Induction hobs

Kitchens come in all shapes and sizes but you almost certainly have some sort of cooker at home. If you have a gas or electric cooker, it’s probably quite obvious how it heats your food. A gas cooker creates a flame and the heat from that flame is transferred to the pan that you’ve placed on top of it, heating the food inside. With an electric cooker, the hob itself heats up and again you have a transfer of heat from the hob to the pan. In both cases, the hob itself gets very hot and this heat is then used to heat the pan. But what about induction cookers, which heat the pan directly rather than the hob?

An induction cooker has a ceramic surface, topped with glass, beneath which are four coils of copper – one for each hob. When a hob is turned on, electricity runs through the coil and this generates an electric field. The French physicist Ampere discovered that whenever and wherever you have an electric field, you also have a magnetic field, and that you cannot have one without the other. At this point no heat is being generated because on induction cookers, the pan generates the heat rather than the hob. It relies on induction, a concept discovered by Michael Faraday FRS. He found that when he wrapped two wires close together and sent an alternating electrical current through one of them, an electrical effect was measured in the second wire. This is what is happening with an induction cooker.

When you put an iron-based pan on top of the glass, the magnetic field generates electric fields in the iron in the bottom of the pan. Since iron is not a very good conductor of electricity, it heats up and this is where the heat that cooks the food comes from. Although switched on, the hob will stay cool until a pan is placed on top; however, the pan does then heat up the hob and it will take time to cool after the pan is removed. It only works with pans that have iron in them; pans made of copper, for example, will not heat up because copper conducts electricity too well. The easiest way to check if a pan contains iron is to use a magnet, which will stick to the pan if it contains iron.

Safety tests have been carried out to see if the magnetic field has any impact on items that will potentially be nearby, such as rings on fingers and pacemakers. The British Heart Foundation advises that people with pacemakers should keep a distance of at least 60 centimetres from an induction hob while it is on, as it can interfere with the device, or avoid them completely.

Because the heat comes from the pan itself rather than the stove, heat is evenly spread throughout the pan making cooking more efficient – induction hobs can heat a pan of water to boiling point in half the time of a gas-powered one. The lack of heat on the hob itself means that food doesn’t burn onto the surface when it boils over, so the surface is easy to clean.

To learn more about the engineering marvels hidden in your home, watch Rhys discuss induction hobs and other engineers’ videos on Twitter at @QEPrize. The videos are part of the QEPrize’s #EngineeringInspiration video campaign, celebrating the launch of the 2021 QEPrize nominations.

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This article has been adapted from "How does that work - Inductions hobs", which originally appeared in the print edition of Ingenia 83 (June 2020).