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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Disappearing Glass Rods - You can make glass objects disappear.

      

Disappearing Glass Rods

 

You can make glass objects disappear. 

 

Glass objects are visible because they reflect some of the light that shines on them and bend or refract the light that shines through them. If you eliminate reflection from and refraction by a glass object, you can make that object disappear.

• Wesson™ oil. (Regular, not lite.)
• One or more Pyrex® stirring rods or other small, clear glass objects.
• A beaker.
• Optional:  glass eyedropper, glass magnifying lens.

(1 minute or less)
Pour some Wesson™ oil into the beaker.

(15 minutes or more)
Immerse a glass object in the oil. Notice that the object becomes more difficult to see. Only a ghostly image of the object remains. If you do this as a demonstration, keep your audience at a distance to make it harder for them to see the ghost object.

Experiment with a variety of glass objects, such as clear marbles, lenses, and odd glassware. Some will disappear in the oil more completely than others.

You can make an eyedropper vanish before your eyes by immersing it and then sucking oil up into the dropper.
Immerse the magnifying lens in the oil. Notice that it does not magnify images when it is submerged.

When light traveling through air encounters a glass surface at an angle, some of the light reflects. The rest of the light keeps going, but it bends or refracts as it moves from the air to the glass. You see a glass object because it both reflects and refracts light.

When light passes from air into glass, it slows down. It's this change in speed that causes the light to reflect and refract as it moves from one clear material (air) to another (glass). Every material has an index of refraction that is linked to the speed of light in the material. The higher a material's index of refraction, the slower light travels in that material.

The smaller the difference in speed between two clear materials, the less reflection will occur at the boundary and the less refraction will occur for the transmitted light. If a transparent object is surrounded by another material that has the same index of refraction, then the speed of light will not change as it enters the object. No reflection and no refraction will take place, and the object will be invisible.

Wesson™ oil has nearly the same index of refraction (n) as Pyrex® glass (n = 1.474). Different glasses have different indices of refraction. In Wesson™ oil, Pyrex® disappears, but other types of glass &emdash; such as crown glass or flint glass &emdash; remain visible. Fortunately for us, a great deal of laboratory glassware and home kitchen glassware is made from Pyrex® glass.

For most Pyrex® glass, the index matching with Wesson™ oil is not perfect. This is because the Pyrex® glass has internal strains that make its index of refraction vary at different places in the object. Even if you can match the index of refraction for one part of a Pyrex® stirring rod, the match will not be perfect for other parts of the rod. That's why a ghostly image of the rod remains even with the best index matching.

The index of refraction of the oil (and of the glass, too) is a function of temperature. This demonstration will work better on some days than others.

Index of refraction is sometimes called optical density, but optical density is not the same as mass density. Two materials can have different mass densities even when they have the same index of refraction. Though Pyrex® glass and Wesson™ oil have similar indices of refraction, Pyrex® sinks in Wesson™ oil because it has a higher mass density than the oil. Wesson™ oil has a higher index of refraction than water (n = 1.33), but it has a lower mass density and floats on water. The index of refraction depends not only on density, but also on the chemical composition of a material.

You can also make Pyrex® glass disappear by immersing it in mineral oil, which is available from pharmacies or chemical supply houses. However, mineral oil comes in light, medium, and heavy weights, and each variety has a different index of refraction. To match the index of refraction of Pyrex® glass, you'll need a mixture of mineral oils of different weights. To create the proper mixture, place a Pyrex® glass object into a large glass beaker and pour in enough heavy mineral oil to submerge it partially. Slowly add light mineral oil and stir. 

Watch the glass object as you pour. Most Pyrex® glass will disappear when the mixture is about 2 parts heavy mineral oil to 1 part light mineral oil. Notice the swirling refraction patterns as you mix the two oils.

Karo™ syrup is another material that has an index of refraction close to that of glass. Karo™ can be diluted with water to match some varieties of glass.

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