Fire from, or at least near, ice

A reader writes:

Got a science question of sorts.

WTF is actually going on here?


The ice cube is not glowing. The induction coil is.

Induction heaters are interesting things. You make a coil out of a sturdy conductor - usually copper bar stock - and you put a whole lot of current through it at, usually, a pretty high AC frequency. The alternating current then induces current in any conductive object you put inside the coil, and the resistance of the object turns the current into heat, which heats the object. It's the same principle that heats up a wire, or an actual heating element, when you put current through it. The source of the current in an induction heater is just less obvious, and the electricity in the heated object isn't going round and round in a circuit; it's just jiggling eddy currents.

(Magnetic braking relies on induced eddy currents as well, and also heats up the object the eddy currents are being induced in.)

The induction coil was actually the first, and worst, kind of transformer. It was the worst because the purpose of a transformer is to turn one voltage of AC into another (or keep the same voltage but isolate two circuits). The more energy a transformer wastes as heat, the less useful it is. Modern transformers have laminated cores made from "electrical steel", specifically to minimise unproductive transformer-heating eddy currents.

A powerful enough induction heater can do all sorts of neat tricks, like heat-treating part of a piece of metal - all the way to glowing hot - so fast that the heat won't have managed to conduct through the metal to other parts of the object before the bit you're heating gets to the right temperature and can be quenched. You can also use an induction heater to melt metal in a crucible without a flame.

Or even to levitate a light enough metal, while it melts!

Induction cooktops work this way too. That's why they'll heat a metal pot, but not glass cookware. If it's conductive, they heat it; if it isn't, they don't.

[UPDATE: As commenters have pointed out, only ferromagnetic cookware actually works on an induction cooktop. I'll fix this properly when I have a moment.]

Ice is very slightly conductive (as I have proved to my own satisfaction), but can generally be considered an insulator, and won't be significantly warmed by an induction heater. So the induction coil in the ice-cube video is essentially being run "empty", and just rapidly heating itself up, and in due course glowing, in a simple resistive way. That ice cube will actually melt pretty quickly, because of radiant heat and air convection from the coil. But it'll last as long as you'd expect it to if it were sitting next to a similarly glowing plain resistive heating element.

(The glow probably isn't really as impressive as it looks, either, because digital cameras of all sorts are sensitive to infrared light. Most digital image sensors have an IR-blocking filter on them to minimise this effect, but the filters aren't completely effective, and so very hot things like this coil or the aftermath of certain pyrotechnic entertainments look hotter than they are. The human eye may see some glowing metal as orange, but most digital cameras will think it's white.)

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

8 Responses to “Fire from, or at least near, ice”

  1. Chipmunk Says:

    Interesting Dan, I was always under the impression with that particular video, that the person had embedded a conductive item IN the ice cube, which was currently glowing, and would shortly drop out through the bottom. Is that possible as an alternate explanation or would the cube explode due to steam pressure?

  2. dave.cock Says:

    While I think Dan's analysis is correct, and the ice is not actually undergoing heating (other than through radiation from the glowing coil), there actually is a closely related technique that *will* heat ice or water directly: Dielectric heating. Rather than the material forming the core of an inductor (or transformer), as in induction heating, it's placed between two plates and forms the dielectric of a capacitor. The constantly realignment of molecules with the applied electric field heats the material. Similarly, when induction heating a ferrous metal (e.g. steel) at high frequencies, resistive losses through eddy currents are actually less important than hysteresis losses. Roughly speaking, every time you magnetize and demagnetize steel, a small amount of energy is dissipated internally (the area of the Hysteresis curve), and the more often you do this, the more power you dissipate. A combination of these is, more or less, how the microwave works (water-containing material e.g. food is heated dielectrically, the utensil you accidentally left in it heats by hysteresis and eddy currents, with a small bonus from plasma arcs).

    For anyone's whose interest is piqued by this stuff, I recommend adding the following to your reading list (after J. E. Gordon's magna opera): abc's (sic) of Radio Frequency Heating and Metals in the Service of Man.

    • guyv Says:

      Thanks for the nice article, I did not know about induction heating of non-magnetic metals.
      Thanks also to dave.cock for the information on Dielectric and Hysteris heating.

