How does an induction hob work?

The principle

Induction hob. Under the glass of each cooking zone is a flat coil of copper wire, and electronics that feed it a very fast flipping current.

Step 1: A coil makes a flipping magnet. Electricity in a coil turns it into a magnet. Here the current flips direction about 20,000 to 100,000 times a second (about 20-100 kHz; most hobs about 25-50 kHz), so the coil’s magnet flips north-south just as fast. In the video it is slowed down a lot.

Step 2: Swirling currents in the pan. A changing magnet pushes on the free electricity in any metal near it. In the flat pan base it pushes it round and round in circles, like little whirlpools in a stream (these are called eddy currents). Every time the magnet flips, the swirl turns the other way. The metal resists these currents, and that resistance turns them into heat, right inside the pan base.

Step 3: Why the pan needs iron. Iron (cast iron, most steel and many stainless pans) is magnetic. It pulls the magnet’s field into itself and squeezes the swirling currents into a thin outer layer of the base, where they meet a lot of resistance and make a lot of heat. Aluminium, copper and glass are not magnetic, so they heat far too little, and the hob notices that there is no suitable pan and stops. Quick test: if a fridge magnet sticks to the bottom of the pan, it works.

Step 4: Fast and efficient. Because the heat is made in the pan itself, about 85% of the energy ends up in the food, compared with about a third for gas, where much of the flame’s heat goes past the pan into the kitchen. A strong 3 kW zone can boil 1 litre of water in about 2 minutes.

Step 5: Putting it together. Flipping current, flipping magnet, swirling currents, hot pan. Lift the pan and the heating stops at once; the glass under it is only warm because the hot pan sat on it.

Induction, ceramic or gas?

• Induction: Upside: very fast, about 85% efficient, quick to turn down, the glass doesn’t glow, easy to clean. Downside: needs pans with a magnetic base, costs more, and some hobs hum or buzz a little with certain pans.
• Ceramic (radiant) electric: Upside: works with any flat pan, cheaper. Downside: the glass gets red hot, slower to heat and to cool down, so less control.
• Gas: Upside: works with any pan, you can see the flame. Downside: only about a third of the heat goes into the pan, and it heats the kitchen and puts exhaust fumes in the air.

Try it yourself: hold a fridge magnet against the bottom of your pans. Those it sticks to will work on induction.