Lenz's law states that when a magnetic field induces an electric current in a conductor, the induced current creates its own magnetic field that opposes the change in magnetic flux that caused it.
Drag the bar magnet towards or away from the coil (or use the ← / → arrow keys or on-screen buttons). Watch the live flux Φ and e.m.f. readings: moving a North pole towards the coil induces an induced North pole on the near end to repel it, whereas pulling it away creates an induced South pole to attract it back.
According to Faraday's and Lenz's laws, an electromotive force (e.m.f.) and current are only induced when there is a change in magnetic flux over time (e.m.f. = −dΦ/dt). When the magnet stays still, the flux through the coil remains constant, so no current flows — you can see this directly as a flat line on the live flux graph.
Moving the magnet faster increases the rate of change of magnetic flux. This results in a higher induced e.m.f., causing a larger current to flow and making the light bulb glow brighter. Try the speed slider to compare slow and fast passes on the graph.
Click the "Flip poles" button or press the Up/Down Arrow Keys (↑ / ↓) on your keyboard to swap the North and South poles of the magnet.