Concepts: thunderstorms, static electricity
Materials: AM radio w/ antenna, inflated rubber balloon
An AM radio makes an excellent lightning detector. During a thunderstorm you
can listen in on individual lightning bolts on your AM radio. Tune the radio to a
station and each crackling sound that you hear is from the radio waves emitted by
a lightning bolt. You will notice that you hear the static crackle at the same
moment you see a flash of lightning. This is because radio waves and visible
light both travel at the speed of light. The sound of thunder travels at the speed
of sound (about 1,000 feet per second) and thus lags behind the flash of light (or
AM crackle)*. You will notice that there will be some lightning bolts that you
detect that are not seen. These are lightning bolts that are high up in the
thunderstorm cloud or even in another thunderstorm cell.
You can make your own lightning strikes (on a much smaller scale) and detect
them on your AM radio. Tune your AM radio to a station with its antenna
extended. Take an inflated balloon and rub it back and forth for awhile on your
own clean, dry, oil-free hair (fur works well also). Slowly bring the balloon
close to the antenna. You may need a quite classroom to hear the effect, but the
static crackle becomes audible as you bring the balloon slowly near the antenna.
* You can use the time lag between lightning and thunder as an excellent way to judge the distance to the lightning. Count the seconds after you see the flash of lightning. Each second equals about 1,000 feet or five second equals about a mile. If you see the flash of lightning and then hear the thunder by the time you count to ten (or sooner) you are within two miles of the lightning and could be at some risk of getting a lightning strike!