Why does a magnet make a nearby coil of wire light a bulb — but only when it's moving?
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Why does a magnet make a nearby coil of wire light a bulb — but only when it's moving?
The short answer
A magnet lights a coil's bulb only while it is moving because electricity is pushed by a *changing* magnetic field, not by a magnet sitting still. A strong magnet parked motionless inside the coil makes no current; sweeping that same magnet in and out makes the bulb glow, and faster sweeps glow brighter.
How it works
A magnet has an invisible field around it, and a coil of wire can carry current that lights a bulb — but only if something pushes the charges along. When a magnet moves toward or away from the coil, the amount of field passing through the loops keeps changing, and that change is what shoves current around the wire. Hold the magnet still and the field through the coil stops changing, so no current flows and the bulb stays dark. Move it faster, or use more loops of wire, and the push gets stronger, so the bulb glows brighter.
What people get wrong
People often think a magnet powers nearby wires just by being strong and close. But strength and closeness alone do nothing: a powerful magnet sitting perfectly still inside a coil produces zero current. What matters is motion — the magnetic field through the coil has to be changing. The moment the magnet stops, the light dies.
The catch
Moving the magnet faster makes more electricity and a brighter bulb, but it always takes effort to keep something moving, and the induced current actually pushes back against your hand, so the harder you crank, the harder it resists — you never get the electricity for free. Leaving the magnet still costs no effort at all, but it also gives you exactly zero power. No motion, no electricity.
Questions kids ask
Why doesn't a still magnet light the bulb even if it's really strong?
Because electricity in the coil is pushed by a *changing* field, not by the field itself. A still magnet gives a steady, unchanging amount of field through the coil, so nothing pushes the current and the bulb stays dark — no matter how strong the magnet is.
How does moving a magnet faster make the bulb brighter?
The faster the magnet moves, the faster the field through the coil changes, and a faster change pushes harder on the current. A bigger push means more current, which makes the bulb glow brighter.
Is this how real power plants make electricity?
Yes. A power-plant generator spins big magnets past coils of wire over and over, so the field through the coils keeps changing and current keeps getting pushed. Whether the spinning comes from steam, falling water, or wind, the core trick is the same moving-magnet idea.
Why do you feel the magnet push back when you move it near the coil?
When the moving magnet makes current flow, that current creates its own magnetic field that resists the change — so it pushes back on the magnet. That push-back is why making electricity always takes a bit of work; the energy in the bulb comes from the effort of your hand.
For grown-ups
This is Faraday's law of electromagnetic induction: the voltage (EMF) induced in a coil is proportional to the rate of change of magnetic flux through it (EMF = −N·dΦ/dt). A stationary magnet gives steady flux, so dΦ/dt is zero and no current flows; moving it changes the flux and drives current, and faster motion or more turns gives a larger EMF. The minus sign is Lenz's law — the induced current opposes the change, which is why you feel the magnet resist and why generating electricity always requires work. Every power-plant generator is this same trick scaled up: magnets and coils spun past each other.