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Mondo Magnets: 40 Attractive (and Repulsive) Devices and Demonstrations

Mondo Magnets: 40 Attractive (and Repulsive) Devices and Demonstrations

by Fred Jeffers

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Surprising and seemingly impossible effects result from the 40 experiments included in this fascinating science resource—all based on real magnetic physics. Each experiment—such as using a common refrigerator magnet to create a three-dimensional image or floating a magnet and carbon sheet in midair—is outlined with step-by-step instructions and diagrams


Surprising and seemingly impossible effects result from the 40 experiments included in this fascinating science resource—all based on real magnetic physics. Each experiment—such as using a common refrigerator magnet to create a three-dimensional image or floating a magnet and carbon sheet in midair—is outlined with step-by-step instructions and diagrams that illustrate the key concepts of magnetism. Even the most experienced science teacher or at-home tinkerer will find dozens of new tricks in this amazing collection.

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Mondo Magnets

40 Attractive (and Repulsive) Devices & Demonstrations

By Fred Jeffers

Chicago Review Press Incorporated

Copyright © 2007 Fred Jeffers
All rights reserved.
ISBN: 978-1-61374-244-0


Experiment 1

Magnetic Money, Rocks, Beach Sand, Total® Cereal, and Dirt

It's shown that money, many rocks, beach sand, Total® cereal, and even common dirt are magnetic.


U.S. $1 bill
Cellophane tape
Very strong magnet (such as an NdFeB magnet)
Rock (see page 2)
Several handfuls dry beach sand
Two small, nonmagnetic jars (such as glass baby food jars) and their lids
Several handfuls Total® cereal
Medium-sized bowl
Pestle or heavy spoon
Several handfuls dirt

Fold the $1 bill tightly into a roughly ½-inch square and hold it together with cellophane tape, as shown. Show that the $1 bill is attracted to the strong magnet.

Next, show that the rock can be suspended under the magnet. You will need to find a suitable rock before you begin — not all rocks are attracted to a strong magnet. Go to a beach or a streambed or anywhere where there is a large variety of rocks, touch your strong magnet to different small stones, and in a short time you should be able to find one that will work for this demonstration.

Place a large handful of beach sand in one of the jars and secure the lid. Turn the jar upside down and tape the magnet to its bottom. Shake the jar briskly for a few minutes, then remove the lid and slowly pour out the sand. A tablespoon or so of coarse, strongly magnetic black powder will remain in the jar, attracted to the magnet.

Remove the magnet from the jar and pour the black powder into another small jar for safekeeping. Using the same sand, repeat the procedure a few times to separate a bit more magnetic material from the sand. Transfer the black magnetic powder to the other small jar and set aside.

Using additional handfuls of sand, repeat the process, transferring the black magnetic powder that's collected to the second jar as you go. (This powder can be used to make a crude but effective Magnetic Mountain demonstration, which is described in Experiment 7.)

Place a large handful of cereal in the bowl and, using the pestle or spoon, grind it into a very fine powder. Pour the powdered cereal into one of the jars and secure the lid. Turn the jar upside down, tape the magnet to the bottom of the jar, and briskly shake the jar for a few minutes. Remove the lid and slowly pour out the powdered cereal. About a teaspoon of magnetic material will remain inside, attracted to the magnet. You may have to repeat this process a few times, using new bowls of cereal, to collect a significant amount of magnetic material. Shown on page 4 is a small amount of magnetic cereal matter suspended in a plastic bag.

Finally, take a few handfuls of ordinary dirt from the garden and refine it in the same manner as for sand. This photo shows the jar of magnetic dirt suspended below the magnet.

The Science Behind It

Why are all these things magnetic?

In each case, the black magnetic material that's collected is iron oxide. Iron is the fourth most abundant element in the earth's crust. The most abundant iron oxide is black Fe3O4, which is strongly magnetic. A $1 bill is magnetic because the black ink on the portrait side of the bill is loaded with black iron oxide Fe3O4 pigment. The bill contains only about 5 mg of the oxide. The bill weighs almost exactly 1 gm, so the Fe3O4 present in the pigment is easily holding up more than 200 times its own weight! This Fe3O4 pigment was originally put in the ink to make it black, not to make it magnetic. The hysteresis loop of a $1 bill is shown below. The coercivity is about 100 Oe and the remanent moment is appreciable.

The image on the next page was made by scanning the surface of a $1 bill with a magnetic read head similar to those used in modern tape recorders and hard disk drives. (The image is courtesy of Fred Chamberlain, who was at San Diego Magnetics at the time that he provided it.)

Comparing the magnetic image above to an actual dollar bill, the observant viewer will note that the treasury seal on the left side of the portrait is black, but is not magnetic. That is because the black ink in the seal contains no Fe3O4. This fact is the basis of some simple counterfeit currency detectors.

The rock shown in this experiment is magnetic because it also contains Fe3O4. About 1 in 20 Southern California beach rocks is slightly magnetic, while about 1 in 100 rocks is magnetic enough to be picked up.

