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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Drawing Board - A pendulum moving in two directions creates beautiful designs.

Drawing Board

 

A pendulum moving in two directions creates beautiful designs. 

 

The Drawing Board consists of a marking pen that remains stationary and a platform that swings beneath the pen, acting as a pendulum. As the platform swings, the pen marks a sheet of paper that is fastened to the platform, generating beautiful repetitive patterns, which grow smaller with each repetition. These colorful designs contain hidden lessons in physics. 

 

Make Your Own Version:

• A ruler.
• One 4 x 4 x 12 inch (10 x 10 x 30 cm) wood post.
• A nonskid base.
• A drill.
• A pivot bolt.
• 2 metersticks or yardsticks.
• Washers.
• Wire.
• Rubber bands.
• 3 large tables or 4 evenly spaced hooks in a ceiling.
• A large board.
• 4 large hook eyes.
• Rope.
• Duct tape.
• Bricks.
• A large sheet of plain paper.
• Masking tape.
• Marking pens.
• Optional: String.
• Adult help.

(5 minutes or less)

Set up the Ellipto™ or Pendul-Art™ according to the manufacturer's instructions. If you want to build your own Drawing Board, see the information here.

(15 minutes or more)

Once the Drawing Board is adjusted, you can create wonderfully intricate designs. Try drawing one to four patterns on the same paper using pens of different colors, changing the direction and force of the push with each new color.

When the platform is displaced from its rest position, the four suspending strings exert forces on it to bring it back. You can think of these forces as acting in two directions perpendicular to each other: "north-south" and "east-west," for example. The combination of these two simultaneous motions can produce a variety of curved forms, in the same way that proper simultaneous manipulation of the two knobs on an Etch-a-Sketch™ toy allows you to draw curves.

The diminishing size of each successive repetition of the pattern is a graphic demonstration of how friction steadily dissipates the energy of a moving object.

Make Your Own Drawing Board

(1 hour or less) One of our teachers put together a large-scale version of the Drawing Board that was dramatic. Rather than attempting to give detailed instructions for assembling this device, we have chosen instead to supply some labeled drawings (see below) and helpful hints. The rest is left to the dedicated experimenter. The penholder must be counterbalanced so that the pen exerts minimum pressure on the moving board while maintaining constant contact with the writing surface. You will have to adjust the length of the suspension ropes, since they stretch with time. Try using a knot called a slip hitch, shown in the diagram. The placement of the weight on the board is critical. Experiment with various positions.         One person pushes the board to start rotational as well as translational motion. Another person controls the penholder, lowering the pen to start drawing and raising it to stop. The Drawing Board should produce a pattern that repeats the same basic shape over and over again, with each cycle getting smaller. If the pattern is not consistent from one cycle to the next, try moving the weight on the board or adjusting the counterbalance weight on the penholder. Also make sure that the penholder is not shifting on the floor.

Some of the shapes you will produce with the Drawing Board are known as harmonograms or Lissajous figures. An oscilloscope can easily produce these figures, since the pattern on the scope face is generated by a single electron beam simultaneously moving vertically and horizontally on the screen. An oscilloscope can be thought of as an electronic Etch-a-Sketch™. One of our teachers had this Snack set up and running during an aftershock of the 1989 Loma Prieta earthquake. The pen traced the pattern of motion generated by the aftershock. The operating principle behind the Drawing Board - a pen directly attached to the earth with a paper only loosely attached to the earth - is the operating principle behind a seismograph.

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Downhill Race - Two cylinders that look the same may roll down a hill at different rates.

Downhill Race

 

Two cylinders that look the same may roll down a hill at different rates. 

 

Two objects with the same shape and the same mass may behave differently when they roll down a hill. How quickly an object accelerates depends partly on how its mass is distributed. A cylinder with a heavy hub accelerates more quickly than a cylinder with a heavy rim. 

 

 

 

• 2 identical round metal cookie tins (such as those from butter cookies).
• 10 large metal washers (about 1/4 pound [112 g] each).
• Double-sided foam stick-on tape (or adhesive-backed Velcro™).
• A ramp

(15 minutes or less)

Arrange five of the washers evenly around the outside rim of the bottom of one tin. Stack five washers in the middle of the bottom of the second tin. In both cases, secure the washers with tape or Velcro™.

(15 minutes or more)

Place both tins at the top of the ramp. Be sure the tops are on. Ask your friends to predict which tin will reach the bottom of the ramp first. Release the tins and let them roll down the ramp. The tin with the mass closer to the center will always reach the bottom first.

At the top of the ramp, both tins have identical potential energy, since both have the same mass and are at the same height. At the bottom of the ramp, each tin will have part of its original potential energy appearing as linear (or translational) kinetic energy and the rest appearing as rotational kinetic energy. Though both tins have the same total mass, each has this mass distributed differently. It is harder to get the tin with its mass distributed along the rim rotating than it is to get the tin with its mass concentrated at the center rotating. The tin with its mass at the rim will use a greater part of its original potential energy just to get rolling than will the tin with its mass concentrated at the center. Therefore the tin with its mass at the rim has less energy available to appear as translational kinetic energy, resulting in a lower linear speed. The tin with its mass concentrated around the rim will lose the race to the bottom of the ramp, and the tin with its mass concentrated at the center will win.

The use of lightweight "mag" wheels on cars is related to translational and rotational kinetic energy. Imagine that you had two cars of equal overall mass, but one had lightweight "mag" wheels and a heavy chassis, and the other had heavy steel wheels and a light chassis. Given the same energy input, the "mag" wheel car would accelerate more rapidly, since less of the energy supplied would be needed to get the wheels rotating, and more would therefore appear as straightline motion of the car as a whole.

It is interesting to experiment with rolling cans of soup down an inclined plane. Solid soups roll down the incline at a slower rate than liquid soups. The liquid does not have to rotate with the can, so the potential energy of the liquid soup can go into linear motion, not into rotation of the soup.

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