CoolStuff Newsletter Article                                                            Vol. 5, April 2002

Elementary Magnetism: Magnetic Momments

A mystery to the ancients and a marvel to Einstein, magnetism is inextricably linked to the operation of motors and generators, the functioning of radio, television and computers, and our understanding of the universe. From the Greeks, who found that certain stones would attract pieces of iron, to Einstein who, as a child, was captivated by the mysterious properties of magnets, humankind's fascination with magnetism has never abated. Your students are certain to feel the same sense of mystery and wonder as they probe one of the fundamental forces of nature.

The eight investigations that follow, allow students to explore the mysteries of magnetism. Students will undoubtedly go beyond the suggested activities by manipulating the magnets in novel ways. Isn’t that what inquiry learning is all about? You may even want to incorporate their discoveries in your collection of magnetic activities next year.

Here’s wishing you and your students many marvelous magnetic moments!
~ Chris Chiaverina

Action and Reaction

Key Concept: Unlike magnetic poles attract, like magnetic poles repel.

Place two identical ring magnets on a smooth surface. Align them so that they repel each other. Bring them close together and then release them. What happens? Does each magnet exert a push on the other magnet? How do you know? Since the magnets have the same mass, what can you conclude about the strength of the force acting on each magnet?

Now arrange the two magnets so that they attract each other. Separate the two magnets so they are one or two centimeters apart and then release them. Describe what happens. Does each magnet exert a pull on the other magnet? How do you know?

You’ve just used magnets to demonstrate Newton’s Law of Action and Reaction. If one object exerts a force on a second object, the second object exerts an equal but oppositely directed force on the first object.

Floatation Devices

Key Concept: Like magnetic poles repel.

Place four or five ceramic ring magnets on a pencil so that when the pencil is held vertically, the magnets levitate above one another. Describe the spacing between the magnets. Is the spacing uniform? If not, explain your observations. Can you think of another situation where material stacked vertically behaves in a similar manner?

First compress, then release the vertical array of magnets. Notice the “springiness” exhibited by the magnets. Why do the magnets behave this way?

Click here for details on the "Floating Magnets Set"

Picking Up Some Cash

Key Concept: Strong magnets attract ferromagnetic materials in inks used in paper
currency.

Hold one end of a bill of any denomination in your hand. Bring a neodymium magnet close to the free end of the bill. What do you observe? Now bring the magnet’s other pole close to the end of the bill. What happens this time? Can you explain your observations?  

An Interesting Plot

Key Concept: Every magnet establishes a magnetic field in the space surrounding it. Field lines may be made visible with iron filings.

Place a sheet of glass or clear plastic over a bar magnet. Sprinkle iron filings onto the sheet above the magnet. Gently tap the glass or plastic. What happens? Make a sketch of the pattern produced by the iron filings.

Repeat the experiment, using two bar magnets. Place the second magnet so the poles of the two magnets are facing each other. Make a sketch of the field lines produced by the pair of magnets. Now rotate one of the magnets by 180 degrees and once again examine the resulting field. If time and materials permit, plot the magnetic field lines produced by a variety of magnets.

You may make a permanent record of the magnetic fields you observe by performing your experiments on the glass plate of a Xerox machine.

There are other methods of observing magnetic fields. One of these involves a magnetic field viewer. The viewer is a credit-card sized laminate containing minute iron particles. Not only is the device convenient, it defines the shape of the magnetic field more precisely.

Click her for details on the Magnetic Field Observation Box 

Click here for details on Magnetic Field Viewers and Viewing Film

Click here for details on the Lenz's Law apparatus 

Putting on the Brakes

Key Concept: A moving magnet will induce an electric current in a conductor. This current in turn produces a magnetic field that opposes the motion of the magnet.

Drop an unmagnetized object through a length of copper tubing. Describe the time required for the object to pass through the tube. Now drop a strong magnet into the tube. Describe its motion through the tube. How do you explain the behavior of the two objects?

Repeat the experiment, but this time use a non metallic tube (e.g., PVC ) with dimensions similar to those of the copper tube. Describe what happens this time. Can you explain your observations?

Many foods are enriched with iron. This is not surprising since iron is essential for good health. However, you may be surprised to learn that in some cases the iron is added to the food in the form of iron filings. One product rich in iron filings is Total® . To see this for yourself, place a handful of the product in a container and cover with water. Allow the mixture to stand until the cereal dissolves. Stirring the resulting mush with a magnet will result in an accumulation of iron filings. Gently rinsing the magnet with water to remove any residual cereal will make the filings visible.

A Simple Motor

Image courtesy of Idaho State University Physics Department; http://www.physics.isu.edu/physdemos/electric/smplmtr.html

Key Concept: A pivoted electromagnet will swing into alignment with a fixed magnetic field. If the current through the electromagnet is reversed each time the electromagnet aligns itself with the fixed magnet, the coil will continue spinning.

To build a very simple motor you will need the following materials:
• 1 - 'D' Cell Battery
• 1 - Wide Rubber Band
• 2 - Paper Clips
• 1 - Ceramic Magnet
• Approximately 1.5 ft of 22 Gauge Magnet Wire (the gauge is not critical but wire should be coated with red enamel insulation)
• 1- Toilet Paper or Paper Towel Tube
• Fine Sandpaper
• Hot glue or tape
Produce a coil of wire by wrapping the enameled wire around the tube. The coil should consist of 3 or 4 turns of wire with approximately 2 inches of wire left over at each end (see fig.1). Slide the finished coil off the tube. 

To keep the coil assembly together, wrap the wire around the coil before straightening it out on either side.

This example shows the 4 coils spread out to view, and the two ends wrapped around the coils. Your coils would not get spread out.

