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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Electrical Fleas - Start your own electric flea circus

Electrical Fleas

 

Start your own electric flea circus! 

 

You're probably familiar with some of the effects of static electricity: Static electricity makes the sparks when you comb your hair on a cold day, and it makes balloons stick to the wall at a birthday party. In this Snack, static electricity makes electric "fleas" jump up and down. 

 

• A sheet of acrylic plastic or other clear plastic (about 1 foot [30 cm] square and 1/s inch [3 mm] thick).
• A piece of wool cloth or fur.
• 4 supports about 1 to 2 inches (2.5 to 5 cm) high (tuna cans work nicely).
• A large piece of white paper, 11 x 17 inches (28 x 43 cm).
• Tiny bits of "stuff." Aluminized ceiling glitter works well, as do grains of rice, puffed rice cereal, spices (dill weed, basil, ground cloves, or nutmeg), or bits of Styrofoam.

(5 minutes or less)

Put the piece of paper on the table. Place the supports on the paper beneath the four corners of the plastic, and scatter the tiny bits of Styrofoam, spices, ceiling glitter, or rice under the plastic. (You can set this assembly up on any tabletop.)

(15 minutes or more)

Charge the plastic by rubbing it vigorously with the piece of wool cloth or fur.

Watch the "fleas" dance! Try different types of material for charging the plastic, including your hand, and experiment with other materials for fleas. Also, try the plastic at different heights.

Both the plastic and the fleas start out electrically neutral. That is, they have an equal number of positive and negative charges. When you rub the plastic with the wool cloth, the cloth transfers negative charges to the plastic.

These negative charges polarize the fleas, attracting the positive charges to the tops of the fleas and pushing the negative charges to the bottoms of the fleas. The attraction between the negative plastic and the positive charge concentrated on the top of the fleas makes the fleas jump up to the underside of the plastic.

When a flea actually touches the plastic, some of the plastic's negative charge flows to the flea. The top of the flea becomes electrically neutral. But since the whole flea was originally neutral, the flea now has some excess negative charge. The negatively charged flea and the negatively charged plastic repel each other strongly, which causes the flea to jump quickly back to the table. As the flea's excess negative charge slowly drains away to the tabletop, or to the air, the flea again becomes neutral and is ready to jump up to the plastic once more.

While the fleas are dancing, put your ear on the plastic plate. Listen to the tapping of the fleas as they hit the plastic. The tapping rate slowly decreases as the charge on the plastic is depleted. The dance of the fleas sounds like the clicking of a Geiger counter measuring a radioactive source that is decaying.

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Eddy Currents - A magnet falls more slowly through a metallic tube than it does through a nonmetallic tube.

   

Eddy Currents

 

A magnet falls more slowly through a metallic tube than it does through a nonmetallic tube. 

 

When a magnet is dropped down a metallic tube, the changing magnetic field created by the falling magnet pushes electrons in the metal tube around in circular, eddy-like currents. These eddy currents have their own magnetic field that opposes the fall of the magnet. The magnet falls dramatically slower than it does in ordinary free fall in a nonmetallic tube. 

 

• A cow magnet or neodymium magnet.
• A nonmagnetic object, such as a pen or a pencil.
• One 3 foot (90 cm) length of aluminum, copper, or brass tubing (do not use iron!) with an inner diameter larger than the cow magnet and with walls as thick as possible.
• One 3 foot (90 cm) PVC or other nonmetallic tubing.
• Optional: 2 thick, flat pieces of aluminum (available at hardware and home-repair stores); cardboard; masking tape; rubber bands or cord.

 

No assembly needed.

Hold the metal tube vertically. Drop the cow magnet through the tube. Then drop a nonmagnetic object, such as a pen or pencil, through the tube. Notice that the magnet takes noticeably more time to fall. Now try dropping both magnetic and nonmagnetic objects through the PVC tube.

In addition to dropping these objects through the tubes, a very simple, visible, and dramatic demonstration can be done by merely dropping the magnet between two thick, flat pieces of aluminum. The aluminum pieces should be spaced just slightly farther apart than the thickness of the magnet. A permanent spacer can easily be made with cardboard and masking tape if you don't want to hold the pieces apart each time. Rubber bands or cord can hold the pieces all together. The flat surfaces need to be only slightly wider than the width of the magnet itself. Thickness, however, is important. The effect will be seen even with thin pieces of aluminum, but a thickness of about 1/4 inch (6 mm) will produce a remarkably slow rate of fall. Allow at least a 6 inch (15 cm) fall.

As the magnet falls, the magnetic field around it constantly changes position. As the magnet passes through a given portion of the metal tube, this portion of the tube experiences a changing magnetic field, which induces the flow of eddy currents in an electrical conductor, such as the copper or aluminum tubing. The eddy currents create a magnetic field that exerts a force on the falling magnet. The force opposes the magnet's fall. As a result of this magnetic repulsion, the magnet falls much more slowly.

Eddy currents are often generated in transformers and lead to power losses. To combat this, thin, laminated strips of metal are used in the construction of power transformers, rather than making the transformer out of one solid piece of metal. The thin strips are separated by insulating glue, which confines the eddy currents to the strips. This reduces the eddy currents, thus reducing the power loss.

With the new high-strength neodymium magnets, the effects of eddy currents become even more dramatic. These magnets are now available from many scientific supply companies, and the price has become relatively affordable. (An excellent source is Dowling Miner Magnetics Corp., P.O. Box 1829, Sonoma, CA 95476. )

Eddy currents are also used to dampen unwanted oscillations in many mechanical balances. Examine your school's balances to see whether they have a thin metal strip that moves between two magnets.

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Duck Into Kaleidoscope - Make multiple images of yourself.

Duck Into Kaleidoscope

 

Make multiple images of yourself.

 

Duck Into Kaleidoscope will create hundreds of images of - whatever you place inside it. The basic kaleidoscope is a triangle, but mirror tiles can beformed into other shapes and angles as well. 

 

• 6 mirror tiles measuring 1 x 1 foot (30 x 30 cm). (Or use plastic mirrors from a plastic supply house.)
• Duct tape.
• 3 pieces of sturdy cardboard measuring 1 x 2 feet (30 x 60 cm).
• Adult help.

(30 minutes or less)

Place the six mirror tiles in a row, as shown below. Tape each tile to the tiles on either side with duct tape (shown as dark stripes on drawing), leaving just enough room for the tape to flex and act as a hinge. Tape over any sharp edges.


Stand the pieces of cardboard on a table so that the long sides are horizontal. Fold the bottom 3 inches (7.5 cm) of each piece of cardboard to form a lip. Tape the 3 cardboard pieces together to form a large equilateral triangle (2 feet [60 cm] on each side), with the lip on the inside.

Form the mirror tiles into an equilateral triangle that is 2 feet (60 cm) on each side, and insert them into the cardboard form so that the bottom edges of the mirrors rest in the cardboard lip. (Be sure that the mirrors are facing inside.)

(5 minutes or more)

Put the kaleidoscope over your head. You will see a million faces!

Take the mirrors out of the cardboard form and make them into various closed geometrical shapes, such as a square, a rectangle, or a hexagon. Put each shape over your head, or place an object in the center of the shape, and see the reflections.

In a kaleidoscope you see reflections of reflections.

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