Gas Model - Caged molecules do their thing.

 

Gas Model

 

Caged molecules do their thing. 

 

The atomic theory of matter tells us that a gas is made up of tiny particles called atoms (or molecules, which are combinations of atoms), which are constantly in motion, smashing into each other and the walls of their container, if there is one. Here is a highly visual model of this idea. 

 

• 12 or more Styrofoam balls, approximately 1-1/4 inches (3 cm) in diameter (available in craft or fabric stores), or table tennis balls.
• A paintbrush.
• Latex paint.
• A small rodent cage with wire mesh on all sides. (Two plastic strawberry baskets or utility baskets with open grid sides may also be put together to form a cage.)
• Short pieces of wire or twist ties.
• A hair dryer, fan, or other blower.

15 minutes or less)

Paint one of the balls a bright color, using latex paint (because oil-based paint dissolves Styrofoam). Place the balls in the cage and secure the door of the cage with short pieces of wire or twist ties.

(15 minutes or more)

Hold the blower under the cage and blow air up through it. The moving air will agitate the balls, simulating the kinetic behavior of a gas. Watching the colored ball will allow you to follow the motion of a single "molecule."

By adjusting the speed of the blower from the cage, you can simulate heating and cooling a gas. The faster the balls are moving, the hotter the gas.

Listen for the clicking of the balls against the walls of the cage. At lower "temperatures," the clicking is quieter and occurs at a slower rate.

Adding heat (simulated by the blower) to a gas increases its internal energy. The molecules of the gas move faster and strike the walls of their container more often, yielding an increase in pressure (force per area). This increased pressure is simulated by the faster motion of the balls, which strike the sides of the cage more often. Cooling the gas (moving the blower farther from the cage) lowers the internal energy, slowing the motion of the molecules, and thus lowering the pressure.

You may want to try this Snack using cages of different volumes or try nesting baskets to change their volume. In this way, you can model the ideal gas law by changing temperature, pressure, and volume.

If you blow air on one side of the bottom of the cage and not the other, the balls will eventually "condense out." That is, they will form a pile on the side away from the blower, where it is "cooler."

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Fog Chamber - Make a portable cloud in a bottle. Now you see it; now you don't!

   

Fog Chamber

 

Make a portable cloud in a bottle. Now you see it; now you don't!

 

Clouds form when invisible water vapor in the air is cooled enough to form tiny droplets of liquid water. In the atmosphere, this usually happens when moist air cools as it rises to higher altitudes. At higher altitudes the pressure is lower, so that the gas expands, loses internal energy, and cools. You can accomplish the same cooling effect by rapidly expanding the air in a jar. 

 

• One 1 gallon (3.S liters) clear glass or plastic jar with a wide mouth (a pickle jar works well).
• A rubber glove (Playtex™ brand works well).
• Matches.
• Tap water.
• Adult help.

(5 minutes or less)

Barely cover the bottom of the jar with water. Hang the glove inside the jar with its fingers pointing down, and stretch the glove's open end over the mouth of the jar to seal it.

(15 minutes or more)

Insert your hand into the glove and pull it quickly outward without disturbing the jar's seal. Nothing will happen. Next, remove the glove, drop a lit match into the jar, and replace the glove. Pull outward on the glove once more. Fog forms inside the jar when you pull the glove outward and disappears when the glove snaps back. The fog will form for 5 to 10 minutes before the smoke particles settle and have to be replenished.

Water molecules are present in the air inside the jar, but they are in the form of an invisible gas, or vapor, flying around individually and not sticking to one another. When you pull the glove outward, you allow the air in the jar to expand. In expanding, the air must do work, which means that it loses some of its thermal energy, which in turn means that its molecules (including those of the water vapor), slow down slightly. This is a roundabout way of saying that the air becomes cooler!

When the water molecules slow down, they can stick to each other more easily, so they begin to bunch up in tiny droplets. The particles of smoke in the jar help this process along: The water molecules bunch together more easily when there is a solid particle to act as a nucleus. When you push the glove back in, you warm the air in the jar slightly, which causes the tiny droplets to evaporate and again become invisible.

In the atmosphere, air expands as it rises to regions of lower pressure and cools off, forming clouds. This is why clouds often obscure mountain tops. Dust, smoke, and salt particles in the air all provide nuclei that help the droplets condense.
Meteorologists consider a falling barometer reading (low air pressure) to be a sign of an approaching storm, whereas high pressure is usually a sign of clear weather. The temperature at which water vapor begins to form droplets on a surface is called the dew point.

