Action and Reaction
Key Concept: Unlike magnetic poles attract, like magnetic
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
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
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
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
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
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
Click her for details on the Magnetic Field Observation Box
Click here for details on Magnetic Field Viewers and Viewing Film
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?
Heavy Metal Breakfast
Key Concept: Iron particles used to enrich foods may be
removed with a magnet.
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;
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.
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.
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,
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.
More about Magnetism:
School for Champions; Succeed in
The Exploratorium: Snacks about
Molecular Expressions; Java tutorials
developed to help students understand topics in electricity and
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.
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