physics//electromagnetism//electromagnetic induction
Electromagnetic induction is the creation of a voltage in a conductor by a magnetic field that changes through it, and it is how electricity is generated, stepped up and down, and read wirelessly: generators, transformers, induction hobs, wireless chargers, guitar pickups and magnetic brain stimulation all run on it. The field itself does nothing; only its change does.
Electromagnetic induction is the creation of a voltage in a conductor by a magnetic field that changes through it, and it is how electricity is generated, stepped up and down, and read wirelessly: generators, transformers, induction hobs, wireless chargers, guitar pickups and magnetic brain stimulation all run on it. The field itself does nothing; only its change does.
Picture a loop of wire and a magnet. Hold the magnet still inside the loop and nothing happens, however strong it is. Push it in, and while it moves the magnetic flux through the loop grows, and a voltage appears around the loop that drives a current; stop, and the current stops; pull it out, and the current flows the other way. Faraday's law says the induced voltage equals the rate of change of the flux through the loop, multiplied by the number of turns:
E=−N dΦdt\mathcal{E} = -N\,\frac{d\Phi}{dt}E=−NdtdΦ
Here Φ\PhiΦ is the flux through one turn and NNN the number of turns, so a faster change or more turns give more voltage. The minus sign is Lenz's law: the induced current flows in the direction whose own magnetic field opposes the change that produced it.
No change, no current.
Induction converts motion or a changing current into electrical push with nothing touching; the price is that it only works while something changes, which is why the grid runs on alternating current.
Lenz's law is felt as a brake. A magnet dropped down a copper pipe falls slowly: as it passes, it changes the flux through each ring of the pipe, the induced eddy currents make fields that push back on it, and the energy of the fall ends up as heat in the copper. Magnetic train brakes and induction hobs use the same currents on purpose; transformer cores are laminated to stop them.
The conductor can be tissue. A brief, strong pulse from a coil held on the head induces currents in the cortex underneath, enough to make neurons fire (transcranial magnetic stimulation in Lobeworks/17); a weaker changing field, around 10 millitesla at 20 Hz, induces currents in the retina that are seen as flickering lights, magnetophosphenes, harmless and the first effect people notice near strong alternating fields.
Induction also works within one coil: a coil opposes changes in its own current, which is the defining behaviour of an inductor.