Microwave ovens

1994/12/01 Otaolaurretxi, Jon Iturria: Elhuyar aldizkaria

• Teknologia

Since Prehistory man is preparing to eat food (meat, for example, placed on fire) with electromagnetic waves. At present it uses in microwave ovens a magnetron that produces waves of somewhat longer than infrared rather than fire, but the base is the same. In both cases the electromagnetic waves are removed energy to heat food.

So let's briefly see what electromagnetic waves are. Electromagnetic waves simultaneously propagate the electric field and the magnetic field, combining the oscillation planes of both. Sinusoidal oscillations with an expansion speed of 300,000 km/s. The frequency or frequency of oscillations, on the other hand, can be very small or slow, very large or faster, from a few periods per second to billions of periods per second.

The magnetron that microwave ovens have for the kitchen has a cylindrical anode (1) full of holes (2). The holes are connected by slots with the central hole where the heated cathode is located (3) where the electrons are released. Electrons are exposed to the magnetic field of a magnet (4) and the high voltage electrostatic field between the anode and cathode. These two fields cause the electrons to spin in the form of clouds and resonate with high-frequency waves in the holes. The coupling ring (5) collects the waves and takes them to the oven.

The magnetic field is able to operate remotely. We know that the magnet can lift a nail at a distance (so it can work). It is said that the magnetic field has potential energy. The electric field also has similar characteristics, although its long distance influence is less. However, the electrified plastic bar can attract wire fragments or fragments of expanded polystyrene. Therefore, the electric field also has potential energy and can work.

The electromagnetic field carries energy, but depending on its oscillation frequency we will not feel it or feel it. The frequency unit is hay, i.e. an oscillation per second, but the periodic phenomenon can also be defined by its wavelength. The wavelength is the distance between two points of maximum intensity (or two of minimum). The wavelength is the distance between two consecutive peaks in the oscillations that occur when pouring the stone to the surface of Urgelko.

In the case of electromagnetic waves that we want to analyze here, we will say that we are very sensitive to wavelengths between 0.4 mm and 0.8 ?m (1 mm = millimeter) and that we detect them visually. We realize that the flash of the camera is carried on power in front of the eyes or that the sunlight raised. The state of transparent glass, however, would feel nothing because it would allow the passage of light and would not absorb energy.

Regarding electromagnetic radiation, there are transparent materials, other opaque materials (radiation is absorbed and energy is taken) and other reflective materials (radiation bounces off the surface). In practice there are three processes in the same material (transmission, absorption and reflection), but normally a process predominates and the other two are disposable. It all depends on the frequency of radiation and the nature of the substance. The same element can be transparent to one frequency, opaque to another and transparent to the third, etc.

Ultraviolet rays, x-rays and finally gamma rays are more frequent than the visual radiations we receive. We do not see ultraviolet rays, but our skin is opaque and very sensitive to them. They produce burns and molecular remodeling can produce tumors.

If we start with visible radiation towards lower frequencies, we first have infrared rays. They penetrate deeper into the skin than visible light and absorbing their energy heats up. The heat from the oven or fire is received remotely and the skin and clothing feel heated.

The powerful infrared radiation emitted by the Labegarai boiler burns the skin to the nearby and burns the garments. Infrared rays are wavelength between 1 mm and 1 mm. From there begins the microwave field, radar, radio telescope and microwave kitchen.

All waves of all frequencies and lengths mentioned so far are electromagnetic waves, that is, of the same nature. However, its effects depend on the electrical and magnetic characteristics of the material they capture, but also on the so-called scale effect. In other words, at very high frequencies the wavelengths are similar to those of molecules and X-rays or gamma directly affect atomic particles, atoms and molecules. They are ionizing radiations. They cross almost all organic bodies (with greater difficulty for metals), but when radiation passes the loads are rearranged in atoms (i.e., the atom is ionized).