Below is an M-H loop of the powdered cereal. Also shown is an almost identical loop of metallic iron powder.

The obvious conclusion from this plot is that Total® fortified cereal is "fortified" with iron that has been added in the form of metallic iron powder. It remains to be seen if metallic iron is digested and absorbed by the body.

The beach sand and dirt both contain varying amounts of Fe3O4. The rock, sand, and dirt all have M-H loops very similar to that of the $1 bill, because all three contain small particles of Fe3O4.


Experiment 2

The Super Strong Magnetic Nail

A common nail is used to pull a string of small steel objects off a very strong magnet. The objects remain suspended under the nail until the magnet is removed, whereupon they fall off.


8-cm long iron framing nail
3 common steel balls measuring 1.3 cm, 1 cm, and 0.8 cm, respectively
1-cm square steel nut
Very strong 5-cm by 5-cm by 2.3-cm NdFeB magnet (magnetized in the 2.3-cm direction)

A nail alone is not a magnet and won't pick up anything. Most people are very surprised to see, then, that a nail will pull small objects off a strong magnet, one at a time, in a long string.

Place the steel balls and the nut on the magnet. Pick up the nail and hold it so that its head is pointed toward the magnet. Place the nail head on the large ball and lift the ball, being careful to pull it up and away from the center of the magnet. Next, touch the large ball to the medium-sized ball and lift it from the center of the magnet. Continue with the small ball and then the nut, as shown below. Carefully raise the string of objects above the magnet. When the objects are a distance of 50 cm (20 inches) or so above the magnet, remove the magnet; the string will remain suspended from the nail head.

Then, slowly place the magnet back under the suspended string and say a few magic magnetic words, such as "gauss" or "oersted." Slowly lower the string of objects toward the center of the magnet. At a distance of about 15 cm (6 inches), everything will abruptly fall off the nail and back onto the magnet.

Here's the trick: while everyone is watching the string of objects hang in the air and wondering why they don't fall off the nail head, discreetly turn the magnet over. As the nail and the objects come closer to it, the inverted magnet demagnetizes the nail; the force disappears, and the small objects fall off the nail head.

The Science Behind It

The field generated by the magnet magnetizes the nail by a process called magnetic induction. The nail used in this experiment is very easily magnetized parallel to its length because it's made of almost pure iron. It has a much higher maximum magnetization, or saturation magnetization, than the magnet below it. In other words, the nail can become even more strongly magnetized than the magnet itself.

When the nail and string of objects are removed from the magnet, they retain some remanent magnetization even in the absence of the magnet's field because they have a low, but nonzero, coercivity. Therefore, they remain stuck together because they are still partially magnetized.

The magnetic field at the head of the magnetized nail is very intense, but its intensity decreases rapidly as distance between the magnet and the nail increases. Hence, the force exerted by the nail is greater than the force exerted in the center of the magnet, and all of the objects can be lifted from the magnet's center quite readily. However, if you tried to pull objects from the edges of the magnet, you would not be able to. The field there is large and it changes rapidly with distance, so the force is much stronger at the edges of the magnet than it is in the center of the magnet.

A Variation

A very subtle extension of this trick is the demonstration that, unlike the behavior of the string of objects, when only the large ball is lifted off the magnet and raised up, at some point it always falls off the nail.

In the earlier experiment, the combination of the nail and objects results in a "string" that is so long that the demagnetizing field at the end of the nail is quite small. This results in a relatively large magnetization in the end of the nail that produces a force strong enough to support the entire string. When only the large ball is picked up, the demagnetizing field at the end of the nail is larger, and as a result the magnetization in the end of the nail is smaller. The resultant force is not strong enough to support the single ball in the absence of the magnet, and it falls off when the magnet is removed.


Experiment 3

The Magic Table Knives

It's shown that common stainless steel table knives will stick together when they are magnetized. Two knives can even be lifted off the table by a third, proving that the force of attraction is very strong. However, there is almost no force of repulsion when one knife is reversed.


Strong magnet

Three ordinary stainless steel table knives with solid handles

Use a strong magnet to magnetize each knife (labeled A, B, and C here for purposes of explanation) in the same direction, parallel to the length of each knife: place the tip of each knife in contact with the "north pole" face of the magnet and slide the knife down the magnet to the end of the knife's handle. The primary field in the center of the magnet is perpendicular to the knife, but the transverse field at the trailing edge of the magnet is also large, so the last field seen as the knife moves over the magnet is parallel to its length and is directed from the knife's hilt to its tip. In the last magnetizing step, place the handle end of the knife in the center of the north face of the magnet and lift the knife straight up. Then place the tip of the knife in the center of the south face and again lift the knife straight up. The idea is to magnetize each knife in the same direction, parallel to its length.

