Waterbender

You can use static electricity to bend water to your will!

What you'll need:
Plastic comb or balloon
Faucet

Turn the faucet on so that only a tiny stream of water comes out (1/16 inch, or 1-2 mm). Run the comb through dry hair or blow up and rub the balloon on your hair. This will build up a static charge. Position the comb or balloon near the water and you will see the stream bend away from the charge. Who knew moving water around was so easy?

A Really Bad Hair Day

Electricity is a force that is created by electrons, tiny bits of negatively charged matter that are usually found in atoms and molecules. However, sometimes the electrons break free and create either current electricity or static electricity.

Current electricity is the kind we usually thing of, in which we get the power from a battery or from plugging into a wall outlet. Electrons flow from one place to another, like a river. We'll explore current electricity more later this month.

In static electricity, the electrons have no where to go, and they just pile up. When they are provided a path to flow through, they rush over all at once. If you have ever walked across a carpet floor, then gotten a nasty shock when you touched something metal, you've had a taste of static electricity. The girls in the photo at left are experiencing static electricity created by a van der Graaf generator (the silver dome in the background).

Here's a fast, simple experiment you can try that will demonstrate some basic ideas about static electricity.

What You'll Need:
Balloon
Facial tissue, torn into small pieces
Antistatic dryer sheet

Blow up the balloon and tie the end. Rub the balloon all over your head (unless you're bald, then find someone with hair!). Slowly pull the balloon away from your head. What happens to your hair? Do the individual hairs on your head stick together or fly away from one another? Place the balloon near the facial tissue bits. Do they react? Now rub the dryer sheet all over the balloon, and try to pick up your hair and the tissue pieces. What happens?

Don't Be Dense!

Here's a quick experiment you can perform to prove to yourself that hot air is less dense than cool air.

What You'll Need:
Dry, empty 2-L soda bottle
Balloon
Hair dryer

Stretch the mouth of the balloon over the top of the soda bottle, and let the balloon hang down over the side. Heat the bottle with the hair dryer. What happens to the balloon?

Pretty quickly, you'll notice that the balloon starts to inflate. Now, you're not blowing into it. The hair dryer isn't blowing into it. There's no chemical reaction going on to create gas. Why is the balloon inflating?

The collapsed balloon, bottle, and the air inside the bottle weigh a certain amount, and there's a certain volume of air in the bottle. If we knew what the exact numbers were, we could calculate the density. When you heat the air, you make it less dense. You can't make it weigh less, because there are the same number of molecules in the cool air as in the hot air. The only way to change the density is to make the volume larger. Since the balloon is free to inflate, that's where the extra volume goes, and you see it begin to grow larger!

Full of Hot Air!

Colorful hot-air balloons are certainly beautiful, as they float through the sky. They stay up as long as the air inside them remains hot. If you're watching the balloon from a distance, they may seem very quiet, and much of the time, the balloons do just float along silently. But every now and then, the balloonist has to turn on the burner to heat the air again, and that's quite noisy if you're in or near the balloon!

Hot-air balloons rise because hot air is less dense than cooler air. Density is the weight of something divided by its volume. A hot-air balloon typically holds about 100,000 cubic feet (2800 cubic meters) of air. Cooler air (68^oF, 20^oF) weighs about 7500 pounds (3400 kilograms); hot air (250^oF, 120^oC) weighs about 5600 pounds (2500 kilograms). That's a difference of 1900 pounds (900 kilograms)! When the air in the balloon is heated, the whole thing becomes lighter, and it rises into the air.

You may notice that you usually don't see hot-air balloons during the day. They're most often spotted in the early morning or just before sunset. That's because it's safest to take off when the air is at its calmest. Also, the air tends to be cooler at these times, so the difference between the cool air and the heated air is greatest.

Another thing about hot-air balloons - you can't really steer them! Balloon flights aren't used to get from one place to another, because where the balloon goes depends on which way the winds blow. Balloons are followed by chase vehicles, cars or trucks that meet the balloon when it lands, driven by people who help the balloonists pack the thing up and take it back to the starting point.

Have you ever been to a balloon festival? As beautiful as one or two hot-air balloons are, it's really amazing to see over one hundred of them take off at once. Some famous balloon festivals are the Quick Chek Festival of Ballooning in Readington NJ (where Dr. B took the photo above) and the Albuquerque International Balloon Fiesta in NM. You can look for a balloon festival near your home here.

Fix a Flat with CO2?

You can buy repair kits for bicycle tires that include a small cylinder of CO₂ gas, which reinflates the tire. It's meant just for short emergency repairs, but some people leave the CO₂ in the tire longer than intended. Based on what you learned about CO₂ in the last two posts, what do you think happens to the tires if the CO₂ is not replaced with air? (Hint: think about what the tire is made of!)

Where Did the Gas Go?

