Inductors & coils

In one line

An inductor is a coil of wire that stores energy in a magnetic field. It is the mirror image of a capacitor: a capacitor resists changes in voltage, an inductor resists changes in current.

How it works

Current through the coil creates a magnetic field; changing the current changes the field, which pushes back (Lenz's law). Two consequences make inductors useful:

• Smoothing & filtering — paired with a capacitor, an inductor forms the heart of power supplies (DC-DC converters) and filters that pass some frequencies and block others.
• Coupling without contact — two coils placed near each other transfer energy through the shared magnetic field. No wires touch. This is inductive coupling.

Where it shows up in digital health

• Wireless charging of implants — a pacemaker, cochlear implant or neurostimulator is charged through the skin by inductive coupling, so there's no wire through the body.
• MRI gradient coils — large, precisely-driven coils create the changing magnetic fields that encode where a signal comes from. (The loud knocking of an MRI is the gradient coils flexing.)
• RFID / NFC tags on consumables and wristbands are powered by the reader's coil.
• Switching power supplies inside every device step voltages up or down efficiently.

Watch for

Inductors hate sudden interruptions — break the current fast and the collapsing field produces a large voltage spike. That's why a coil-driven load (a relay, a solenoid valve) needs a flyback diode to absorb the kick and protect the switch.

Key takeaways

• An inductor stores energy in a magnetic field — the mirror of a capacitor; it resists changes in current.
• Used for smoothing/filtering (power supplies) and contactless coupling (energy through a shared field).
• In health: wireless implant charging, MRI gradient coils, RFID/NFC, switching supplies.
• Interrupting its current makes a voltage spike — coil loads need a flyback diode.