Magnets

The Chinese knew that if you rubbed a steel needle against a lodestone, in a fixed direction, it also became a magnet. Around the year 1000, they furthermore found that if a magnet or lodestone was suspended, floating in a bowl of water, it always pointed in a fixed direction - north-south. You could rotate the bowl, but the magnet would keep pointing in the same direction.

The reason, we now know, is that the Earth, too, is magnetic. From that discovery came the magnetic compass, quickly copied by Arab navigators.  The magnetic compass soon spread to Europe. Columbus used it when he crossed the Atlantic Ocean, noting not only that the needle deviated slightly from exact north (as indicated by the stars) but also that the deviation changed during the voyage. Around 1600 William Gilbert, physician to Queen Elizabeth I of England, proposed an explanation: the Earth itself was a giant magnet, with its magnetic North Pole some distance away from its geographic North Pole.

Chemically and mineralogically, the lodestone is magnetite, a type of iron ore (or iron oxide, Fe₃O₄). Magnetite is  common in nature, but lodestones are relatively rare. Why are a few rare pieces of it different from the rest?

You can demonstrate the floating compass needle to your class on an overhead projector.  Float a small magnetized sewing needle in a Petri dish containing water. The needle should be the thinnest you can get--a small needle of about 3 cm is OK, heavier ones tend to sink. It should be dry, and handling it helps, as it rubs off some grease from the fingers. Magnetize it by stroking with a magnet.  (Get several needles, just in case the first one sinks; it may be harder to float a needle again after it gets wet.)

Drop the needle carefully onto the water's surface. Hold it horizontally at its middle between two fingers, just above the water, then let go.  The needle will point north-south, as you can show by placing a compass (a transparent compass, if you have one) on the overhead  projector.  Don't let the compass get too close to the petri dish, or the two magnetic needles will interact. (Also, make sure that all magnets are well away, or else the needle will point toward the magnet.)

Why does the needle float?  Remember that water has surface tension, which is like a thin "skin" of the top layers of water molecules, which stick together.  Remember water striders, insects which scurry over the tops of quiet ponds?  They also use the surface tension of water.

By 1580, the use and manufacture of compass needles was a well known art. The maker would take a flat steel needle, find its middle by balancing it, install a pivot there, and then magnetize it by stroking it against a magnet or a lodestone. But that was not enough. The north-pointing end always seemed heavier, and a tip had to be snipped off, to make the needle balance again.

The story goes that a compass maker named Robert Norman once snipped off too much and ruined a needle, so he devised an experiment, to find what was happening. Before magnetizing the needle, he balanced it not on a vertical pivot but on a horizontal one, lined up in the east-west direction. Before the needle was magnetized, it stayed horizontal. Afterwards, its north end slanted down. Aha! The north-pointing magnetic force on the needle was not horizontal, but pointed into the Earth.

It was a classical scientific experiment, and was published in 1581. Norman's contemporary was William Gilbert, distinguished physician and later physician to Queen Elizabeth I. Gilbert devoted much of his energy and money to study magnetism, and in 1600 published his research in a book "De Magnete" (Latin for "On the Magnet").

If you lived in London in 1600, you could have purchased "De Magnete" for seven shillings and sixpence. To read it, of course, you would have to know Latin, the language of science in 1600. You might have had the rare privilege of attending first runs of Shakespeare's plays in the "Globe" theatre. However, you might have had to weigh this pleasure against the peril of bubonic plague, which usually spread in the city during summer months.

Gilbert devised an experiment which suggested a reason for the properties of the compass: the Earth itself is a giant magnet. Using a lodestone fashioned into a sphere as a model of the Earth (he named it "terrella" or "little Earth"), Gilbert reproduced not only the north pointing properties of the horizontal needle, but also the downward slanting of the needle which Robert Norman made.

The Earth acts like a giant magnet.  It behaves as if it had a powerful bar magnet at its center. Just as a bar magnet has two poles where its attraction is strongest, so does the Earth, and its two magnetic poles are near the two geographic poles, which define the Earth's axis of rotation. ("Near" here means about 1000 km away.) The needle of a magnetic compass points towards the magnetic North Pole, which is how the Earth's magnetism was first noticed.

Michael Faraday had the bright idea of describing magnetic fields in 3-D by imaginary lines which are everywhere lined up with the magnetic force, the direction shown by a magnet freely suspended in 3-D. He called them "lines of force." Where the lines lie on a flat plane, you can outline them by iron filings in the classroom.

