In daily life, the heating methods we are very familiar with are mostly heat conduction or heat convection. Put an iron pot on a hot fire. The thermal energy of the flame is first transferred to the metal atoms at the bottom of the pot, and then transferred from the metal to the surface of the food close to the bottom of the pot. The heat penetrates step by step into the center of the food. This heating mechanism from the outside to the inside often takes a long time.
The emergence of microwave ovens completely overturned this traditional kitchen thermodynamics law. Inside the microwave oven, there is no resistance wire or open flame that generates heat. Its core component is an electronic component called a magnetron. When the magnetron is energized, it can convert electrical energy into electromagnetic waves of a specific frequency, thus starting a process of precise control of microscopic particles inside food.
The frequency of the electromagnetic waves mentioned here is usually set at 2.45 GHz. At this specific frequency, microwaves can easily penetrate insulating containers such as glass, ceramics and plastics and reach directly into the food. To understand how electromagnetic waves heat food, we must focus on the most common ingredient in food: water. From a chemical structure point of view, water molecules are composed of one oxygen atom and two hydrogen atoms. Since the attraction of oxygen atoms to electrons is much stronger than that of hydrogen atoms, the electron distribution inside water molecules shows a serious imbalance. Oxygen atoms have a weak negative charge on one end, while hydrogen atoms have a weak positive charge on one end. Such molecules with positive and negative poles are defined in physics as polar molecules.
Polar molecules have an extremely special physical property, that is, like the magnetic needle on a compass, they will automatically adjust their arrangement as the direction of the external electromagnetic field changes. Since the microwave emitted by the magnetron is a high-frequency alternating electromagnetic field, the direction of its electric field will rapidly alternate 2.45 billion times per second. The water molecules in this high-frequency oscillating electric field completely lost their calm. They are forced to make billions of U-turns and flips per second as the direction of the electric field wildly switches.
In this wild dance of the microscopic world, countless water molecules collide, squeeze, and rub against each other. The principle of frictional heat generation in the macroscopic world also applies here. The huge microscopic kinetic energy generated by the violent movement of water molecules is quickly converted into heat energy. This kind of heat does not slowly seep in from the outside, but actually bursts out from around every water molecule inside the food, thus achieving extremely efficient overall heating.
It is precisely based on this unique heating mechanism that we will find that microwave ovens show great differences when facing different materials. A piece of fresh meat rich in moisture can be heated to boiling in a short time, but a completely dry piece of wood or a dry paper towel will hardly heat up in a microwave oven because they lack enough polar molecules inside to dance with the electromagnetic field.
In contrast, if you put metal in a microwave, the situation becomes extremely dangerous. There are a large number of free electrons inside the metal. High-frequency electromagnetic waves will drive these free electrons to surge violently on the metal surface, instantly generating a strong high-frequency current, and even ionizing the air at the edge of the metal, erupting with dazzling sparks. In severe cases, the magnetron may be destroyed or cause a fire.
The design of the microwave oven is an example of the application of modern physics to daily life. It cleverly takes advantage of the inherent polarity weakness of water molecules and uses invisible electromagnetic waves to overcome traditional space heat transfer barriers. Every time we press the start button of a microwave oven, we are actually witnessing a feverish dance involving tens of billions of water molecules. The most basic electromagnetic laws of nature are transformed into the steam on the table.