Faraday's Law
Move magnet through coil. Builds on Lorentz Force.
You push a bar magnet toward a coil of wire connected to a galvanometer (a sensitive current meter). The magnet has a fixed strength. You try pushing it slowly, then quickly.
What determines the size of the induced voltage (EMF) in the coil?
It’s not the flux itself — it’s how FAST the flux changes! Hold a powerful magnet perfectly still next to a coil and you get zero voltage. But shove it through quickly and the galvanometer needle swings hard. Faraday discovered that nature cares about the rate of change: EMF = -dΦ/dt.
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Drag the bar magnet toward and through the coil. Move it slowly, then quickly — watch how the galvanometer responds to the SPEED of your motion, not just the magnet’s position.
ε = -N dΦ/dt
No change in flux, no EMF. It doesn’t matter how strong the field is — only how fast it’s changing. Move a magnet twice as fast, get twice the voltage.
Every power plant — coal, gas, nuclear, wind, hydro — uses Faraday’s law. Spinning coils in magnetic fields create the changing flux that drives the electricity powering civilization.
A rapidly alternating magnetic field below the glass surface induces currents directly in your metal pot. The pot itself becomes the heater — the cooktop stays cool.
Your phone’s wireless charger uses a coil to create a changing magnetic field. The coil inside your phone picks up that changing flux and converts it back to current.
A vibrating steel string changes the magnetic flux through a coil wrapped around a small magnet. That tiny induced EMF becomes the signal that gets amplified into rock and roll.
“A changing magnetic field creates an electric force. That’s it. That’s the law that powers the modern world.”
Faraday’s Lab
Explore electromagnetic induction with adjustable coils and magnets. Watch real-time graphs of magnetic flux Φ(t) and induced EMF(t) = -dΦ/dt. See how coil turns, magnet strength, and magnet shape all affect the result.
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Start with a single-turn coil and push the magnet through slowly. Watch the Φ(t) curve rise and the EMF spike. Now double the turns — the EMF doubles! Try the ring magnet vs the bar magnet. Pull the magnet back out and watch the EMF reverse direction. Can you make the EMF as large as possible?
Faraday’s law EMF = -NdΦ/dt is the foundation of electromagnetic induction. The EMF depends on how fast the flux changes, not on the flux itself. More turns multiply the effect. The negative sign means nature always opposes the change.