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This chapter is from the book

Silent Radio

It's really easy to mute a radio playing at maximum volume.

Step By Step

Supplies

  • battery-operated radio

  • cardboard box with a lid (like a shoebox)

  • empty juice carton lined with aluminum foil

  • aluminum foil

  • microwave oven

Tune the radio to a station with a strong signal at maximum volume (with adult supervision). Now, place it inside the shoebox. With the lid on, can you still hear the radio? Next, open the juice carton enough to fit the radio inside and close it up again, as shown (with adhesive tape, if necessary). What happens to the radio?

Alternative

Place the radio inside a metal cooking pot (with a metal lid), or wrap it (including the antenna) in aluminum foil and see what happens. Repeat the experiment with a cellular phone inside a metal pot (lid on) or wrapped in foil. Does it ring when you call its number? You can also wrap aluminum foil around a remote control (to TV, VCR, garage door, or car alarm) and see if these devices still work. What do these remote controls, the radio, and the cell phone all have in common?

A Step Further

Fill a cup with water and pour an equal amount into the empty juice carton (lined with aluminum). Place them both in a microwave and turn it on for about 30 seconds. Compare the water in the cup with that inside the carton. What is the connection between this experiment and the silent radio experiment?

Fun Facts

A metal pot or aluminum foil has lots of "free electrons," which can move around inside the metal walls. These electrons act like tiny antennas as an electromagnetic wave hits the metal's surface. The wave's electric field pushes free electrons and causes them to accelerate. These accelerating electrons redirect (absorb and reemit) the wave in a new direction, producing a mirror reflection. Just as a box made of metal mirrors keeps visible light out, a box made with metal walls keeps radio waves and the microwaves of a cell phone out. The same holds when the electromagnetic wave is generated inside a metal box. The wave becomes trapped inside it, as it is reflected by the metal walls. That's exactly what happens in a microwave oven. (The holes in the metal grid on the oven's glass window are smaller than the microwave wavelength, so the grid also reflects the waves back.) When you turn on a radio or receive a call on a cell phone, external electromagnetic signals activate these devices. When they are inside a metal pot (lid on) or wrapped in aluminum foil, the free electrons in the pot or in the foil respond quickly, reflecting back the incoming signals. This does not happen with the shoebox, since cardboard is an insulator and does not have free electrons. (Try using strips of paper or cardboard to connect a small lamp from a flashlight, for example, to a battery and see if the lamp lights up. You can also try wetting the paper or cardboard. Try wetting it with salt water. What happens then?)

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