1. How a Cooktop Makes the Pan Itself the Heat Source
Under the glass-ceramic top of an induction cooktop sits a coil of copper wire. Alternating current, which keeps reversing direction, runs through it, and the coil makes a magnetic field that changes very quickly. One encyclopedia entry puts the rate at roughly 25 to 50 thousand changes of current direction each second (25–50 kHz). [1]
When a pan with a suitable metal base sits above the coil, the changing field makes circular electric currents flow inside the base. These are called eddy currents. Metal resists the flow of electricity a little, and that resistance turns the currents into heat. [1] [4]
So the heat starts inside the pan, not in the cooktop. This is why the glass does not get very hot from the cooktop's own heating, although it warms up when a hot pan sits on it. [1] It also means a cooktop with no pan on it has almost nothing to heat.
But why would a changing magnetic field push electricity around in the first place? To answer that, we need to go back to a basement laboratory in London.
2. Faraday's Iron Ring in 1831: Where Making Electricity from Magnetism Began
In August 1831 the English scientist Michael Faraday built a ring of iron, about 170 millimetres (roughly 6.7 inches) across, in his basement laboratory at the Royal Institution in London. He wound copper wire around two opposite sides of the ring, with cotton to keep the wires from touching. The Institution's museum notes that winding it was a tedious job that probably took about ten working days. [2]
On 29 August 1831 it worked. When he sent current through one coil, a current appeared in the other coil, but only for a very brief moment. [2] Nothing was connected between the two coils. The change in the magnetic field was what pushed the current through the second wire. This is now called electromagnetic induction.
Faraday's experiment made a current, not heat. Turning that current into heat in a pan came later. In a cooktop the current in the coil reverses tens of thousands of times a second, so the field never stops changing and the eddy currents in the pan never stop either. That link between the 1831 ring and a modern cooktop is how we can explain it today. It is not something Faraday described.
3. Why the Pan Has to Be the Right Kind of Metal
Induction cookers work best with pans made of a magnetic metal, such as cast iron or magnetic stainless steel. [1] A quick, safe check: a fridge magnet should stick firmly to the bottom of a cold pan that suits an induction cooktop. This is a rule of thumb, and the pan's manual is the better guide, because some makers also add special layers.
Copper and aluminium pans usually do not work on a standard induction cooktop. [1] Some newer cookers are said to handle these metals too, but the sources we read do not explain how, so this article leaves that question open. It would be a good one to investigate yourself.
The idea of changing the frequency leads to something that sounds odd: the number of changes per second also decides how deep the heat goes into a piece of metal.
4. Why a Higher Frequency Keeps the Heat Near the Surface of Metal
When the current in metal changes direction very quickly, the induced currents crowd into the outer layer of the metal instead of spreading through all of it. This is called the skin effect. The higher the frequency, the stronger the effect and the thinner the heated layer. [4]
An engineering education site gives these rough depths for heating steel by induction: [4]
- Mains frequency (50 Hz): about 10 to 20 millimetres deep
- Medium frequencies (about 1 to 10 kHz): about 1 to 5 millimetres
- Very high frequencies (up to several MHz): only a few tenths of a millimetre
A simple way to picture it is a frozen lake where only a thin top sheet of ice is warm while the water below stays cold. That picture is only a comparison. In real metal there is no sharp boundary, and the depth also depends on time, the material and the shape. [4]
So frequency works like a dial for how deep the heat goes. Who wants heat only on the surface of a piece of metal? Factories that make gears.
5. How Factories Use Surface Heating to Harden Steel Parts
Parts such as gears and shafts rub against other parts as they turn, so a hard surface wears away more slowly. But a part that is hard all the way through can crack. Induction hardening gives a hard skin and leaves the middle as it was. [3]
The steps are short. A coil heats only the surface layer of the steel until it is very hot. Then water, oil or a polymer liquid cools it quickly, which turns the surface into a harder structure called martensite. The reported surface hardness is roughly 37 to 58 on the Rockwell C scale, and the core keeps the properties of the original steel. [3] [4] The hardened surface also ends up under compressive stress, which is said to help the part last longer under repeated stress. [3]
The thickness of the hard layer is strongly affected by the frequency, as in the list above: lower frequencies for thicker layers and very high frequencies for thin ones. The actual depth also depends on the material, the power and the heating time. [3] [4] The kitchen and the factory use the same principle, with different settings. A cooktop wants to heat a pan base evenly. A hardening machine wants to heat just the outer layer and then cool it right away.
6. How to Spot Induction Heating Around You
If you have an induction cooktop at home, or can look at one in a shop, check the label and the manual for the pan types it allows. Test a cold pan with a fridge magnet and compare it with the manual's list. Pans stay hot after cooking even when no flame was involved, so always ask an adult before touching a pan or the glass.
Some questions are left open here. Exactly how much deeper does the heat reach at one frequency than at another, in the pans we use? Why do copper and aluminium need different settings? The sources at the bottom of this page are a good place to start looking.