PHY 242 Net Research Project for 10/19/98

Electromagnetic Waves - Microwaves

We begin our study of the electromagnetic spectrum this week. An electromagnetic wave consists of mutually perpendicular (and mutually "perpetrating") electric and magnetic fields. Because they are mutually perpetrating, electromagnetic waves can propagate through empty space. Unlike all other types of waves we have studied, no medium is required.

The range of detectable frequencies is vast, from a few Hz (recall the ELF controversy of a previous week of Net Research Project) to gamma rays with frequencies up to 10²² Hz and beyond. We will concentrate on the visible region for several weeks and discuss x-rays and gamma-rays near the end of the semester. But this week's topic concerns frequencies roughly from 10⁹ to 10¹³ Hz with corresponding wavelengths of a few cm to fractions of a mm. This is the microwave region.

The Microwave Oven

The microwave oven is one of the more ubiquitous symbols of modern life. It can warm up or cook food much faster than conventional means. (A recent poll indicates that it is used most often for reheating coffee!) The process is quite simple. Polar molecules have electric dipole moments. In the presence of an external electric field, there is a torque which tends to align the dipole moment with the external field. (Review your text on page 691). When bombarded with microwaves, polar molecules will oscillate back and forth at the frequency of the electric field. Most foods contain water, a very polar molecule. The frequency of a microwave oven is tuned to maximize the heating that is produced by the vibrating water molecules. If a substance is composed mostly of non-polar molecules, the microwave oven cannot efficiently heat the substance. Anyone who has attempted to melt bakers chocolate in a microwave oven knows it takes an unusually long time. Bill Beaty has put together a nice microwave info page. There you will find useful info, FAQ's, and some microwave experiments. I hereby disavow any responsibility should you choose to try some of the experiments ... but if you do decide to try them, be sure to tell us what happens in class!

Microwave Communications

Almost all types communications such phone, cable television and radio have some component in microwave transmission. Because of its shorter wavelength, microwave transmission can be focused more tightly than those for standard radio and television broadcast. This is better for point to point type transmissions. (In addition, microwave frequencies can carry far more information than radio and TV broadcast frequencies.) Most cable radio and television programs are sent via microwave to the various users who pay for the programs. St. Thomas and St. Croix telephone service is maintained via the microwave transmitting/receiving towers on Crown Mountain and Blue Mountain, respectively.

Microwave frequencies are too high to generate with LC circuits. (Try a few values of L and C required to produce a 10¹² Hz frequency!) Instead, they are produced by creating a resonant standing wave in a cavity, such as the one shown in the picture from the National University of Singapore. Essentially, the standing electromagnetic wave is created in a fashion similar to that of a standing sound wave created in an organ pipe.

The FCC controls some frequency regions for far-reaching systems, such as satellite TV systems, and regulates their use to minimize interference. Other regions are left for more locally confined use ( 5 miles or less) and are not controlled. One example, the Local Multipoint Distribution System (LMDS) operates at about 28 GHz. Nortel maintains an LMDS tutorial that can provide details. UVI has a microwave system for distant learning (Room T101 has this technology) that operates at at 6 GHz.

The Cosmic Microwave Background

Arno Penzias and Robert Wilson won the 1978 Nobel Laureate in Physics for their discovery of cosmic microwave background (CMB) radiation. They found it by accident. They had constructed a microwave antenna to study specific objects in the sky. But they had this annoying background noise whose origin they could not locate. Eventually, they realized that it was coming from outside the earth and was essentially uniform in all directions. The picture to the left is a color enhanced plot of the distribution of the background radiation.

As it turned out, this was just the sort of thing astrophysicists had been looking for to verify the current big bang theories of the universe. As all big bang theories require, the early universe was hot and full of electromagnetic radiation. The peak frequency of this electromagnetic spectrum was directly related to the temperature. (We'll discuss this later in the semester.) As the universe expanded and cooled, the radiation frequencies dropped accordingly. Today, those frequencies represent a temperature of about 3 degrees Kelvin and are in the microwave region. Check out Wayne Hu's Introduction to CMB for more information on the CMB and its relation to the big bang model.

Be sure to enter your full name. Enter your email address ONLY if you want the number of points awarded e-mailed to you. Responses should be brief but complete. You must click the SUBMIT button to submit your entry. Entries are due by noon on Monday of next week.