Lenz's law

An induced current always flows in a direction such that its magnetic field opposes the change in flux that produced it.

The approaching N pole induces a current that makes the coil face an N pole too — repelling it.

the minus sign in EMF = −N × dΦ/dt

—: gives the direction of the induced current, not its size

In plain English: Nature resists change: push a magnet into a coil and the coil pushes back; pull it out and the coil tugs it back in. Energy conservation wearing a magnetic costume.

Where you'll meet it: Explains the flyback voltage spike when you switch off a relay coil (and why it needs a flyback diode), eddy-current brakes, and why motors draw less current once spinning (back-EMF).

Calculators that use this law

Synchronous Speed and Slip CalculatorN_s = 120f / P Faraday's Law EMF Calculatore = −N × (dΦ / dt)

Components this law governs

Inductor / CoilThe minus sign explains the direction: the induced voltage always fights whatever you are trying to do to the current.Choke / Common-mode ChokeCommon-mode noise on both lines induces opposing flux in the shared core and gets choked; the wanted differential signal cancels and passes.Transformer (power)Load the secondary and its current opposes the flux, forcing the primary to draw more — the mechanism by which power crosses the gap.Current / Potential Transformer (CT/PT)The secondary current opposes the core flux — and if you open-circuit a live CT nothing opposes it, so the flux and secondary voltage rise to lethal levels.TVS / Transient SuppressorThe spikes it exists to eat are Lenz's law in action — a relay coil or motor refusing to have its current interrupted.IGBTIt nearly always drives inductive loads (motors, induction heaters), so the freewheel path for the collapsing current is not optional.

More in Electromagnetism & induction

Ampère's circuital law Biot–Savart law Ampère's force law Faraday's law of induction Lorentz force law Fleming's left-hand & right-hand rules Hopkinson's law