An electromagnetically charged challenge from Science Buddies

STOP AND GO: How can you block invisible waves? With a radio-controlled car and a few basic household materials, you can give this challenge a whirl and learn about electromagnetic waves in the process. Image: George Retseck

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Key concepts
Physics
Electricity
Electromagnetism
Wireless communication

Introduction
Have you ever wondered how a radio can receive music and news broadcasts over thin air? Radios, as well as radio-controlled cars and cell phones, all receive information via invisible waves. Some of these waves are called radio waves.

Radio waves are a type of electromagnetic radiation, a kind of energy that also includes visible light. Radio waves are not harmful but are in fact extremely useful for communicating across long distances. To send information using radio waves, a transmitting antenna sends out a radio wave at a certain frequency (which can tell us the size of the wave), and this is picked up by a receiving antenna. Some materials can block, or interfere with, radio waves. Have you ever noticed that you lose reception on a car radio or cell phone when you drive into a tunnel or an underground parking garage? What materials block the radio waves, and which ones allow the waves to easily pass through?

Background
A transmitting antenna, or transmitter, generates and sends out radio waves thanks to a specific electrical current. This is possible because inside transmitters are wires that allow negatively charged particles, called electrons, to flow through them, which makes an electrical current. When current flows within a wire, it generates an electromagnetic field around the wire. This electromagnetic field radiates out of the antenna in all directions, creating invisible radio waves.

When the electromagnetic radio waves hit a receiving antenna, or receiver, such as a radio, it generates a current inside of a wire in the receiver. (The reverse process of what happened in the transmitter when the current generated the electromagnetic field in the first place.) The receiver then processes the current back into the transmitted information, which, for a radio, allows you to hear music or other broadcasts. Some materials can block the radio waves that the transmitter generates, which can be tested by seeing whether a receiver can process and respond to information sent by a transmitter.

Materials
•    Radio-controlled car and its corresponding remote control (both with working batteries)
•    Cotton (such as a piece of clothing made from cotton)
•    Aluminum foil
•    Plastic wrap
•    Wax paper
•    Rubber glove
•    A smooth, wide open space to test drive your radio-controlled car

Preparation
•    First check the radio-controlled car and its remote control to make sure they both have fresh batteries. Check that the car runs well on the open space that you will be using.
•    Make sure you have enough of each material you want to test (the cotton, aluminum foil, plastic wrap, wax paper, and rubber glove) to completely cover the RC car remote control. Every covering should be loose enough so you can still operate the remote control buttons through it.

Procedure
•    Wrap the remote control in cotton. Make sure that the remote control is completely and securely covered so that there are not any openings or holes in the covering, and that it is covered loose enough so that you can still operate the controls.
•    Try to operate the radio-controlled car using the cotton-covered remote control. Does it work? Does the car move at all?
•    Remove the tested material from the remote control. Test to make sure that the car still works when it is not covered with any material.
•    Repeat this process using each different material separately. For which materials does the car still operate, and for which materials does the car not operate? Why do you think this is?
•    Extra: Are the wireless signals transmitted by other devices blocked by the same material(s) that you found could block the signals from the remote control in this activity? You can try this activity again but using other remote control devices, such as your TV or stereo remote. How do different wireless devices respond? Do you think they use similar or different types of waves?
•    Extra: Do you need to completely cover the radio-controlled car remote control to block its signal? Using the material(s) you found to block the signals from the remote control to the radio-controlled car, completely cover the remote control as you originally did in this activity, and then slowly remove the covering from the remote control while trying to operate the car. How much covering is needed to block the signal? Are some parts of the remote control more important to cover in order to block the signal?

View
1. 1. styman 03:17 PM 12/26/11

how do you block cell phones say at church were people forget to turn them off or down?

2. 2. moi45 12:56 AM 1/11/12

Keeping noise on electronic machines working specially with antenna is available with contacting two steel bars with each other by wire and sink the free ended steel bars into a lemon while avoiding touching each other inside the lemon. That makes a flux of magnetic field which you made and near to you more than the source signal that influences the electronic device so neighborhood takes order.

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