How an Induction Cooktop Heats the Pan, Not the Hob
An induction cooktop uses a magnetic field to make heat inside the pan itself. See how the coil works, why only some pans suit it, and how efficient it is.
In this article
An induction cooktop does not heat its glass surface and then pass the heat to a pan. Instead, a coil under the glass creates a changing magnetic field, and that field makes electric current flow inside the metal of the pan. The pan's own resistance turns that current into heat. This article explains each step, why the pan material matters, and what published tests say about efficiency.
The coil under the glass
Below the ceramic or glass surface sits a coil of copper wire. When the cooktop is on, an alternating current runs through this coil. Alternating current reverses direction many times a second, and the Wikipedia article on induction cooking gives a typical frequency of about 25 to 50 kHz, meaning tens of thousands of reversals every second.
A current in a coil creates a magnetic field around it. Because the current keeps reversing, the field keeps rising, collapsing and flipping. A changing magnetic field is the key ingredient, because it can push current through any nearby piece of metal. This effect is called electromagnetic induction.
How the pan becomes the heater
When a suitable pan sits on the surface, the changing field passes into its base and drives circular currents around inside the metal. These are called eddy currents. Engineers describe the whole arrangement as a transformer: the coil is the primary winding, and the pan acts as a single turn of wire that is shorted out, so it carries a large current.
Metal resists the flow of current, and a current pushed through resistance produces heat. This is the same effect that warms the element in a kettle or toaster, but here it happens inside the pan base. The heat then moves into the food by ordinary conduction. The glass surface is not the source of heat, although it can still become warm because the hot pan touches it.
There is a second, smaller source of heat in iron and steel. Magnetic materials lose a little energy each time their internal magnetic regions flip direction, a loss called hysteresis. According to Wikipedia, this accounts for less than 10 percent of the total heat in ferrous pots, so most of the heating comes from the eddy currents.
Why the pan material matters
Most induction cooktops need cookware that is ferromagnetic, which means a magnet sticks to it. Cast iron and many stainless steels qualify. Aluminium and copper pans usually do not work on a standard cooktop. A common explanation is that these metals conduct current so well that their low resistance produces little heat. Another explanation sometimes given is that the field couples to them less well.
- Cast iron and carbon steel pans normally work.
- Many stainless steel pans work, but not all grades are magnetic.
- Plain aluminium, copper and glass pans usually do not work.
- A magnet that sticks firmly to the base is a practical quick check.
There are exceptions on the market. Wikipedia notes that in 2009 Panasonic developed an all-metal induction cooker running at 60 kHz or higher, which can heat non-ferrous pans. Such designs are less common than the standard type.
The electronics that control it
An article on induction cookers in Electronics Online explains that a modern cooktop uses a resonant inverter, a circuit that switches power on and off very quickly to produce the high-frequency current for the coil. Switching is timed to cut wasted energy. The power delivered is set by changing the switching frequency, and a microcontroller watches the mains voltage and the current to do this.
The same article describes two common layouts. A quasi-resonant design uses one switching transistor, costs less and suits home cooktops up to about 2 kW. A half-bridge design uses two, is more stable and reaches about 5 kW, and is typically found in commercial equipment. The figures describe the reference designs in that article, and a particular product may differ.
Pan detection and safety
With no pan on the surface, the coil has nothing to transfer energy to, and the controller can detect this. The Electronics Online article says the unit then drops into a sleep mode with standby use below 1 W. Wikipedia adds that cooktops monitor whether a pan is present and switch off if none, or one that is too small, is found. A surface sensor can also cut the power if the pan overheats or boils dry.
Wikipedia states that the voltages induced in the pan are too low to be a shock hazard, and that induction burns no gas, so it releases no combustion fumes into the room. It also notes that people with pacemakers or other electronic implants are usually advised to avoid strong magnetic field sources. This is general information, and anyone with an implant should follow their doctor's advice and the maker's manual.
How efficient is it?
Efficiency here means the share of the electrical or fuel energy that ends up in the water or food. Wikipedia summarises several tests. In a 2014 study by the American Council for an Energy-Efficient Economy, induction heated water at about 76 percent efficiency, while electric coil hobs ranged from over 80 percent with large pans down to 40 to 50 percent with small ones, and gas from 35 to 40 percent down to 30 percent. A US Department of Energy test block from 2014 gave 70.7 to 73.6 percent for induction, 71.9 percent for electric coil and 43.9 percent for gas.
Results therefore vary with the test, the pan size and the amount of water. A 2023 study cited by Wikipedia found induction between 54 and 81 percent for boiling water, depending on volume. The fair summary is that induction usually lands in the mid-70s per cent in these tests, and it is generally ahead of gas, though not always ahead of a well-matched electric coil.
A short history
The idea is older than most kitchens. Wikipedia records that Arthur Berry applied for a British patent in 1906 and Simon Hohlfeld received a German patent in 1909. A commercial product followed much later: the Westinghouse Cool Top 2 went into production in 1973 and ended in 1975, with four burners of about 1,600 W each and a price of $1,500 in 1973 dollars. Sears Kenmore sold a range with four induction surfaces in the mid-1980s.
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