After a couple of hours, you may observe that the CO₂-filled balloon is noticeably smaller than the air-filled balloon. By the next day, there won't be any doubt. Where is the gas going?

"Aaahhh," you might say, "I remember talking about effusion, which explained why helium-filled balloons deflate faster than air-filled ones. Tiny helium atoms can escape faster through itty-bitty holes in the balloon than the molecules in air can. It must be the same effect!" That's a very good connection to draw, so let's see if it fits with what we know about the sizes of gas molecules.

Here are some gas molecules, drawn more or less to scale:

Most of air is nitrogen, which exists as N₂ (shown in blue). Oxygen is O₂ (red) and carbon dioxide is CO₂ (gray and red). Other gases shown for comparison are argon (aqua), water (red and black), and methane or natural gas (gray and black). Helium (He) is the little orange circle!
Does it still make sense to say that He and CO₂ effuse quickly because they are so much smaller than the other molecules in air? Of course not! CO₂ is actually the biggest molecule shown! What is going on?

I have to admit that I was puzzled by this at first, because I was thinking just like you are. But it turns out that the speed with which gas molecules escape from a thin-walled container like a balloon isn't just determined by the size. And carbon dioxide is known to move through many kinds of rubber very quickly indeed (rubber is said to be very permeable to CO₂).

But why? It turns out that CO₂ is very comfortable mixing with the rubber. So these molecules get absorbed into the balloon wall, and work their way through to the outside (a process called diffusion). Other molecules, such as nitrogen and oxygen, don't mix so easily, and they stay in the balloon longer, although they eventually diffuse out as well.

So, the moral of this story is, you can't always assume that one scientific principle applies in every case - sometimes you have to look deeper. And frankly, that's one of the things that makes science so interesting!

The Amazing Self-Inflating Balloon!

Are you getting tired of blowing up balloons for these experiments? Here's a way to get chemistry to do the work for you!

What you'll need:
1 empty plastic bottle (about 20 ounces in volume)
Measuring spoon (1 tablespoon)
Funnel
Balloon
Vinegar
Baking soda

Pour about one inch of vinegar into the bottle. Using the funnel, put one tablespoon of baking soda into the balloon. Carefully stretch the balloon opening over the mouth of the bottle, letting the balloon hang down over the side, as shown in the photo above. When you're ready to go, lift the balloon up so that the baking soda falls down into the vinegar. You'll immediately see lots of bubbling, and will observe the balloon grow larger! What's going on?

Baking soda (chemical known as sodium hydrogen carbonate) reacts with vinegar (or acetic acid) to give off lots and lots of carbon dioxide gas, or CO₂. The gas has nowhere to go but into the balloon! When the balloon is at its largest, remove it from the bottle and tie the end off to seal the CO₂ in. Blow up another balloon of the same type using your breath to about the same size and tie it off. Can you tell the difference between the two balloons? Put them aside for a few hours, and we'll come back to them later.

Stop the Pop!

1. Take one inflated balloon.

2. Take one sharp needle.

3. Stick needle into balloon.

Do you expect to hear a loud POP???

Most of the time, yes. But as the photo at right shows, it is possible to stick a needle into a blown-up balloon without popping it. The secret lies in knowing where and how to insert the needle.

First of all, only fill the balloon about half-way with air. Select a spot at the top or bottom of the balloon where the rubber isn't stretched out as much as it is in the middle. Push the needle in slowly, while twisting it a little. The rubber will resist at first, then give way. You may need to try a few times, but eventually you will be able to do this. Make sure that you have a sharp needle, and grease it with some vegetable oil if you are having trouble.

How is this possible? Rubber is composed of long, stringy molecules (picture a bowl of wriggly worms - yuck!). By pushing the needle in slowly, you allow the molecules to wriggle away from the point. If you insert the needle too quickly, they don't have time to get out of the way, and the balloon will pop. In the middle of the balloon, where the rubber is stretched thin, the molecules are also stretched out, and they can't move as easily. While it is possible to insert a needle into a balloon in the middle parts, it is much harder.

World's Easiest Balloon Animal

Courtesy of John Cassidy, here's the fastest and easiest balloon animal you can make! All you need is a rubber glove. If you, or anyone in your family, are allergic to latex, make sure that the glove is made from something other than latex rubber.

First you need to blow the glove up like a balloon. This is being demonstrated in the picture at left by Dr. B's young friend, Steven C. Make sure that all the fingers are inflated. Then have an adult tie a knot in the glove (where your hand would normally go) to keep the air from escaping.

Next, hold the the glove-loon by the knot with one hand. Grab the middle finger of the glove and pull sideways, as shown below by Matthew C. What kind of animal did he make? Here's a hint - it swims and is pretty scary. That's right - a shark! Now wasn't that fast and easy??

"Top" Balloon Animals

Have you ever seen someone make balloon animals or other kinds of balloon sculpture? Take look at the work of balloon genius John Cassidy of Mont Clare PA (click on "TV" from the home page). John makes all kinds of animals, plants, and even hats! He even holds several world's records for speedy balloon sculpting. He once created 654 balloon sculptures in one hour!

