29 Sep Why can’t copper and aluminum pots and pans be used for induction cooktops
Let’s first look at how induction cookers work .
When a lump of metal is placed in a time-varying magnetic field, the electrons in the metal are induced by the force of the induced electric field to produce an induced current, which is called an eddy current or eddy current. As shown in Figure 1, when a variable current is passed through the coil, a variable magnetic field is generated around the coil, and the metal is equated to an infinite number of closed loops, and the changing magnetic field generates eddy currents in the loops.
Eddy currents emit Joule heat just like ordinary currents. The method of heating by the thermal effect of eddy currents is called induction heating. In induction cooktops, a new type of stove, the principle of eddy current heating is used. As shown in Figure 2, an alternating magnetic field is generated by the components of the electronic circuit board inside the stove. When the bottom of the cookware is placed on the stove surface, the cookware cuts the alternating magnetic field and generates eddy currents in the bottom of the cookware to heat the cookware for cooking food. Since induction cooktops work by using eddy currents, copper and aluminum pots and pans on the induction cooktop should also generate eddy currents; however, no heat from eddy currents was observed in the experiment. Is it because the eddy currents generated in copper and aluminum pans are particularly small? But according to the characteristics of eddy currents, the smaller the resistivity of the bulk metal, the larger the eddy currents in the conductor should be. The resistivity of copper and aluminum is smaller than that of iron, so it seems that the eddy currents generated by copper and aluminum pans on an induction cooker should be larger than those of iron pans. In fact, copper and aluminum pots and pans on the open induction stove basically do not produce eddy currents. This is because the size of the eddy current is not only related to the change of magnetic field and metal resistivity, but also the magnetic permeability of the conductor plays a key role.
In fact, copper pots and pans, aluminum pots and pans placed on the open induction cooker basically do not produce eddy currents. This is because the size of the eddy current is not only related to the change of the magnetic field and the resistivity of the metal, but also the magnetic permeability of the conductor plays a key role. According to the classification of magnetic media, copper is an antimagnetic medium with a permeability slightly less than 1, aluminum is a paramagnetic medium with a permeability slightly greater than 1, and iron is a ferromagnetic substance with a permeability between 2,000 and 100,000 (Figure 3). The magnetic permeability of antimagnetic and paramagnetic media is much smaller than that of ferromagnetic media.
As shown in Fig. 4, the B′ of the antimagnetic medium is reversed from B 0 by the induced molecular current under the action of the external magnetic field B0, and the vector sum of the electron magnetic moments of the antimagnetic mass is nearly zero. This effect is also found in paramagnetic media, but the effect is relatively small, and the vector of electron magnetic moments of paramagnetic media is also small. Therefore, copper and aluminum pots and pans of antimagnetic and paramagnetic qualities are placed on the induction cooker, and the magnetic induction lines passing through the pots and pans are very weak. Therefore, induction cookers can not use copper and aluminum pots and pans.
Iron is a ferromagnetic medium, and the B′ formed by the induced molecular current under the action of the external magnetic field B0 is the same direction as B0, so the magnetic field B = B0 + B′ in the iron medium, and B′ is much larger than B0, so the combined magnetic field strength of the iron pot on the induction cooker is much larger than the magnetic field strength generated by the coil of the induction cooker, so the iron pot can get a large eddy current, which can play a fast heating requirements.