Why does an induction cooktop heat iron and steel pots but not copper or aluminum ones?
Because, as metals go, iron and steel – ferrous metals, to get technical – are crappy conductors. Not saying that’s the most elegant explanation or that it’s sufficient. However, it’s positioned at the expert/knucklehead interface, and that’s where we need to start.
This is not, sad to say, standard explanatory practice. For example, in the Wikipedia article on induction heating, we find the following:
The rapidly alternating magnetic field penetrates the object, generating electric currents inside the conductor called eddy currents. The eddy currents flow through the resistance of the material, and heat it by Joule heating.
This explanation was written by an expert for other experts. One imagines the assembled sages murmuring, “Yes, Gandalf, this is wise.” Whereas the reader of average (i.e., minimal) scientific knowledge is thinking: Eddy currents? Joule heating? And that’s before the article gets into ferrimagnetic materials and magnetic hysteresis losses – they’re in the next sentence.
If we turn to the Wikipedia article on induction cooking, we find an equally uninformative treatment. Mention of Joule heating has been mercifully delayed until the fifth paragraph (it just means resistance heating), but the nontechnical reader will still trip over eddy currents, ferromagnetic materials, low radio frequency alternating electric current, electromagnetic coupling, the skin effect, magnetic permeability, hysteresis losses, litz wire, and numerous other inscrutable concepts, plus sentences such as “skin depth is inversely proportional to the square root of the frequency,” many of which – I’m not saying all – could probably have been buried too.
You think the coming age of artificial intelligence will make things better? Don’t get your hopes up. When I put the question to ChatGPT – I’ll spare you the details – I got back a trifecta of an answer: incomprehensible, incomplete, and wrong.
So let’s try it the old-school way, starting with what a nontechnical reader might reasonably be expected to know and working out from there. And yes, some jargon is unavoidable, but we need to keep it to a minimum. Let me count the terms:
We start with the pivotal fact that the pots that work best on induction cooktops are made of metals that magnets will stick to, namely iron and steel, collectively known as ferrous metals. You prefer to say ferromagnetic materials? Free country. Either way, that’s jargon item #1.
We then note the AC current in the copper coil in the cooktop – and no, we don’t have to point out it’s made of litz wire – generates an electromagnetic field that induces an electric current in the pot, a process known as (duh) induction. Everybody knows what AC current is, so I’m not counting that. Induction? Lots of people have induction chargers for their phones and laptops, and we are after all talking about an induction cooktop, so I’m not seeing that the concept is entirely foreign. But never mind – jargon item #2. Electromagnetic field? Come on, people understand radio waves well enough. But fine, be a stickler. Jargon item #3.
The electromagnetic field induces eddy currents in the pot. I debated whether we could just call them electrical currents, but they’re different from your regular AC currents – they don’t oscillate between one end of the pot and the other, but rather move in circles (eddies). Also, eddy currents show up in just about every description of induction cooking above the level of “it’s done with magnets,” so we need to accept reality. Jargon item #4.
Now we start to home in on the core issue. (Engineering pun, slipped in, sorry.) Eddy currents can be induced in any metal, including copper and aluminum. However, in non-ferrous metals, the currents are small and don’t amount to much. Ferrous metals, on the other hand, are readily magnetized by the coil in the induction stove, resulting in a monster electromagnetic field that in turn produces intense eddy currents. For reasons we needn’t get into, the eddy current flow concentrates in a thin layer on the iron pot’s surface – the so-called skin effect, which is a bit techie but has the advantage of being a good visual. Jargon item #5.
Almost there. The more intense the eddy current and the higher the AC frequency, the thinner the skin layer where the current flows. (That’s just how eddy currents work, take my word for it.) Other things being equal, the skin depth of iron is much less than that of copper. All that juice flowing through a tiny thickness of metal results in resistance heating, same as you’d get in a toaster, hot plate, or space heater. Do we need to call this by its technical name, Joule heating? I spit on your Joule heating. Eh, I take that back, English physicist James Prescott Joule was a worthy individual. But “resistance heating” you don’t need to be an engineer to understand.
The main thing is, because they’re prone to strong eddy currents, ferrous metals tend to be poor conductors that are useless for, say, AC transmission lines. But for induced resistance heating, the drawback becomes an advantage, so in cookware they’re great.
To wrap it all up: You want iron or steel pots and pans on an induction cooktop because they’re easily magnetized and produce strong eddy currents that increase the internal resistance of the metal and generate a lot of heat. Nonferrous metals aren’t and don’t. QED.
Is there more to it? There’s always more to it. For instance, one might get into magnetic hysteresis losses, which contribute about 10% of induction heating. But don’t look at me.
You may say: Why make such a big deal out of this? It’s just stoves.
It’s the principle of the thing. We live in an amazing age, surrounded by extraordinary technology. Without some basic understanding of what makes this stuff work – and you’re going to be seeing a lot of induction cooktops – it might as well be magic. From there it’s a short leap to magical thinking, and who wants more of that?
– CECIL ADAMS
After some time off to recharge, Cecil Adams is back! The Master can answer any question. Post questions or topics for investigation in the Cecil’s Columns forum on the Straight Dope Message Board, boards.straightdope.com/.