For an added treat, shine a slide projector through the cloud you make in the jar. When the smoke is fresh, the droplets will be large compared to all wavelengths of visible light, and the light they scatter will be white. As the smoke dissipates, the water drops will become smaller, and the light scattered will create beautiful pastel colors at some viewing angles. Light of different colors diffracts around the small droplets, going off in different directions. If you look at clouds near the sun, you can often see bands of these pastel colors. (Remember, you should never look directly at the sun.)

For a longer discussion of this effect, see the book Clouds in a Glass of Beer  by C. Bohren (John Wiley & Sons, 1987).

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Far Out Corners - Your experience of the world influences what you see.

Far Out Corners

 

Your experience of the world influences what you see. 

 

When they first glance at this exhibit, many people say, "What's the big deal? It's just a bunch of boxes." But there are no boxes at all. A closer look reveals that the Far Out Corners exhibit is a cluster of corners lit from below. When you walk past the exhibit with one eye closed, the cubes will seem to turn mysteriously so that they follow your movement. 

 

• A large cardboard box measuring about 19 x 15 inches (48 x 38 cm).
• Flat black spray paint.
• Thick, white, nonflexible posterboard measuring at least 15 x 15 inches (38 x 38 cm).
• X-Acto™ knife or matte knife.
• Masking tape or transparent tape.
• A bright free-standing lamp.
• Adult help.

(1 hour or less) You can cut the inside corners from square-cornered containers, such as clean milk cartons or tissue boxes, or you can make your own corners from posterboard. To make your own, use an X-Acto™ knife or matte knife to cut the posterboard into nine squares, each of which measures 5 x 5 inches (13 x 13 cm).
Now use three of the squares to construct a partial cube or corner in the following fashion: Tape two squares together at one edge; open each of the two squares into a right angle; tape the third square on top of the first two squares. Make three partial cubes, or corners.



Spray-paint the inside of the large cardboard box black. When the box is dry, arrange the corners so that two are side by side on the bottom of the box, as shown. Make sure the hollow open sides of each corner are facing out toward you and down. Tape them so they are tilted up at a small angle. Place the third corner as far forward as possible on top of the original two, also tilted upward. Tape all three corners in place. Now position the light so that it shines directly into the box.

(15 minutes or more)Stand back ten feet and close one eye. With a little mental effort, you can see the corners that you have constructed as three-dimensional cubes rather than hollow corners.

Walk back and forth parallel to the box. Notice that the cube on top seems to be following you as you move.

The first step to successfully seeing the top partial cube turn with you lies in your ability to perceive it as a complete six-sided figure. This perception has a lot to do with being raised in a society that recognizes cubes as a common shape. Your brain is used to seeing cubes, so it fills in the rest of the cube shape, even though this partial cube only has three sides.

As you move past the exhibit, your view of the corners changes in a way that would not make any sense if the corners were stationary cubes. Your eye-brain system is used to seeing things that are near you move faster than things that are farther away. When you are riding in a car, for example, nearby objects seem to whiz by, whereas distant objects seem to follow you at a slower pace. Since you perceive this inside corner to be the outside of a solid cube, your brain "sees" the corner farthest from you as being the closest. To maintain this misconception, your brain perceives a rapid rotation of the cube as your angle to the corner changes.


The diagram above shows how this illusion works. In the real situation, as your eye moves to the right, it sees more of side A. In order to see more of side A of the imagined corner, the perceived cube must be seen to rotate as you move.
 

 

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Falling Feather - Prove to yourself that Galileo was right!

Falling Feather

 

Prove to yourself that Galileo was right! 

 

In a famous demonstration, Galileo supposedly dropped a heavy weight and a light weight from the top of the Leaning Tower of Pisa to show that both weights fall at the same acceleration. Actually, this rule is true only if there is no air resistance. This demonstration lets you repeat Galileo's experiment in a vacuum. 

