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# Boom!: What Causes Thunder?

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Source:
Author: Janice VanCleave

### Problem

What causes thunder?

### Materials

• 9-inch (23-cm) round balloon
• Glove
• Straight pin

### Procedure

1. Inflate the balloon to about half of its normal size, about 4 inches (10 cm), and make a knot.
2. Lay the inflated balloon on a table.
3. Place the glove on one hand.
4. Hold the pin with the gloved hand.
5. Stand at arm's length from the balloon.
6. Stick the pin into the balloon.

### Results

When the pin is inserted into the balloon, the balloon rips. At the same time a loud popping noise is heard.

### Why?

When your lungs force air inside the balloon, the rubber stretches and the balloon inflates. The air inside the balloon pushes outward. The stretched rubber pushes the air inside the balloon.

Sticking the pin into the balloon makes a tiny tear. The stretched rubber immediately starts to pull at the tear. At the same time, the compressed air rushes out and pushes on the tear. The balloon breaks apart.

As the compressed air rushes through the tear, it expands (moves apart). This quick expansion of air pushes outward against the air surrounding the balloon. This creates sound waves that reach your ears as a popping sound. Thunder is produced in a similar way. As lightning strikes, it gives off energy that heats the air through which it passes. This heated air quickly expands, then cools and contracts. The fast expansion and contraction of air around lightning causes air molecules to move back and forth, which in turn produces sound waves that you hear as thunder.

### Let's Explore

1. Does the amount of air in the balloon affect the results? Repeat the experiment twice; the first time, inflate the balloon to about 2 inches (5 cm); and the second time, inflate the balloon to its full size of about 9 inches (23 cm).
2. Does the shape of the balloon affect the results? Repeat the experiment using balloons of different shapes. Science Fair Hint: Inflate and photograph the different balloons used. Display the photographs along with the results of the experiments.
3. How does the content of the balloon affect the results? Repeat the original experiment. Fill the balloon with water by stretching the mouth of the balloon over an outside water faucet. Lay the balloon on a table outdoors. When the balloon is pricked, compare the sound made by the balloon filled with water to the sound of the balloon filled with air. When explaining any differences in the experiments, remember that water is much more difficult to compress than air.

### Show Time!

1. How does the speed of the expanding air affect the results? Inflate a 9-inch (23-cm) balloon to about half its normal size. Place a 1-inch (2.5-cm) piece of duct tape over a section near the neck of the balloon. Be sure the tape sticks smoothly to the balloon's surface. Put gloves on both hands. With one hand, hold the balloon at arm's length against a table. Stick the pin through the tape with the other hand. Remove the pin and allow the compressed air to slowly leak out. If the balloon breaks, as in the original experiment, the tape did not stick to the balloon. Try again.
2. There are other ways to show the fast expansion of air:

• Fill a sealable plastic bag with air and close it. Hold the bag in your hands and squeeze firmly until the bag opens.
• Fill a paper lunch bag by blowing into it. Twist the open end and hold it closed with one hand. Quickly and forcefully hit the bag with your free hand.
• listen as you open a warm can of soda. The gases inside the can expand quickly when the can is opened, producing the fizzing sound you hear.
3. You can figure out how far away a lightning stroke is from you. Count the seconds from the instant you see the lightning until you hear the thunder. Divide the seconds by five to calculate the number of miles away the lightning was. For kilometer distances, divide the seconds by three.

### Check it Out!

1. Lightning is a flash of light, and thunder is a booming sound. Why is thunder always heard after the lightning flash is seen? Find out about the speeds of light and sound. Does air temperature affect either one?
2. Often lightning flashes are seen but no thunder is heard. Find out how far from a flash of lightning the sound of thunder is carried. How would wind force and direction affect the distance? Is the type of lightning stroke a factor in this distance?