Problem — The induction hob

Problem

An induction hob only cooks if the pan is made of a suitable material. Explain in five lines which physical principle is at work and why steel works but glass ovenware does not.

Solution

Beneath the glass-ceramic plate there is a coil carrying high-frequency alternating current (typically $\sim 25\ \text{kHz}$), which generates a time-varying magnetic field.

This field, passing through the base of the pan, induces eddy currents in it (closed parasitic currents in the metal). The currents flowing in a resistive material heat the base through Joule heating ($P=Ri^2$): the pan itself becomes the source of heat.

Steel is ferromagnetic and conducting: it concentrates the field and develops intense currents, so it heats up a lot. Glass (ovenware) is an insulator: no currents flow in it, so it does not heat up. For the same reason the glass-ceramic plate stays cold: it only heats up through contact with the pan.

$\ev{\text{alternating field}\to \text{eddy currents}\to \text{Joule heating; requires a ferromagnetic/conducting base}}$

Links

Topics: Induzione elettromagnetica Concepts: Correnti di Foucault · Effetto Joule · Legge di Faraday-Neumann-Lenz