Changing magnetic fields induce voltage — Science, 14–17 years
A changing magnetic field can push charges through a wire, explaining how generators produce electricity.
What induction is
Move a magnet through a coil of wire and the magnetic field through the coil changes. That change creates a voltage across the wire, which can drive a current if the circuit is closed. The magnet is not simply giving the wire a permanent charge; motion is the important part.
Why induction was needed
Early scientists wanted to turn mechanical motion into electrical current, rather than rely only on chemical cells. Induction provided the missing link: a rotating magnet or coil continually changes a magnetic field and produces voltage. The direction follows a rule that opposes the change, which is why generators need continuous input of motion.
A coil and a magnet
Connect a coil to a sensitive meter and push the north pole of a magnet into it. The meter needle moves, showing an induced voltage. Hold the magnet still and the needle returns to zero; pull it out and the needle moves the opposite way. Faster motion gives a larger deflection.
The tempting mistake
It is easy to think that a strong magnetic field always creates a current. A stationary magnet beside a stationary coil produces no induced voltage, because nothing is changing through the coil. A stronger magnet, more turns of wire or faster motion can increase the effect, but none replaces the required change.
Where it is used
Power-station generators use turbines to rotate coils or magnets, producing the electricity sent through the grid. Transformers use changing current in one coil to induce a different voltage in another, allowing efficient transmission. Induction cookers also create changing magnetic fields that heat suitable metal pans directly.
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