Straight Dope 2/17/2023: Why does an induction cooktop heat iron and steel pots but not copper or aluminum ones?

Cecil's Columns/Staff Reports
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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:

  1. 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.

  2. 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.

  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.

  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.

  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.

  6. 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.

  7. 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/.

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Great article and feels more like a classic to me. I really enjoyed this one.

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The kind of explanation that makes the non-techies say, “Ah, I get it.”

5

Excellent column. Now I finally understand how those stoves work!

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Yup.

I mean, I’m non-techy enough that I probably couldn’t tell if it were wrong; but it makes sense to me.

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Good explanation. So what is the advantage of an induction stove over a traditional electric stove? Faster heating? For what I can assume is a hefty price increase in the former, I can wait an extra minute.

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The stove top is safer. It doesn’t get hot, just warm from the pot it is heating.

It heats the pan/pot quicker as mentioned.

Precise temperature control is a bonus for some, I wouldn’t know what to do with that.

They are actually more energy efficient so that is a real plus to me.

Very hard to start a fire with induction heating is a small bonus.

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And it stops heating all on its own as soon as you remove the pan/pot, no risk of leaving the stove on by accident.
In the same vein: they don’t heat before putting the pot/pan on. No danger of the cat burning her paws.
And mostly they can be programmed, both for starting and for ending time. Very practical when you leave home. I miss this function in my current flat.
And the really fancy ones heat the pot/pan wherever you position them, they sense where it is and only heat there. Granted: those are more expensive.

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Agreed! With this article, it finally feels like Cecil is back.

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Another advantage to induction is it allows for some really wild countertop integration. I’ve seen a couple installs of something called Invisacook under the countertop. All you see above is four faint dots or rings etched on the surface to show where the “burners” are.

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Aside from @What_Exit and others’ excellent summaries, we are discovering that Natural Gas doesn’t burn quite as cleanly as we’ve thought, and that some of its byproducts can be hazardous over time. Induction stovetops are completely clean.

There’s also an efficiency question to consider. Much of the energy given off by a stovetop flame goes into the air around the pot or pan, heating the kitchen. You may even need to use more energy to cool things back down, or air out the area above the stove. None of that happens with induction.

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When we got married, we bought a nice set of Revere Ware, copper bottom, but they’re mainly steel. Would they work with induction?

We need a new stove soon anyway and might go induction. It would be nice if our pots still worked.

The glassware pots we have will be off to Goodwill, but I can’t remember the last time I pulled one out anyway.

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Stupid question that I was hoping this article would answer, but it didn’t - if induction heating works by inducing an electric current in the pan, why is there no danger (at least, I assume) of getting an electric shock from the pan if you touch it?

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I’m wondering exactly this, as well. We got a set of copper-clad Revere Ware skillets and pots as wedding gifts; 30 years later, they’re still in everday use.

We’re not looking to replace our range in the near future, but induction does seem to be a potentially smart choice when we get there.

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While cleaning the garage, I came across a magazine from 2012 bemoaning the fact that a new 64” television cost $2500 in Canada but could be cheaply purchased across the border for only $1800. Easily 80% of Canadians live close to the US border. Canada is a cold country (see previous point) and we understand heating. Some of us have magnetic personalities. :wink:

The prices of televisions have fallen fast. The price of appliances? Not so much. And we are not going to be seeing a lot of induction stoves until they do. Which possibly will not happen until government restricts less energy-efficient alternatives. If indeed they are. Are they?

With all the recent disasters, Canadian provinces have even been reluctant to improve building codes to say, attach a roof to a house so it can withstand inclement weather. This involves using nails which are very slightly more expensive and so would add a pittance on to the cost of every new home. You think a thrifty people are going to fork out for a fancy saucepan?

I loved this well-written column. Thanks for reviewing some half-forgotten concepts. But engineering has nothing on the Wu-Tang Clan, who stated in their doctoral thesis that “Money has a large effect on decisions, including the purchase and use of kitchen stoves and associated cookware”. Or something like that.

More seriously, what are the differences in price and energy use? What would make me upgrade?

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If a magnet will stick to the base of the pot, it will work. So, you can test them with a fridge magnet.

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Just to paint the whole picture I will tell one disadvantage I perceived in the model I once had. That was 12 years ago and the technical features may have changed, but it was quite curious to me:
when you regulate the power of a gas flame you make it bigger or smaller, so it is hotter or less hot in a constant manner. Not so with an induction stove, that is always the same power. To regulate the heath the stove clicks audibly on and off, regulating the heat amount by switching on and off longer or shorter periods in a given time. So, for instance, when I wanted to thicken a sauce gently, the induction stove would click on and the sauce would immediately start to boil, just to switch of shortly afterwards and remain off for a second or two, during which time the sauce would cool down again while I whisked it furiously to avoid it sticking to the pan. This is not good when you want to make a sauce without boiling it, the stove would make it briefly boil repeatedly. I may have imagined it, but I thought it changed the taste of bechamel, for instance.
I think I understand why this is so: the current must be strong enough for the induction heating to work, it cannot be regulated any other way. But perhaps I am wrong and they have found better ways to regulate the strength of the heating. I found it a small minus point back then.
That, and the constant click (on), click (off), click (on), click (off)… every couple of seconds. It can get on your nerves in a very silent room.

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Using a chatbot confirms the facts in your columns. It said^:

“Let 𝑝(𝑛),∀𝑛≥𝑛0,𝑛,𝑛0∈ℤ+ be a statement. We would show that p(n) is true for all possible values of n.

Show that p(n) is true for the smallest possible value of n: In our case 𝑝(𝑛0) and

For Regular Induction: Assume that the statement is true for 𝑛=𝑘, for some integer 𝑘≥𝑛0
Show that the statement is true for n = k + 1.

OR For Strong Induction: Assume that the statement p(r) is true for all integers r, where 𝑛0≤𝑟≤k for some 𝑘≥𝑛0. Show that p(k+1) is true.”

Now magnetic fields sound pretty strong. So where do I plug in the stove?

^ Not really.

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We have a Jenn-air top that is new, but the model has been in production for maybe 10 years. It does cycle as you say on lower settings, but there is only a little buzz, no clicking. Aside from the speed at which they heat, the ease of cleaning cannot be overstated. Quick wipedown and it’s immaculate. You can clean immediately after using a burner although it will be warm. Spills don’t cook on, either.