# Lenz's law

Lenz's law is an important concept in electromagnetism. It states that when a voltage is created by a change in magnetic flux, the induced voltage must create a current whose magnetic field is in opposition to the change which produces it.[1]

The induced magnetic field is always arranged so that a loop of wire with the induced current has constant magnetic flux. The practical use of this law is almost endless, and is how electrical generators work. With the sole exception of photovoltaic cells, every man-made power source such as hydroelectric facilities, nuclear power plants, and natural gas power plants transfer the energy from their fuel or flow into useful electricity by these generators. A generator produces an alternating current, which is what 99% of the world's electricity grid uses.[2]

## PhET Simulation

Lenz's law can be explored using the PhET simulation below that was graciously provided to us by the University of Colorado. As the magnet's velocity varies near the inductor (the loop of wire), a voltage is induced across it which drives the current, and lights up the light bulb.

Generators spin which changes the orientation of the magnet's poles, in order to produce alternating current. This is analogous to placing the magnet inside the loop and repeatedly pressing the pole-reversing button, near the bottom right of the simulation.