      One correction to the main article: Induction cooktops will not work with conductive materials that are not also magnetic (ferromagnetic). This is why they do not work with aluminum (aluminium) cookware.

      I suspect that Induction Cooking needs ferromagnetic cookware because it works mostly by Hysteresis heating and that eddie currents are relatively insignificant part of the operation.

      • Popup Says:

        Induction cooktops will not work with conductive materials that are not also magnetic (ferromagnetic).
        I suspect that Induction Cooking needs ferromagnetic cookware because it works mostly by Hysteresis heating and that eddie currents are relatively insignificant part of the operation

        Not true. (Or at least not entirely true.)

        Read the Wikipedia article on induction cookers - it's really quite well written. The reason induction cookers don't work with Al or Cu pots has nothing (intrinsically) to do with ferromagnetism, but rather with resistivity and skin depth (which in turn depends on the magnetic permeability). I have never tried, but induction cookers ought to work with 18/10 stainless steel, despite it not being the least ferromagnetic.

        When using a steel pot, the resistance as seen by the cooker is moderately high, and almost all the energy is dissipated in the pot. When using Cu or Al, however, even if they are thick enough to contain the majority fields (thicker than the skin depth), the resistance is much smaller. A larger proportion of the energy will be dissipated in the coil rather than the pot, possibly overheating the cooker.

        • guyv Says:

          Well I have definitely tried 18/10 stainless steel and it does not work. I have used induction cooking for 9 years and have not yet seen it work with non ferromagnetic pans.

          The Wikipedia article on induction cooking does not back what you are saying.

          "To be used on an induction cooktop, a cooking vessel must be made of a ferromagnetic metal, or placed on an interface disk which enables non-induction cookware to be used on induction surface."
          "An induction cooker transfers electrical energy by induction from a coil of wire into a pot made of material which must be electrically conductive and ferromagnetic."
          From dave.cock comment I believe that the usual explanation based on current might be incorrect, and that the explanation based on Hysteresis might be a better explanation, albeit one I had not seen anywhere before.

          Remember that in order to be able to say that a statement is not true, it is not enough to offer a differing opinion (yours or wikipedia's or any other) but you must prove it to not be factual. I don't believe that without experimental facts any of us can prove my conjecture on Hysteresis to be true or untrue, however if induction currents were the correct explanation then induction cookers would not need ferromagnetic materials. I do believe that there is ample factual evidence that induction cookers do not work with non-ferromagnetic materials, but you are welcome to hold a healthy critical view on this as long as you don't state that it is known to be untrue without facts to back you up.

          • guyv Says:

            Also I believe your math on resistance might need precision on what the constant parameters are. If, as it is with a normal power supply, the voltage (or voltage field in this case) is constant and enforced by the apparatus, then the lower the resistance, the higher the current. The formula is Power=((Voltage)squared)/Resistance or P=U * U / R. So for a set voltage and resistance, the power is higher with a smaller resistance. Thus the lower resistance AL or CU would produce a higher power dissipated in the pan.

            If the current "I" was constant then you would be correct because the formula is then P=R * I * I. However I don't think the induction cooker has a way to enforce a constant/known current in the pan, so I don't believe this formula applies.

  3. Popup Says:

    Hmmm. This is definitely a subject that deserves more attention. If (as you say) 18/10 steel doesn't work on an induction stove - then it must indeed be the case that the main heat transfer mechanism is the magnetic hysteresis.

    It's been a couple of years since I did the maths, but I once tried to calculate the penetration of the EM fields in the cookware, and I believe that even regardless of the magnetic hysteresis the eddy currents would ensure that all the energy is absorbed in the first fraction of a mm. Admittedly I used values for magnetic permeability of 'normal steel' rather than explicitly 18/10. Maybe it's wildly different?

    Anyway - The subject deserves more attention.

    Regarding the resistance vs power conundrum, one way is to view the oscillator-coil-pot system as a powersupply-transformer-resistor. Then one can apply standard rules for transforming the system into a voltage-source - resistor with series resistance in the source. If the final resistance is too small all the power will be dissipated in the power supply, and if it's too large no power will be dissipated at all. Iron cookware seems to be ideally situated somewhere in between these extremes and absorb the vast majority of the energy in the saucepan.

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