We call such lines "magnetic field lines" and they are extremely important in magnetic fields in space.  Most space is filled with a "gas" (usually rather rarefied) consisting of freely floating electrons and positive ions--atoms which have lost one or more electrons, giving them a positive charge. This "gas" is called plasma.  It conducts electricity and becomes attached to magnetic field lines

Sometimes "magnetic storms" occasionally disturb the Earth's magnetic field. Unlike changes of the internal field, they happen quickly, over hours or days--and magnetic variations from the aurora borealis ("northern lights") are even faster. Their cause is outside the Earth, not in the core.

Magnetic storms are subtly connected to the sunspot cycle.  Large storms appear more frequently during times of high sunspot activity, and such storms also cause auroras at unusual locations, much further from the magnetic poles--even in the middle of Europe and of the United States.

Kristian Birkeland, a Norwegian physicist, took in 1895 a hint from William Gilbert and placed a terrella--a magnetized sphere representing the Earth--in a vacuum chamber made of glass. He then aimed a beam of electrons at the terrella (somewhat like the beam of electrons in a TV picture tube) and observed their path by means of the glow they produced in the residual air left in the chamber. The glow followed magnetic field lines (lines of force) and converged near the magnetic poles of the terrella. Was this a clue why the polar aurora is usually seen only within a limited distance of the magnetic poles?

Indeed it was. The French mathematician Henri Poincaré--and 50 years later, in more detail, Hannes Alfvén in Sweden--analyzed the motion of such electrons and concluded that they were guided by magnetic field lines, like beads strung on a wire. From his terrella experiment, Birkeland guessed that the aurora was caused by electrons from the Sun, which were guided by magnetic field lines to the Earth's magnetic poles/  They produce a glow as they interact with the upper atmosphere. As it turned out, the Sun was not the source, but the rest of the guess was pretty close to the truth.

In 1958 artificial satellites orbiting the Earth observed that "radiation belts" (the Van Allen radiation belt) are permanent features of the Earth's magnetic environment in space. In 1959 that environment was named the "magnetosphere" by Tom Gold of Cornell University. Two types of belts were found. An inner, small but intense proton belt turned out to be a secondary product of the cosmic radiation, of the diffuse background of high-energy particles which seems to fill our galaxy. But it was the "outer belt" that carried the ring current, a belt of ions and electrons with moderate energy but in large numbers.

The energy source of the ring current and of all its associated phenomena turned out to be the solar wind - a continuous flow of ions and electrons, spreading out in all directions from the Sun's topmost layer, the million-degree hot corona. Magnetic storms arise from unusually fast flows in the solar wind, especially those connected to explosive events associated with active sunspots. The solar wind compresses the magnetic field lines facing it on the day side of the Earth and confines those lines into a rounded cavity. In the opposite direction, on the night side, the same solar wind stretches field lines into a long "magnetotail".

Induced Magnetism

If you dip a bar magnet into a cup of nails, nails will stick to it. But exactly, why? You know that magnets attract iron, but then you also note, some nails stick to other nails. Why is that?

William Gilbert guessed that the reason was that ordinary iron turned into a magnet whenever it touched another magnet. Soft iron (not steel) loses its magnetism once it is taken away, but while in contact with a magnet, he proposed, it too acts like a magnet, and its polarity is always in a direction which helps it stick to the first magnet. Thus one nail attracts another.

He proved his hunch by showing that such temporary magnets not only attracted, but could also repel. He did this by an experiment.  The soldier on the right holds two pieces of iron suspended together from strings, just above a magnetized sphere. Their free ends seem to repel. In actual practice this does not work too well, so we can demonstrate the same effect in a somewhat different way.

Here is how you perform the experiment in class. Put a magnet on top of the projector, and allow two small nails to attach themselves to adjacent spots on one of the poles, with your fingers holding them parallel to each other.  (Try holding the ends together by a small wire strand wound around them?)

Both nails are now temporary magnets with the same polarity--say, north-seeking or N--at the ends next to the magnet. The polarities of their other ends therefore must be the same--here, south-seeking or S, and those ends should repel each other. By spreading your fingers and allowing the ends of the nails to move apart, you can show that in fact they do.

You might also recognize that this is a magnetic analog of the electroscope. Details of this experiment are at http://www.phy6.org/earthmag/inducemg.htm

Electroscope.  Image courtesy of NASA.

This is why iron filings on a paper sheet above a magnet line up the way they do. Each little piece becomes a temporary magnet, and like a tiny compass needle, it lines up with the direction of the magnetic force.

Oersted's Discovery of Electromagnetism

Now we will jump to the year 1820, not long after the invention of electric batteries. Electric currents were a hot scientific topic, but no one suspected they had anything to do with magnetism.

