Instructor Information
JCE Classroom Activity: #71
Investigating the Invisible: Attenuation of Radio Waves
Have you ever been inside a building and found it impossible to tune in a radio station? If so, you were trapped inside what scientists call a “Faraday cage”. A Faraday cage can be visualized as a hollowed-out conductor of any shape. If you rubbed your feet on a carpet to generate excess charge (i.e., static electricity) and transferred the charge by touching the hollow conductor, the excess charge would be transferred to the outer surface of the conductor. The inner surface would not possess any excess charge. Excess charge placed anywhere on a conductor travels to the outermost surface and distributes itself evenly. Such a surface is an equipotential surface. An end result of an equipotential surface is that no electric field exists anywhere inside the conductor, hollow or otherwise. So, if a radio were placed inside a hollow conductor, no radio signals could reach the radio! This is because radio waves, like all electromagnetic radiation, are composed of fluctuating electric fields. When a non-fluctuating electric field is incident upon a conductor, the surface charges redistribute themselves to maintain equal charge distribution and a net-zero electric field exists inside the conductor. When a fluctuating electric field like a radio wave is incident upon a conductor, the surface charges make it difficult for the signal to be transmitted through the conductor. This is because the charges continuously redistribute themselves to maintain equal charge distribution resulting in a net-zero electric field inside the conductor. As long as the charges located on the surface of a conductor can evenly redistribute themselves rapidly enough, incident electromagnetic radiation will be blocked. What if a hollow conductor consisted of holes, like a ball made out of metallic screen? As long as the holes are much smaller than the wavelength of the incident electromagnetic radiation, the screen tends to block the radiation. An example of this is the door of a microwave oven. A metal screen on the door helps to prevent microwaves from being transmitted through the door because the holes are smaller than the wavelength of a microwave. (Microwave radiation has wavelengths on the order of centimeters while the door’s screen hole diameter is on the order of millimeters.) Now back to the original question of why radio stations cannot be received and tuned inside certain buildings. Some building materials contain rebar, wire mesh, or other metallic reinforcement that creates an effective Faraday cage.
Integrating the Activity into Your Curriculum Students often think of light as only being the visible region of the electromagnetic spectrum. It is often difficult for students to visualize the electric and magnetic fields that compose light and that different frequencies of light have different wavelengths. This Activity helps students to investigate these concepts in an interactive and novel fashion.
perforated
About the Activity This Activity leads students to interactively investigate properties of something that cannot be seen (radiation). The Instructor Information discusses the electrical underpinnings of what is occurring. The authors leave the decision up to the teacher to decide how much of these “electrical aspects” are discussed in the classroom. In More Things to Try, when the radio is placed inside the cage, weak AM stations are often completely attenuated whereas FM stations offer a lower degree of attenuation (softer volume while in the cage compared to volume while outside cage), if any. It will be difficult for students to completely attenuate a radio signal with household appliances. Some attenuation should be observed in appliances that basically surround the radio in metal.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
photo A. A. Smith and C. A Smith
Anthony A. Smith and Charles A. Smith* Department of Chemistry, Our Lady of the Lake University, San Antonio, TX 78207; *
[email protected] In this Activity, students investigate properties of radiation using a handheld radio. Students compare the abilities of conductive and dielectric materials to attenuate, or block, radio waves, and compare the attenuation of AM versus FM radio waves.
Answers to Questions 1. Conductors (metal pot with metal lid, foil) that surround the radio prevent radio waves from reaching it. Nonconducting materials (paper, glass, plastic) do not block radio waves. 2. No, foil with holes added still blocked the radio waves. The holes in the foil are smaller than the radio wavelengths. 3. Doors on microwave ovens have metallic screens that are roughly 2–3 mm in diameter. These help to block microwaves from leaving the microwave oven.
