How does an eddy current generate heat
Physics of the induction cooker
If you cook on an induction stove, you make use of a few other physical effects in addition to electromagnetic induction.
In 1831, the scientist Michael Faraday discovered that a changing magnetic field creates a current. This electromagnetic induction made electric motors, generators and ultimately induction cookers possible many years later. In contrast to other stove types, the new stoves do not generate heat below the cookware, but directly in the bottom of the pot or pan.
This is made possible by a tightly wound, disc-shaped copper coil under the hob through which an alternating current flows with a frequency of 20,000 to 100,000 Hertz. A few years before Faraday, his colleague André-Marie Ampère had already discovered that a time-varying current flow creates a magnetic field around a coil. The direction of this field changes at the same rate as the sign of the current, i.e. between 20,000 and 100,000 times per second for an induction cooker.
According to Faraday, such a magnetic alternating field in turn induces an electrical voltage - in the coil itself, but also in its surroundings. In contrast to conventional voltage sources, in which the electric field lines run between negative and positive charges, the electric field lines are self-contained. If there is a pot or pan made of a conductive material on the hob, the bottom of the hob is also penetrated by the field lines and a force is exerted on the free electrons in it: so-called eddy currents are formed.
Different heat sources
These ring-shaped induction currents do not flow without loss in the bottom of the pot - part of the electrical energy is converted into thermal energy and the bottom of the pot is heated. The electrical resistance varies from material to material: stainless steel, for example, conducts significantly worse than copper or aluminum, so the heat released is correspondingly higher with stainless steel. The heat output of induction cooking is increased by a further effect.
The physicist Emil Lenz discovered in 1833 that the induced eddy currents in turn generate a magnetic field that counteracts the cause of their formation. In the case of the induction cooker, this field counteracts the rapidly varying magnetic field of the coil, which is thereby displaced into the lower layer of the bottom of the pot or pan. As a result, the eddy currents in the cookware can only flow in a layer a few millimeters thick.
This so-called skin effect reduces the effective conductor area in the bottom of the pot, which is why the electrical resistance and thus the heat output seem to increase. The penetration depth of the primary field depends on the frequency of the alternating current as well as on the pot material. For example, the field penetrates more deeply into aluminum or copper than into stainless steel. Pots made of aluminum or copper are therefore less suitable for an induction cooker.
In addition to eddy currents, ferromagnetic cookware has another source of heat: these materials contain microscopic areas in which the magnetic moments of the atoms are aligned in parallel - similar to tiny compass needles that all point in the same direction. Neighboring Weiss areas, as these ordered areas are called, are randomly oriented without an external magnetic field and the entire body appears non-magnetic. However, as soon as an external field is applied - such as the high-frequency alternating field in an induction cooker - all magnetic moments are uniformly aligned with it.
Principle of an induction cooker
Repeated magnetization reversal, i.e. the rapid flipping back and forth of the magnetic moments in the bottom of the pot, also converts energy into heat - i.e. it is lost. How large these so-called hysteresis losses turn out to be depends on the material of the cookware and the frequency of the alternating field. In the case of commercially available stoves and a standard ferromagnetic pot, for example, around thirty percent of the heat output is caused by the reversal of magnetization. "But there are many pots made of different materials in complex geometries - for example made of several layers or with an aluminum core - and the hysteresis losses are very low here," explains Sergio Llorente from the development center of BSH Hausgeräte GmbH in Zaragoza, Spain. In some materials, the hysteresis losses are even negligible. "In fact, you can only specify a range between zero and thirty percent, it depends on the materials."
The heat generated in the bottom of the pot is transferred on the one hand by the free electrons and by lattice vibrations of the atoms in the metal and transported to the food. In order to increase the thermal conductivity from the heated lower edge layer of the pot upwards, an aluminum core can be used, for example, which conducts the heat better than stainless steel. The induction hob, on the other hand, consists of a glass ceramic such as Ceran, which conducts heat very poorly and therefore only heats up slightly.
Induction cookers are more energy efficient than electric stoves, especially when they are heated up: For example, a liter of water boils around twice as quickly as with a halogen hob or electric hob. The efficiency of the induction cooker is around eighty percent, says Llorente. In the case of conventional stoves, on the other hand, it is only around sixty percent. After that, however, the energy consumption adjusts: after more than an hour of cooking, both stoves have an efficiency of around ninety percent.
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