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The Physics of .....(almost) burning down the house!

We redid our kitchen recently and as part of that job we installed an induction hob - which has been a big success. Not only is it great for cooking - fast, responsive and environmentally friendly - but it's also interesting physics. I had only been partly aware of them before but I'm a big fan now: a changing magnetic field inside the hob induces eddy currents in the pot you're using, and those currents then cause that pot to heat itself up. With far less waste than could ever be the case with a traditional electric cooker, or gas hob,

I was confused at first as to why it only works with steel pots. We know from our leaving cert physics that a changing electric field will create eddy currents in copper or aluminium, so I wondered why we can't we use them with the induction hob. But then I came across a simple explanation: the relatively high resistance of steel means that more heat is created when the eddy currents flow and as that is the purpose of the whole thing, they're designed to only work with steel. Fair enough. (I'm a little confused still about one other point: if there are magnets inside the hob and if the pot is made of steel, why do I feel no magnetic attraction between the two? I'll leave that to the back of my mind to figure it out in time.....*)

The other attraction, I thought, of an induction hob is safety. With no naked flame, there is surely no realistic chance of my cooking oil going on fire, I thought. But as you can see from the attached video, I thought wrong. Thankfully, I had a teenager on hand to help out and avoid a catastrophe. And, of course, to video the whole thing!

But after the oil had burnt itself out, I was left perplexed as to how it had started burning in the first place. I had poured oil on the wok only a minute before. I had given it a bit of time to heat up and then I'd gone back to add in my mushrooms, whereupon the flames leapt up. Where had they come from?

At one level it's not hard to explain. Hot oil will burn and the wok was very hot. Numbers are hard to find because there is more than one type of olive oi, but the flash-point is generally given as a little over 200 oC, and the flame-point a little over 400 oC. The difference between the two seems to be that the flash point is the temperature at which the oil will burn if exposed to a naked flame. But that's not relevant here because there was no naked flame. That makes the flame-point the relevant figure: the temperature at which the oil will ignite regardless of an external flame.

That seems fair enough. The hob is very efficient and the currents inside the metal obviously brought it to the 400 oC temperature needed to cause the fire. But that still left me confused. Surely those sort of temperatures are reached all the time when we fry with oil. Why doesn't this always happen?

It took a bit of reading to find what I think might be the answer. Which is that in between the flash-point and flame-point lies the boiling-point, at a little over 300 oC.

Generally the oil will start to boil at the b.p. and once that happens, any additional heat energy will be absorbed as latent heat, and will be used as the liquid turn to a gas. Only after all the oil has gone will the temperature start to rise again - but at that point there is no oil left to burn, so we don't usually get a fire.

So why did I have a fire then? I'm guessing that it comes down to the efficiency of the hob and the cleanliness of the wok. I think the oil heated up very quickly and reached its b.p. But the smooth surface of the wok and the rapid addition of heat meant that, instead of boiling, it superheated. Bubbles of gas tend to form on a nucleus, and on smooth surfaces there is nothing there to form that nucleus. When that happens, the liquid will often reach temperatures far in excess of the b.p. - but will boil with great rapidity when moved, as the vibrations allow a nucleus to form. So when I took hold of the wok, it immediately began to boil, a vapour quickly formed over its surface - and because the temperature was now over the flame-point - it immediately began to burn.

That's my theory at least. I still love the new hob. But I've also learned to fear it a little.

* but if anybody can tell me the answer, that would be great!

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