Fig. 1

Fig. 2

Using sandpaper, remove the enamel from just one side of both straight segments of wire.  In the greatly exaggerated figure below, the enamel has been removed from just the top side of both wires. Important: remove the enamel from same side of both wires.

The two large paper clips should be fashioned into the shape shown at the right.

Fig.3

Use the rubber band to hold the two paper clips in place and attach the magnet to the battery with either hot glue or tape (see figure). Place the two arms of the coil in the clips as shown and give it an initial spin with your finger. If the coil does not continue spinning, you may need to move the coil closer to the magnets. This may be accomplished by simply adjusting the paper clips.

Click here to get details on the "Worlds Simplest Motor" kit

An Even Simpler Motor

An extremely simple motor can be assembled from just a battery, a nail, a neodymium magnet, and a length of copper wire. The arrangement of these elements is shown in the image to the left. The motor is activated by touching the tip of the copper wire to the side of the magnet. An electric current flows through a circuit consisting of the wire, magnet, and battery. Charge flowing through the magnet experiences a force due to the field of the magnet. This produces a force on the magnet causing it to spin. (Note: this motor is based on a design presented in the December 2004 issue of The Physics Teacher magazine.) 

A Very Cool Magnet

By now, most people have heard the term superconductivity. What many people don’t know is that it means the flow of current with absolutely no electrical resistance. Often associated with technological advances of the future such as levitated trains, zero-resistance power transmission, and super computers, superconductivity only occurs at extremely low temperatures. Therefore it’s a phenomenon not seen by many. However, with a superconductive disc, a small cube-shaped neodymium magnet, and some liquid nitrogen, superconductivity may be readily demonstrated. The first two items are available for $39 from Arbor. Small amounts of liquid nitrogen may often be obtained for educational purposes from university and commercial research laboratories, animal breeders, and hospitals.

Click here for details on the Arbor Scientifics' Superconductor kit

The superconductive disc is placed in the lid of a plastic Petri dish and covered with liquid nitrogen. After the disc has been allowed to cool for approximately one minute, it becomes superconducting. At this point, plastic tweezers are used to bring the neodymium magnet close to the surface of the disc. Once released, the magnet will levitate above the surface of the disc. [ Tech Notes: The Meissner effect in superconductors like the superconducting disc acts to exclude magnetic fields from the material. Since the electrical resistance is zero, supercurrents are generated in the material to exclude the magnetic fields from a magnet brought near it. The currents which cancel the external field produce magnetic poles which mirror the poles of the permanent magnet, repelling them to provide the lift to levitate the magnet.]

Students are amazing to see the little cube floating in air. If the cube is given a tap on one of its corners it will spin for quite some time. Students will likely have difficulty seeing the tiny magnet from their seats. You can either walk around the room with the demonstration or use a flex cam to project the image on a screen. Either way, this is a demonstration that your students will not soon forget!

Did you know....

Over 2000 years ago, the Greeks observed that lodestone, a naturally occurring material, was capable of attracting pieces of iron. Lodestone’s unusual properties were first observed in a region of Greece known as Magnesia, so it comes as no surprise that materials with the similar properties are still referred to as magnets.

While the Greeks are credited with discovering magnetism, the Chinese are believed to be the first to use lodestone as a compass, an application of magnetism that changed the world. The compass not only permitted the circumnavigation of the globe but led to the modern science of magnetism.

Four hundred years after the Chinese first took to the sea with compass in hand, Englishman William Gilbert found that some materials can acquire magnetic properties by rubbing them with lodestone. Noting that a compass always points in the same direction, Gilbert also suggested that the earth itself is a magnet.

Prior to the nineteenth century, scientists believed that electricity and magnetism were distinct phenomena, even though they share some common characteristics. Then in 1820, a Danish scientist Hans Christian Oersted made a remarkable discovery. He found that an electric current affects a magnetic compass. Later it was observed that moving a magnet through a coil of wire induced an electric current. There was clearly a link between electricity and magnetism, but what was it?

James Clerk Maxwell realized that electricity and magnetism are different manifestations of the same phenomenon called electromagnetism. Maxwell produced a set of four equations that govern every aspect of classical electric and magnetic phenomena.

Perhaps one of the most astonishing ramifications of Maxwell’s work was that light was an electromagnetic wave. And the surprises kept on coming. With the 1905 publication “On the Electrodynamics of Moving Bodies,” Einstein revealed that magnetism was not a unique force but rather the natural consequence of viewing electricity through the lens of the theory of special relativity. 

Interesting Links: 

More about Magnetism: http://www-istp.gsfc.nasa.gov/Education/Imagnet.html

School for Champions; Succeed in Physical Science: http://www.school-for-champions.com/science/magnetism.htm

The Exploratorium: Snacks about Magnetism: http://www.exploratorium.edu/snacks/iconmagnetism.html

Molecular Expressions; Java tutorials developed to help students understand topics in electricity and magnetism: http://micro.magnet.fsu.edu/electromag/java/

Next Issue:

Static electricity will be the subject of the next edition of Coolstuff. The electrical force is so pervasive that it is difficult to name many aspects of the physical world not affected by it. At the most fundamental level, the attractive electrical force between electrons and protons holds atoms together. On a bit larger scale, electrical interactions between atoms are responsible for the formation of molecules. On a much grander scale, a rapid discharge of atmospheric electricity manifests itself as a flash of lightning. All these phenomena are governed by the same basic principles.

Though every student has had direct experience with static charges (who hasn’t been zapped after walking across a carpet?), few understand the mechanisms underlying electrostatic phenomena. Join us next time for a collection of activities designed to spark interest in learning about electricity.

Regards,

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