Electromagnetic induction and transformers
Physics · GCSE · The School
Moving a conductor in a magnetic field induces a voltage
Move a wire through a magnetic field and a voltage is induced across it. Moving the field past the wire instead works too. If the wire is part of a complete circuit, a current flows. This is the generator effect. It is the reverse of the motor effect: motion makes a current, instead of a current making motion.
Three ways to induce more voltage
A bigger voltage is induced if the wire moves faster. It also grows if the magnetic field is stronger. Winding the wire into a coil with more turns has the same effect. A bicycle dynamo, a wind-turbine generator and a power-station alternator all use this one effect, just at very different scales.
Generators produce alternating current
Picture a coil turning inside a magnetic field. Each side of the coil cuts through the field in a different direction as it spins. So the current keeps swapping direction too. That swapping current is called alternating current, or AC — and it is what mains electricity is. A different design, using a split-ring commutator, keeps the current flowing one way instead. That gives direct current, or DC.
A transformer changes the size of an AC voltage
A transformer has two coils wound around one iron core: a primary coil and a secondary coil. The two coils are not connected to each other. An alternating current in the primary coil makes the magnetic field in the core keep changing. That changing field induces a voltage in the secondary coil. A step-up transformer has more turns on the secondary coil, so it raises the voltage. A step-down transformer has fewer turns, so it lowers it.
Transformers only work on AC
A transformer needs the magnetic field to keep CHANGING. Only that induces a voltage in the secondary coil. Direct current makes a constant field instead — one that never changes. A constant field induces nothing at all. So a transformer plugged into a DC supply simply does not work. This is one reason mains power is generated and sent out as AC, not DC.
Why the National Grid uses very high voltage
The same power sent at a higher voltage means a lower current. And it is current, not voltage, that heats up a cable and wastes energy. So the Grid steps voltage up very high for the long journey from a power station. Nearer homes, it steps the voltage back down to a safe level. Less energy is lost as heat along the way.