Inductors & Electromagnetism

Every current makes a magnet

In 1820 Ørsted noticed a compass twitch beside a current-carrying wire: moving charge creates a magnetic field. Wind the wire into a coil and the field concentrates — an electromagnet, strengthened by more turns, more current, or an iron core. This is half of the deepest link in physics (electricity ⇄ magnetism), and the other half is just as good: a changing magnetic field pushes charges — induction. Generators spin coils near magnets to make electricity; motors run the trick backwards; transformers pass power between coils with no moving parts at all.

The inductor: inertia for current

A coil used as a component is an inductor. Its magnetic field stores energy, and that field resists being changed — so an inductor resists changes in current, the perfect mirror of the capacitor resisting changes in voltage:

Close a switch on an RL circuit and the current doesn’t jump — it ramps along the same exponential curve you met last lesson, reaching 63% of V/R after one τ = L/R. Steady current eventually flows as if the inductor were plain wire.

The inductive kick

Now open the switch. The current must stop almost instantly — so dI/dt is huge, and V = L·dI/dt means the coil generates a huge voltage spike (hundreds of volts from a 9 V circuit) trying to keep its current flowing. The spike arcs across switch contacts and kills transistors. The standard cure is beautifully simple: a flyback diode across the coil gives the current a safe loop to decay through, clamping the spike to under a volt. Every relay and motor-driver schematic you ever see will have one — now you know why.