Magnetic ‘magic’ offers a cool way to charge, cook

Charging a cellphone on an induction pad seems like magic, and in a way it is. Induction is a feature of electromagnetism that Michael Faraday stated in 1831, now known as Faraday’s law. It is the basis for much of modern electrical and electronic technology.

Faraday’s law explains the workings of electric motors, generators, transformers and inductors. These are useful to us even if we are unable to say exactly what magnetism is, other than it is a fundamental force like gravity and it is a close relative of electricity.

Faraday discovered that when he inserted a magnet into a coil of wire, an electric current flowed in the wire. Surprisingly, the current flowed only when the magnet or the coil moved. With both the magnet and coil stationary, there was no current even if the magnet was inside the coil.

Faraday later reasoned that the relative motion between the magnet and coil is equivalent to changing the magnetic field inside the coil. As the magnet moves into the coil, the strength of the magnetic field inside the coil changes from zero when the magnet is outside to a maximum when completely inside.

Faraday also discovered that moving the magnet in and out of the coil caused the electric current to change direction, flowing one way as the magnet entered the coil and the opposite direction as it exited.

Faraday also experimented with using electricity to generate magnetic fields. Hans Christian Orsted discovered in 1819 that an electric current generates a magnetic field encircling it. Bending the wire into a coil enhances the magnetic field, and inserting a piece of iron creates an electromagnet with the same properties as a permanent magnet as long as current flows through the coil.

When an alternating current passes through the coil, it generates an oscillating magnetic field, which can then interact with another coil.

Just like moving a magnet in and out of a coil, the changing magnetic field caused by the alternating current in the first coil induces an alternating current in the second coil.

To charge a battery, a coil inside the charging plate generates a changing magnetic field that induces electricity to flow through the battery circuit of the device it is charging. There must be special circuitry in the device that contains a coil for this type of charging.

Induction stovetops work in the same way, except the target device is not a battery to be charged, but rather a pot or pan to be heated.

A powerful alternating current flows through the burner pads on the stove. The magnetic field that this induces interacts with the magnetic material in the pot or skillet. Similar to the way a microwave causes molecules of water or fat to shake, the magnetic field from the coil causes the molecules of the iron to vibrate, thus giving them thermal energy, which appears as heat and raises the temperature of the cookware.

Induction cooking is energy-efficient because the cooking surface does not get hot even as it heats the cookware. The downside is that it works only on iron or stainless-steel cookware, not aluminum.

If you are considering an induction stovetop, think first about your favorite aluminum pots, pans and skillets that will be useless for inductive cooking.