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Q & A: induction stove tops

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Most recent answer: 09/23/2013
Q:
In an induction oven, heating is due to hystersis and only a limited area(say the base of the ferromagnetic pan) is heated. Now, that area has electrons that are free(in metals) and possessing thermal energy. Why don't they move to other areas like the top of the cookware and heat them up?
- Souptik Dhar (age 19)
Bangalore, India
A:

On a side point, the heating isn't mainly due to magnetic hysteresis. Even a non-magnetic metal pan will be heated by eddy currents stirred by the ac field. The magnetism helps keep the fields located in a small enough layer of metal to work efficiently with a standard pot and a standard ac frequency. (See )

Now for your question. The electrons do carry energy and do cause heat to flow from the hotter parts of the pot to the colder parts. They do that for induction heating just as they do that for heating on an ordinary burner. However, the electrons only move short distances, say about 10 nm, before bouncing off in a new direction.  So the freedom that electrons have to respond to the ac field and generate heat is not the freedom to quickly hop long distances. That means that the heat diffuses rather slowly from one part to another. The process is described by the ordinary heat conduction equation, and looks just like any other sort of thermal conductivity, even like in a glass where no free electrons are present. The electrons just speed it up some compared to other thermal conduction. That's one of the main reasons most cooking is done in metals.

Aluminum is a much better thermal conductor than iron, but doesn't work well with most induction stoves because it's not magnetic. Good stainless steel also isn't very magnetic. So for some of us replacing the stove with an ordinary induction type would require replacing most of our cookware.

Mike W.


(published on 09/18/2013)

Follow-Up #1: electron heat conduction

Q:
So, the basically the top of the utensil does not heat up in case of induction heating simply because the alternating current does not allow the electrons to move freely. Is that right?
- Souptik Dhar (age 19)
Bangalore, India
A:

No, the cause of the limited heating is not that the ac current keeps changing directions. It's that the "mean free path" of the electrons, the typical distance that they travel before bouncing off a defect or a sound wave, is very short.

 Perhaps here's a key point for background: the electrons are always moving around very fast, even when there's no applied field. The applied field, whether ac or dc, only slightly tilts the distribution of velocities one way or the other. The thermal conduction doesn't come from the field carrying the electrons from one place to another but rather from the wandering they were doing already. The applied field only causes heat generation in the layer where there are net electrical currents.

Mike W.


(published on 09/23/2013)

Follow-up on this answer.