wire moving through a magnetic field — induces an EMF (the generator effect). These two principles, discovered by Faraday and summarised by Lenz's Law (induced effects oppose the change that caused them), underpin every electric motor and generator on Earth.
Transformers exploit electromagnetic induction to change AC voltages. The National Grid uses step-up transformers to transmit electricity at very high voltages (and low currents), drastically reducing resistive heating losses in cables (P = I²R). Step-down transformers then reduce the voltage to safe levels for homes and businesses. Transformers only work with AC — a constant DC current creates no changing flux, so no EMF is induced.
A transformer has 400 turns on the primary coil and 20 turns on the secondary. (a) If the input voltage is 230 V, what is the output voltage? (b) If the primary current is 0.5 A, what is the secondary current (assume 100% efficiency)? (c) Why does the transformer not work with DC?