Chapter 7: Microwaves


7.1 Introduction

It was discussed in Chapter 6 that the electromagnetic spectrum consists of various types of radiation, characterized by wavelength (λ) and frequency (ν). The frequency of the wave can be visualized as the number of wave crests that move by an observer in a second (Figure 7.1). Microwave radiation refers to the region of the spectrum with frequencies between ~109 Hz to ~1011 Hz. This type of radiation lies between infrared radiation and radio waves.


Figure 7.1 How to measure the frequency of a wave.
In the case shown here, the frequency of the wave is 2 Hz, or 2 cycles per second.

Recall that waves with shorter wavelength (lower frequency) have higher energy. The frequency of visible (green) light at λ = 500 nm is approximately 61014 Hz. The frequency of radiation used in microwave ovens is approximately 2.5109 Hz, with a wavelength of 12 cm. This means that the microwave radiation used to heat food is actually less energetic than visible light. Why, then, are microwave ovens so efficient at heating food?

7.2 Water Molecules and Microwaves

Water molecules (H2O) are polar; that is, the electric charges on the molecules are not symmetric (Figure 7.2). The alignment and the charges on the atoms are such that the hydrogen side of the molecule has a positive (+) charge, and the oxygen side has a negative (-) charge.

Electromagnetic radiation have electric charge as well; the "wave" representation shown in Figure 7.1 is actually the electric charge on the wave as it flips between positive and negative. For a microwave oscillating at 2.5109 Hz, the charge changes signs nearly 5 billion times a second (2 times 2.5109 Hz).

Electric charges are similar to magnetic charges: opposites attract. When oscillating electric charge from radiation interacts with a water molecule, it causes the molecule to flip (Figure 7.3). Microwave radiation used in ovens are specially tuned to the natural frequency of water molecules to maximize this interaction. Therefore, as a result of the radiation hitting the molecules, the water molecule flips back and forth 5 billion times a second. Because temperature measures how fast molecules move in a substance, the vigorous movement of the water molecules raises the temperature of water.


Figure 7.2 Water molecule


Figure 7.3 Interaction between water molecule
and microwave

7.3 Inside a Microwave Oven

Microwave radiation is produced by a device called the magnetron. The magnetron consists of four major components: an anode block, a cathode filament, a pair of permanent magnets, and an antenna.

The anode block is a hollow cylinder with fins coming out to the inside. The top view is shown in Figure 7.4. The fins are called the anode vanes, and the spaces between them are the resonant cavities.

The cathode filament is a cylindrical rod located at the center of the anode block. It serves as the source for electrons during the emission of microwave radiation.

Two permanent magnets are located at the top and bottom of the anode block, as shown in the side view in Figure 7.5. These magnets create a magnetic field inside the anode block that are parallel to the cathode filament.


Figure 7.4 Anode block inside a magnetron

Figure 7.5 Side view of the magnetron

An antenna is positioned so that one end goes into one of the resonant cavities in the magnetron. The other end is located in the waveguide, which transfers the microwave radiation to the cooking chamber, much like how fiber optics is used to transfer light to remote locations.

The events which lead to the production of microwave radiation begins when an electron is emitted by the cathode filament. Since the filament and the electron both have negative charge, the electron accelerates outward, toward the anode vanes (Figure 7.6). The magnetic field due to the permanent magnets around the anode block are pointing out from the paper (represented by +).

Moving electric field from the electrons induce a magnetic field around itself. If you were traveling with the electron, facing forward, the induced magnetic field appears clockwise. On Figure 7.6b, this is denoted by the (+) above and (-) below the electron. The positive and negative magnetic field below the electron cancels each other, while the positive magnetic fields above the electron builds up to create a net force downward, perpendicular to the electric force. This net force pushes the electrons clockwise around the cathode filament.

When electrons are continuously emitted all around the cathode tube, they collectively move clockwise around the cathode filament, forming a cloud (Figure 7.7). When an electron approaches an anode vane, a positive charge is induced within the vane as electrons in the vane are repelled by the approaching free electron. Meanwhile, negative charge is induced in the neighboring anode vanes due to the accumulation of the repelled electrons. The negatively charged vanes then repel electrons rotating within the electron cloud. The effect of this is that the electron cloud forms into a pinwheel shape, as shown in Figure 7.8. As the cloud rotates, the anode vanes closest to the "spokes" of the pinwheel has an induced positive charge, and the neighboring vanes are negatively charged. As the cloud rotates, the positive and negative charges in the vanes rapidly oscillate. The oscillation of charges create an alternating current within the resonant cavities of the anode block. This current is carried by the antenna, and released as microwave radiation inside the waveguide.

The final step in a microwave oven is to release the radiation within the cooking chamber. When the radiation exits the waveguide, it is often reflected by a rotating fan blade to evenly distribute the radiation throughout the cooking chamber (Figure 7.9). Once entering the chamber, the radiation is reflected by the chamber walls until it is absorbed by the food.


Figure 7.7 Clockwise motion of
electrons in the anode block

Figure 7.8 Cloud of electrons and
the induced current

Figure 7.9 Microwave chamber
7.4 Characteristics of a Microwave

Microwave heats food by agitating the water molecules. Most foods we consume contain over 70% water by weight, making this an effective way for heating foods. However, the negative side is that food with low water contents take longer to heat in a microwave. Furthermore, frozen foods take longer to heat because the water molecules are not moved as much as in liquid water. One must also remember, when using a microwave, that foods heated in a microwave cannot become hotter than the boiling point of water, or 100C. This is why foods cannot be browned in a microwave, and pies reheated in a microwave do not have a crisp crust like a freshly baked pie would have.

One often hears the statement, "microwaves cook foods from inside." This is because foods heated in a microwave tend to heat most rapidly in the center, and cooks much faster than what we intuitively believe from conduction of heat from outside. The statement, however, is not entirely true; microwave radiation simply has the ability to penetrate several inches into the food. Radiation with longer wavelengths tend to penetrate deep into matter, evident by the ease by which radio waves go through concrete walls. Since microwaves penetrate into food several inches down, it can heat foods quicker than if it were being heated through conduction by boiling water on the outer surface. If the food were larger than several inches, however, the middle of the food still needs to be heated by conduction.

One of the major drawbacks of microwave is the hot spots. Microwave radiation, as it reflects around the cooking chamber, interact with other reflected radiation in such way that hot and cold spots are formed inside the microwave. The phenomenon responsible for this is the interference of waves, shown in Figure 7.10. Waves overlapping in such way that the crests match one another interfere constructively, forming a hot spot. Waves which interfere destructively result in a cold spot. The uneven heating caused by these effects can be reduced significantly by using a rotary platform, included in many of the newer models of microwave ovens.


Figure 7.10 Interference of waves
Useful Links
  • How Microwave Ovens Work by HowStuffWorks.com
  • An interactive tutorial on Microwave Ovens from University of Colorado, Physics 2000 project.

    Click here for References

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