More detail from :
Induction heating takes place in an electrically conducting object (not necessarily magnetic steel) when the object is placed in a varying magnetic field. Induction heating is due to the hysteresis and eddy-current losses.
Hysteresis losses only occur in magnetic materials such as steel, nickel, and very few others. Hysteresis loss states that this is caused by friction between molecules when the material is magnetized first in one direction, and then in the other. The molecules may be regarded as small magnets which turn around with each reversal of direction of the magnetic field. Work (energy) is required to turn them around. The energy converts into heat. The rate of expenditure of energy (power) increases with an increased rate of reversal (frequency).
Eddy-current losses occur in any conducting material in a varying magnetic field. This causes heating, even if the materials do not have any of the magnetic properties usually associated with iron and steel. Examples are copper, brass, aluminum, zirconium, nonmagnetic stainless steel, and uranium. Eddy currents are electric currents induced by transformer action in the material. As their name implies, they appear to flow around in swirls on eddies within a solid mass of material. Eddy-current losses are much more important than hysteresis losses in induction heating. Note that induction heating is applied to nonmagnetic materials, where no hysteresis losses occur.
So what I mentioned was hysteresis losses. This seems to say that eddy-current losses are a bigger source of the heat. Basically the metal, even a solid chunk, acts like a transformer/generator, where the changing magnetic field generates electric current in the metal. Since it's all effectively shorted out, it turns into heat as it's just a big metal resistor.
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