The coercivity of a common stainless steel table knife is usually higher than 100 Oe, and the knives retain a good deal of magnetization, like a permanent magnet when the magnetizing field is removed. The mutual force of attraction is so strong that it's possible to pick up two knives, hilt to tip, with the third, as is shown here.

However, if you rotate one knife 180 degrees, you'll see that its force of repulsion is not even close to being strong enough to push another knife away. (This effect was discovered by Dr. Tom Howell.)

The Science Behind It

How can the force of attraction be so strong and the force of repulsion be so weak?

The explanation is that the fields near the tips of the knives are comparable to the coercivity. In the attraction mode, the field of Knife A enhances the magnetization of the tip of Knife B, and vice versa. As a result, the respective magnetizations at the tips are large, and the mutual force of attraction is strong.

In the repulsion mode, the field of one knife tends to demagnetize the tip of another. Hence, the magnetizations and the forces of repulsion are weak. However, the force of repulsion is not zero; this can be easily shown by balancing one knife on the edge of a table. (The balanced knife may tend to rotate to the north in response to the earth's field.) Bringing another knife, in repulsion mode, within a centimeter or so of the first knife easily causes the balanced knife to rotate away. This occurs because the fields present at that distance of separation are so small that they do not demagnetize the tips.

A Variation

A final mystifying bit of table knife physics follows. In repulsion mode, if a finger is used to prevent the balanced knife from rotating, and the tip of the knife is carefully touched by the tip of another knife, a very weak force of attraction may be observed. But this effect doesn't happen with all pairs of knives. In this experiment, Knives A and C and Knives B and C exhibit the effect, but Knives A and B do not!

What is happening? In each case where two knives show a weak force of attraction, one of the knives has a higher coercivity than the other. Its field is thus large enough to locally reverse the magnetization of the other knife at the point of contact. This produces the very weak attractive force. Because the fields involved are so small, the effect is reversible, and it can be repeated at will without causing a change in the magnetization of the knife.

The reason that pairs A and C and B and C exhibit the effect is that Knife C has a higher coercivity than either Knife A or B. The reason why the A-B pair doesn't show the effect is that Knife A has about the same coercivity as Knife B. The tip of each knife is demagnetized to nearly zero when one is brought near the other, and no force of attraction occurs.


Experiment 4

The Mysterious Magnetic Arch

Magnetic balls form an arch on a magnetic plate. The arch is shown to be stable even when the plate is held vertically or upside down.


18 very strong 8-mm-diameter spherical NdFeB magnets (available from Engineered Concepts at www.engconcepts.net) 5-inch by 7-inch, 40-mil-thick steel plate

Place the magnets on top of the steel plate as shown below. They will form a sturdy, symmetric arch.

You will find that this arch is surprisingly stable and that it can be poked and prodded without collapsing. The arch even remains stable when the plate is oriented vertically, though gravity will cause it to sag slightly.

The Science Behind It

Each magnet in the arch is oriented in a very stable "head-to-tail" arrangement. When the balls are placed on the magnetic plate, a magnetic image is created inside the surface of the high-permeability (easily magnetized) steel plate, similar to the reflection one sees in a mirror.

Away from the plate, the magnetic field generated by the magnetic image of the balls is identical to the field that would exist if the image balls were real, and the physical balls and the plate were not present. Because of the field emanating from the magnetic image balls, the actual balls on the plate and the image balls are effectively arranged in a complete and stable circle, although only the arch (the "half circle") of the actual balls can be seen.


Experiment 5

The Strange "Zinging" Magnets

Two oblong, football-shaped magnets are held in the hand, separated by a small distance. When they are tossed in the air they snap together and emit a loud "zinging" noise.


Two oblong, ceramic barium ferrite magnets (available from Cyberguys, Inc., at www.cyberguys.com/templates/searchdetail.asp?T1=250+0680 and many other Web site stores)

Hold the two magnets in the palm of your hand, using your thumb to keep them about an inch apart.

Toss them into the air. As they snap together they will emit a loud "zinging" noise.

The Science Behind It

The magnets are made of ceramic barium ferrite and are magnetized through the small dimension. They are, of course, strongly attracted to one another. The ceramic material from which they are made is very hard. If they are separated by an inch or so and thrown up into the air, they will snap together, bounce apart and snap back together very rapidly, with only a small amount of energy loss per bounce. Because the force of attraction is strong and the ceramic is very hard, the initial frequency of the magnets bouncing apart and coming back together is a few hundred hertz, or Hz (cycles per second). The frequency increases as the amplitude of the bouncing decreases. This causes the bouncing magnets to emit a loud ZZZZZZZiiiiNNNNGGG noise.

For fun, try tossing the magnets to an unwary victim, saying, "Here, catch!" The zinging noise is very disconcerting, and most people will fail to make the catch. (It's best to pull this joke over a carpeted floor to avoid breaking the magnets.)


Excerpted from Mondo Magnets by Fred Jeffers. Copyright © 2007 Fred Jeffers. Excerpted by permission of Chicago Review Press Incorporated.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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