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Re: induction or hysteresis?



In the latest issue of The Physics Teacher, Brahmia and Horton raise the
question of whether the "induction cooktop" really utilizes the induction
of current to warm up the pan. They note that a ferromagnetic pan is
needed and this implies an hysteresis effect.

Later, someone in the same thread pointed out that if it was due to the
induction of eddy currents, any metallic pan would do and thus the heating
must be due to magnetic domains "flip-flopping".

Back as a postdoc (ten years ago already!) I worked with induction heating.
I don't remember the exact equations right now, but the size of the induced
current depends on the size & frequency of the external field, on the size
and geometry of the sample, and on the resistivity and permeability of the
sample. Basically, as you go from a realtive permeability of 1 for a
"normal" metal to perhaps several thousand for a soft magnet, the heating
also goes up by several thousand (or maybe square root of a several
thousand, but you get the point). Thus it is easy to inductively heat a
chunk of iron to just below the curie temperature, but much more difficult
to go higher.

In fact, another option is to wrap a non-magnetic metal around a ceramic
magnet. You get a bigger induced current because there is a bigger field
to oppose. The heating comes from these induced currents. There are no
currents in the ceramic, and no appreciable heating from flipping the
domains.

So any pan will show some effects, but the magnitude depends strongly on
magnetic properties.

Tim Folkerts
FHSU Physics