Ultraviolet rays are less penetrating. Metals are almost completely reflective, but penetrate organic bodies in shallow depth. From visible light, metals are absolutely reflective for all wavelengths, but this does not occur in animal tissues. They can reach more depth than ultraviolet rays and penetrate a few centimeters before their total absorption.

From there, radiations of wavelengths greater than several meters, like x-rays, cross organic bodies. Radio waves, for example, cross the tree or the clear animal like glass. The wavelength from the wavelength to the micron of the infrared to the wavelength of the microwaves of 30 cm is therefore the greater depth and absorption of radiation in the organic bodies.

When radiation is absorbed, energy is also absorbed, which in animal tissues is transformed into heat. Since the wavelength of infrared radiation is the same size as the cell, absorption begins in the first cells found, i.e., at shallow depth. Electromagnetic energy is transformed into molecular agitation, increasing the surface temperature. This has happened every time since prehistory man has placed the piece of meat next to the heap of clouds (without touching the fire). In fact, the infrared radiation emitted by these flames is the one that has been absorbed and heated on the surface of the flesh. Infrared ones do little to the inside and that's why the meat burns outside, but it stays raw and red inside. The inside of the meat never burns, although the heat of the skin is slowly transmitted by conduction to the whole piece. This is how meat or fish is prepared today on skewers and grills.

Since Prehistory human beings prepare to eat food with electromagnetic waves. At present it uses in microwave ovens a magnetron that produces waves of a length slightly higher than the infrared instead of the fire, but the base is the same.

If you want to heat the inside of the piece of meat like the skin, you should use a radiation of a wavelength of about ten centimeters, but this type of radiation is not emitted by fire or by the electrical resistances that are placed at the height of the Joule effect. This requires a special generator that emits radio waves, called magnetron.

The magnetron was first used in radars in 1935, but in 1947 the military also began to use it in the kitchen. The magnetron is like a vacuum lamp. It consists of an anode or positive cylindrical body and a negative cathode heated inside by a low tension filament. Between the anode and the cathode a voltage is applied between 4,000 and 5,000 volts and a permanent magnet creates a magnetic field of great intensity on the axis of the anode.

Without this magnetic field, the attraction of the high-voltage electric field would allow radially attracting electrons emitted by the high cathode. But if there were only a megetic field, the electrons would describe the curve and return to the cathode. When there are both fields (electric and magnetic), the electrons have compound movement and like the helix of different arms the electron clouds are formed. As seen in the image, the anode has holes inside that are connected by slots with the central hole, where the cathode is located. Spinning, the electron cloud produces electromagnetic waves. The length of these waves is determined by the diameter of the holes (resonance holes). These waves collect pieces called coupling rings and are sent to the oven where food is found.

In practice, a frequency of 2,450 megahertz is used, corresponding to a wavelength of 12.24 cm. This type of radiation captures food inside the oven, but food (meat, milk, potatoes, etc.). ), mainly, contain water, from 75% to 90%. Therefore, the effect of microwaves on water is fundamental. The water molecule is not electrically neutral but called the polar molecule. In them coexist two electric charges equal but opposite sign forming an electric dipole. In the electric field, the compass behaves as in the magnetic field, that is, it is oriented in the direction of the field lines.

When microwaves cross the water or the very hydrated body, all molecules are oriented in the direction of the electric field of these waves, but as this field changes billions of times per second, the molecules are based on that frequency. These oscillations mean millions of collisions, that is, an increase in the agitation of molecules, both in frequency and in amplitude. As a result, body temperature increases.

The energy contained in microwaves is transformed into heat, but this heat also occurs a few centimeters deep and not only on the surface (as with infrared).

With infrared waves, when the person feels radiation moves away. For nothing to happen, it will happen to your skin, but if the microwave radiation catches you, to realize it will also heat the internal tissues. That is why it is dangerous to circulate around the radars and therefore the microwaves that are manufactured for use in the kitchen have safety mechanisms.