References, Additional Related Activities, and Demonstrations 1. Faraday Cage. http://www.fact-index.com/f/fa/faraday_cage.html (accessed Feb 2005). 2. Delaney, P.; Greer, J.C. C60 as a Faraday Cage. Applied Physics Letters, 2004, 84(3), 431–433. JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 82 No. 4 April 2005 •
Journal of Chemical Education
560A
JCE Classroom Activity: #71
Student Activity
Investigating the Invisible: Attenuation of Radio Waves
Try This You will need: a small, battery-powered, handheld AM/FM radio, aluminum foil, sharpened pencil, paper or newspaper, glass or plastic container with glass or plastic cover, and a metal container with a metal cover. Both containers need to be large enough to hold the radio with the cover fitting tightly. __1. Turn on a small battery-powered, handheld AM/FM radio. Tune the radio to an AM radio station and set the volume to a soft level. __2. Place the radio under your shirt. Are you still able to hear the station clearly or is there only static or silence? Does the volume change? Record your observations. __3. Wrap the radio in a thick roll of paper or newspaper. Record your observations. __4. Place the radio in a plastic or glass container, first without its plastic or glass cover, then with the cover. The cover should fit tightly when the radio is inside the container. Record your observations. __5. Place the radio in a metal container, first without its metal cover, then with the metal cover. The cover should fit tightly when the radio is inside the container. Record your observations. __6. Place a sheet of aluminum foil on a flat surface. Place the radio in the center of the foil. Slowly bring up the sides of the foil and gently wrap them around the radio. Record your observations. __7. Using the sheet of foil from step 6, have someone hold the foil while you use a sharpened pencil to carefully punch holes all over the foil. Repeat step 6, this time using the foil with the holes. Record your observations. __8. Tune the radio to an FM radio station and try these steps again using the materials or objects that effectively attenuated radio waves for the AM station. Record your observations and make note of any differences.
More Things To Try You will need: chicken wire, wire cutters, electrical tape, and other items to place the radio in. __1. Using a wire cutter, cut two squares of chicken wire, one slightly larger than the other. The smaller square will be a base and the larger will form a cage to contain the radio. Fold the larger square and connect the loose ends to form a cage with one side open. Wrap electrical tape around any sharp ends. Place the radio tuned to a weak AM station at the center of the base square. Slowly place the wire cage over the radio. Record your observations. __2. Tune the radio to an FM station and place inside the chicken wire cage. Does it appear that the chicken wire cage can attenuate AM radio waves better than FM radio waves? __3. Place the radio in as many different containers as you can conveniently and safely find, such as a drawer, microwave oven, refrigerator, etc. (Do not turn the appliances on!) In each case, record the type of material surrounding the radio (e.g., wood, metal, plastic, glass, etc.) and the degree of attenuation.
Questions 1. Based on your observations, what types of materials prevented radio waves from reaching the radio? Which materials did not block the radio waves? 2. Did punching holes in the aluminum foil have any effect on blocking the radio waves? Explain. 3. Examine the door of a microwave oven. What do you see? Why do you think the door was designed this way?
Information from the World Wide Web (accessed Feb 2005) Electromagnetic Spectrum—Introduction. http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html This Classroom Activity may be reproduced for use in the subscriber’s classroom.
560B
Journal of Chemical Education
• Vol. 82 No. 4 April 2005 •
www.JCE.DivCHED.org
photos A. A. Smith and C. A Smith
Did you know that X-rays, microwaves, gamma rays, radio waves, and visible light are all forms of electromagnetic radiation better known as light? The only difference among them is their wavelengths. The wavelength of radiation determines its energy and uses. Whether we can use radiation to see, cook hot dogs, communicate, or grow plants depends on its wavelength. What exactly is radiation? How can radiation be blocked? (Scientists call this “attenuated”.) Is attenuating radio waves related to wavelength or the type of material the radiation encounters? In this Activity, you will use a handheld radio to investigate how to effectively attenuate radio waves. After the radio is turned on and tuned to an AM radio station, the radio will be placed inside different objects. When only static or silence can be heard from the radio, then the radio waves are being prevented from reaching the radio. In the process you will discover which types of materials attenuate radio waves.