OK, so they're cool, but you might be wondering how balloons animals are related to science. Dr. B must admit to you that John is a member of her family (a cousin-in-law). So, is this post just an attempt to get you to look at his site? No, actually, the making of balloon animals is related to an area of mathematics called topology. Topology is the study of the way that three-dimensional shapes can be transformed from one into another, without cutting or gluing. Also, the beginning and ending shapes must have the same number of holes (which is obviously zero for a balloon!). So why do mathematicians study topology? It helps them plan efficient road, train, and computer systems.

You can experiment with topology using clay. First, make a ball. Then try smushing it into different shapes, like a cube, a flat sheet, or a box. Remember that you can push and pull all you want, but that you can't break the clay apart, stick two parts together, or make a hole. Next, make a clay donut. Can you figure out how to change it into the shape of a coffee mug? Or, you can just blow up some long, skinny balloons and make some animals! The next post will show you what John calls "the world's easiest balloon animal"!

Flying Child

After writing the last post, Dr. B remembered that the Mythbusters actually inflated a bunch of balloons that could lift a small child. The video is here. This probably gives you an idea of whether you did your calculations right!

Payload

So, helium rises because it's less dense than air, but you know that there's a limit to how much weight a helium balloon can lift. Obviously, it can lift the weight of the actual balloon and the string, but it won't lift you up into the sky (which is probably a good thing!). How much weight can a helium balloon lift? And could you gather enough helium balloons to lift yourself? Here's a experiment to try and some calculations that will let you figure that out!

What You'll Need:
Helium-filled balloon with string
Standard-sized metal paper clips (about 1-1/2" long)

Do this experiment indoors! Make a loop in the end of the string; the balloon should still float. Then attach one paper clip to the loop. Does the balloon still float? Keep attaching paper clips until the balloon can no longer lift its payload (the amount of weight that a device can lift). How many paper clips did it take?

Now for some math! We're going to do these calculations in the metric system because it's easier, and it's what scientists use! The weight of one standard-sized metal paper clip is about 0.5 grams. Grab a calculator and multiply the largest number of paper clips the balloon could lift by 0.5 grams. This tells you the total weight that the balloon can lift. So, if the balloon payload was 3 clips, that gives you 4.5 grams.

How much do you weigh? If you know your weight in pounds, multiply that number by 454 (the number of grams in a pound). If you know your weight in kilograms, multiply by 1000 (the number of grams in a kilogram). Let's say you weigh 60 pounds - that's 27,240 grams. Or, if you're 18 kilograms, that's 18,000 grams. Don't worry about how big those numbers are - grams are really small!

That's all the information you need to figure out how many balloons it would take to lift you off the ground. Can you do the calculation on your own? Think about it, and read the next paragraph only if you need help.

In the example above, the balloon lifted 4.5 grams (your number may be different). You should be able to see that two balloons could lift 9.0 grams (2 x 4.5). How much weight could 10 balloons lift? The answer is 10 balloons x 4.5 grams/balloon = 45 grams. Now we'll ask the question in another way - how many balloons would it take to lift 45 grams (about one pound)? Wait, we already know that answer (10), but let's see how the calculation is done. You don't multiply in this case, you divide: 45 / 4.5 = 10. How many balloons are needed to lift 100 grams? The calculator shows 100 / 4.5 = 22.22222..., but we really can't have a fraction of a balloon, so we'll raise the number to 23. You're probably beginning to see that it will take a lot of balloons to lift you. The last calculation is up to you. Divide your weight (in grams) by the payload weight of your balloon, and you'll have your answer!

Hee-Hee... Helium!

What's in those balloons that float up into the sky? Helium! What's so special about helium, that it makes balloons lighter than air?

It's all about density. Density is defined as the weight of a substance divided by its volume. For example, the density of iron is 491 pounds per cubic foot (a cube 12 inches on each side), or 7870 grams per liter. It's pretty obvious that, if you had a piece of iron smaller than a cubic foot, it would weigh less. But the ratio of the weight of the smaller piece to its volume would be the same as the larger piece.

If you mix substances with two different densities, the more dense one will sink, or looking at the the other way around, the less dense substance rises. If you dropped a chunk of iron (whether large or small) into water, it would sink. But if you put a piece of wood (most kinds, at least) into water, it floats because its density is less than that of water. Helium gas is less dense than air (0.011 pounds per cubic foot or 0.18 grams per liter for helium versus 0.078 pounds per cubic foot or 0.078 grams per liter for air), so balloons filled with helium float!

Up, Up, and Away!

Who doesn't love balloons? Whether they're the small party kind, filled with air or helium, or the giant hot-air balloons that you can ride in, balloons just make you smile! This month, you'll learn all kinds of experiments that you can do with balloons!