Why can’t metal utensils be used in the microwave?

Jaroslav Kores, Ph.D.

A microwave oven uses electromagnetic waves of a specific frequency for heating — 2.45 GHz. This frequency is not chosen randomly, it is the resonance frequency of water molecules. Resonance is an action in which we supply the oscillator (a body that oscillates) with such a frequency that its oscillation will continue to increase. This can be imagined using the example of a swing — only if we oscillate on its seat with a certain frequency (resonance) the deflection of our swing will increase.

Electromagnetic waves with a frequency of 2.45 GHz vibrate water molecules, which we observe externally as an increase in its temperature. The temperature of a substance is directly related to the speed of the particles of which it is composed. If there is anything in the microwave that contains water, the waves will be absorbed by the water molecules, otherwise the waves will bounce off its inner walls. This reflected wave meets a new wave that sends the source of this wave (called a magnetron) into the microwave space. The reflected and new waves will add up and create a so-called standing wave (the same way sound is created in musical instruments). A standing wave is specific because some of its places still have zero deviation (in these places the water molecules do not “heat up” at all) — the so-called nodes and places in which the deviation is the largest (in these places the water molecules heat up the most), so-called antinodes. In order for the food to be heated evenly, there is a revolving dish in the microwave so that the antinodes cross the largest possible volume of the heated food. If the antinodes are places with maximum deviation (i.e. with the most intense electromagnetic field) and nodes with zero electromagnetic field, there is a big difference in the intensity of the electric field between these places. If we put something metallic in the microwave, the electrons in the antinodes will be affected the most (they will accumulate in them) and in the nodes, the electromagnetic field will not affect them at all. The energy of the electromagnetic wave will not be absorbed by anything and thus the deviation of the antinodes will increase. This will create places where there will be much more electrons (antinodes) compared to places where their number will not change (nodes). This will result in one part of the metal having a greater charge than the other, which is the same as lightning — the cloud has a significantly different charge than the Earth. If the difference in charges is large enough, an electrostatic discharge, or lightning will occur. So if we put metal dishes in the microwave, we will see lightning in it.

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