 

• A clear, plastic, rigid-walled tube with at least a 1 inch (2.5 cm) inner diameter and at least 3 feet (90 cm) long. Available at your local plastic store. (Longer tubes show the effect more clearly.)
• A solid rubber stopper and a one-hole rubber stopper to fit in the ends of the plastic tube.
• A section of copper tubing about 4 inches (10 cm) long that fits tightly in the hole in the rubber stopper (glass tubing can be used if care is taken).
• A thick-walled flexible plastic or rubber vacuum tubing about 6 feet (180 cm) long.
• A coin and a feather (or a small piece of paper).
• A vacuum pump (use a regular lab vacuum pump if available; if not, use a small hand pump such as Mityvac®).
• 2 hose clamps.
• Adult help.

(30 minutes or less)

Insert the solid stopper firmly into one end of the plastic tube. Put the coin and feather in the tube. Push the copper tube through the one-hole stopper, and firmly insert the stopper in the other end of the plastic tube. Push the vacuum tubing over the copper tube and secure it with a hose clamp, if needed. Attach the other end of the vacuum tubing to the pump; again, use a hose clamp if needed.

(15 minutes or more)

Invert the tube and let the objects fall. Notice that the feather falls much more slowly than the coin. Now pump the air out of the tube and invert it again (the pump can remain attached while you invert the tube). Notice that the feather falls much more rapidly than before - in fact, it falls almost as fast as the coin. Let the air back into the tube and repeat the experiment. (Try to avoid rubbing the wall of the tube; otherwise, static electricity may make the feather stick to it.)

Galileo predicted that heavy objects and light ones would fall at the same rate. The reason for this is simple. Suppose the coin has 50 times as much mass as the feather. This means that the earth pulls 50 times as hard on the coin as it does on the feather. You might think this would cause the coin to fall faster. But because of the coin's greater mass, it's also much harder to accelerate the coin than the feather - 50 times harder, in fact! The two effects exactly cancel out, and the two objects therefore fall with the same acceleration.

This rule holds true only if gravity is the only force acting on the two objects. If the objects fall in air, then air resistance must also be taken into account. Larger objects experience more air resistance. Also, the faster an object is falling, the more air resistance it feels. When the retarding force of the air just balances the downward pull of gravity, the object will no longer gain speed; it will have reached what is called its terminal velocity. Since the feather is so much lighter than the coin, the air resistance on it very quickly builds up to equal the pull of gravity. After that, the feather gains no more speed, but just drifts slowly downward. The heavier coin, meanwhile, must fall much longer before it gathers enough speed so that air resistance will balance the gravitational force on it. The coin quickly pulls away from the feather.

The terminal velocity of a falling human being with arms and legs outstretched is about 120 miles per hour (192 km per hour) - slower than a lead balloon, but a good deal faster than a feather!

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Fading Dot - Now you see it; now you don't. An object without a sharp edge can fade from your view

Fading Dot

 

Now you see it; now you don't. An object without a sharp edge can fade from your view. 

 

A fuzzy, colored dot that has no distinct edges seems to disappear. As you stare at the dot, its color appears to blend with the colors surrounding it. 

 

• Pink paper (1 sheet).
• Blue paper dot (about 1 inch [2.5 cm] in diameter).
• Waxed paper.

(5 minutes or less)

Use the blue paper to make a 1 inch (2.5 cm) dot, and place the dot in the center of the pink paper. Cover the paper with a sheet of waxed paper. Look through the waxed paper at the colored papers below. Lift the waxed paper from the pink paper until you see very faint blue color in a field of pale pink.

(15 minutes or more)

Stare at a point next to the fuzzy dot for a while without moving your eyes or your head. The blue will gradually fade into the field of pink. As soon as you move your head or eyes, notice that the dot reappears. Experiment with other color combinations.

Even though you are not aware of it, your eyes are always making tiny jittering movements. Each time your eyes move, they receive new information and send it to your brain. You need this constant new information to see images.

Your eyes also jitter when you look at this dot, but the color changes at the edge of the dot (as seen fuzzily through the waxed paper) are so gradual that your eyes can't tell the difference between one point on the dot and a point right next to it. Your eyes receive no new information, and the image seems to fade away. If the dot had a distinct border, your eyes would immediately detect the change when they jittered, and you would continue to see the dot.

You may have noticed that, although the dot fades, just about everything else in your field of vision remains clear. That's because everything else you see has distinct edges.

For more information, we suggest you read the sections on lateral inhibition and chromatic lateral inhibition in Seeing the Light, by David Falk, Dieter Brill, and David Stork (Harper & Row, 1986).

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