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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Electroscope - What's your (electrical) sign?

Electroscope

 

What's your (electrical) sign? 

 

A commonly available brand of plastic tape can gain or lose negatively charged electrons when you stick it to a surface and rip it off. By suspending pieces of tape from a straw, you can build an electroscope, a device that detects electrical charge. A plastic comb will enable you to identify whether the pieces of tape are positively or negatively charged. 

 

• 4 plastic drinking straws with flexible ends.
• 2 plastic 35 mm film cans.
• Enough modeling clay to fill the film cans halfway.
• A roll of 3-M Scotch Magic™ Tape, 3/4 inch (2 cm) width. (Don't substitute other brands of tape the first time you try this Snack. Once you know what to expect, you can experiment with other tapes.)
• A plastic comb and hair or a piece of wool cloth.

(5 minutes or less)

Press enough modeling clay into both film cans to fill them halfway to the top. Press the inflexible ends of two drinking straws into the clay in each can, and bend the flexible ends to form horizontal arms that extend in opposite directions. The heights of the straws should be the same.

(15 minutes or more)

Tear off two, 4 inch (10 cm) pieces of tape. Press each piece firmly to a tabletop or other flat surface, leaving one end of each tape sticking up as a handle. Quickly pull the tapes from the table and stick one piece on an arm of a straw in one film can, and the other piece on an arm of a straw in the other film can. Move the cans so that the two tapes are face to face, about 6 inches (15 cm) apart. Then move the cans closer together. Notice that the two tapes repel each other.

Tear off two more pieces of tape and press the sticky side of one against the smooth side of the other, leaving one end of each tape sticking out as a handle. Quickly pull the tapes apart and stick them to the two remaining arms. Bring the arms close together. Notice that these two tapes attract each other.

Run the comb through your hair, or rub the comb with the wool cloth. Then hold the comb near the dangling tapes. Notice that the comb repels the piece of tape whose smooth side was in the middle of the "sandwich" and attracts the tape whose sticky side was in the middle. When you hold the comb near the tapes pulled from the flat surface, the comb will repel both tapes if they were pulled from a Formica™ surface; the comb may attract tapes pulled from other surfaces.

Try pulling other kinds of tape from various surfaces, or rubbing various objects together, and then bringing the tape or objects near the tapes on the arms. Bring your hand near the tapes and notice what happens.

When you rip the two pieces of tape off the table, there is a tug-of-war for electric charges between each tape and the table. The tape either steals negative charges (electrons) from the table or leaves some of its own negative charges behind, depending on what the table is made of (a positive charge doesn't move in this situation). In any case, both pieces of tape end up with the same kind of charge, either positive or negative. Since like charges repel, the pieces of tape repel each other.

When the tape sandwich is pulled apart, one piece rips negative charges from the other. One piece of tape therefore has extra negative charges. The other piece, which has lost some negative charges, now has an overall positive charge. Since opposite charges attract, the two tapes attract each other.

When you run a plastic comb through your hair, the comb becomes negatively charged. Tapes repelled by the comb have net negative charge, and tapes attracted by the comb either have net positive charge or are uncharged.

You may have found that your hand attracts both positively and negatively charged tapes. Your body is usually uncharged, unless you have acquired a charge -- by walking across a carpet, for example. An uncharged object attracts charged objects. When you hold your hand near a positively charged tape, the tape attracts electrons in your body. The part of your body nearest the tape becomes negatively charged, while a positive charge remains behind on the rest of your body. The positive tape is attracted to the nearby negative charges more strongly than it is repelled by the more distant positive charges, and the tape moves toward your hand.

Since some table surfaces will not charge the tape, be sure to test your surfaces before trying this Snack with an audience.

Charge leaks slowly off the tape into the air or along the surface of the tape, so you may have to recharge your tapes after a few minutes of use.

You can use your electroscope to test whether an object is electrically charged. First use the comb to determine the charge on a piece of tape, and then see whether an object whose charge is unknown repels the tape. If the tape is negatively charged and an object repels it, then the object is negatively charged. 
Don't use attraction to judge whether an object is charged: A charged object may attract an uncharged one. If tape is attracted to an object, the tape and the object may have opposite charges, or the tape may be charged and the object uncharged, or the object may be charged and the tape uncharged. But if the tape is repelled by the object, the tape and the object must have the same charge. The only way that tape and an object will neither repel nor attract is if both are uncharged.

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