A Danish professor, Hans Christian Oersted, prepared for some friends a science demonstration in his home. It included the heating of a wire by an electric current from a battery, and also some demonstrations of magnetism, using a compass on a stand.

While carrying out the heating experiment, Oersted noted that every time he connected the current, the compass needle moved, too, something completely unexpected. No one else took notice. In the four months that followed he tried hard to make sense of the phenomenon; but he couldn't! The compass needle was neither attracted nor repelled, but tended to stand perpendicular to the wire. In the end he could only report his observations in an article--written in Latin, no less.

Following Oersted's article, the effect was confirmed all over Europe, and André-Marie Ampére in France figured it out, using some clever experiments. Magnetism, he claimed, was basically the force between electric currents. Parallel currents attract, anti-parallel currents repel. Loops of current attract or repel like magnets, coils with many loops multiplied the magnetic force, and Ampére guessed that iron atoms were magnetic because electric currents (soon named "Ampére currents") circulated in them. An electron orbiting in an atom can carry such a current, although we know now that the actual explanation, involving electron spin, is more complicated.

The Experiment

You can perform Oersted's experiment on top of an overhead projector, using a transparent compass, a D-cell battery, and a short (about 9"), thick, flexible insulated wire (insulated in case it gets hot). The battery should be fresh. You will have to draw a large current from it, a short circuit really, though only for a very short time.

Put the compass on the glass, and let everyone see that it points north. Then with your thumb press one end of the wire against the bottom of the D-cell. The wire should form a short loop, coming back to the other terminal of the battery, but not touching it.

Maneuver the wire so that the middle of the wire passes over the compass needle and is parallel to it. Then touch the other end of the wire to the other end of the cell--just a short touch (1-2 seconds), it's a short circuit and not good for the battery, also it generates a lot of heat at the contacts. The compass needle will immediately pivot to stand at 90 degrees to the wire.

Reverse the electrical contacts by turning the battery around. The needle will swing to stand at 90 degrees in the other direction. Details may be found here: http://www.phy6.org/earthmag/oersted.htm.

Gilbert might have had a clue to this effect.  He read a report from Italy, of an iron rod set in a steeple, which was found bent. It was taken down and brought to a blacksmith to be straightened out, and was then found to be strongly magnetic. We today would guess it was magnetized (and also bent) by lightning, which carries a strong (but brief) electric current. Magnetization by lightning (when it strikes some special iron-bearing minerals) may be what produces lodestones. 

Andre-Marie Ampére in France felt that if a current in a wire exerted a magnetic force on a compass needle, two such wires also should interact magnetically. In a series of ingenious experiments he showed that this interaction was simple and fundamental--parallel (straight) currents attract, anti-parallel currents repel. The force between two long straight parallel currents was inversely proportional to the distance between them and proportional to the intensity of the current flowing in each.

Here is how this can lead to the notion of magnetic poles. Bend the wires into circles with constant separation (figure below):

Replace each circle with a coil of 10, 100 or more turns, carrying the same current (figure below) and the attraction or repulsion increase by an equal factor. In fact, each coil acts very much like a magnet with magnetic poles at each end (an "electromagnet"). Ampere guessed that each atom of iron contained a circulating current, turning it into a small magnet, and that in an iron magnet all these atomic magnets were lined up in the same direction, allowing their magnetic forces to add up. (Nowadays one could claim that the electrons circling the nucleus carry such a current, but the actual situation is more complicated).

The magnetic property becomes even stronger if a core of iron is placed inside the coils, creating an "electromagnet"; that requires enlisting the help of iron, but is not essential.

Magnets and Electricity

(From the Energy Information Administration)

In most objects, all of the forces are in balance. Half of the electrons are spinning in one direction; half are spinning in the other. These spinning electrons are scattered evenly throughout the object.

Magnets are different. In magnets, most of the electrons at one end are spinning in one direction. Most of the electrons at the other end are spinning in the opposite direction.

Bar Magnet

This creates an imbalance in the forces between the ends of a magnet. This creates a magnetic field around a magnet. A magnet is labeled with North (N) and South (S) poles. The magnetic force in a magnet flows from the North pole to the South pole.

Have you ever held two magnets close to each other? They don’t act like most objects. If you try to push the South poles together, they repel each other. Two North poles also repel each other.

Turn one magnet around and the North (N) and the South (S) poles are attracted to each other. The magnets come together with a strong force. Just like protons and electrons, opposites attract.

These special properties of magnets can be used to make electricity. Moving magnetic fields can pull and push electrons. Some metals, like copper have electrons that are loosely held. They can be pushed from their shells by moving magnets. Magnets and wire are used together in electric generators.

Image courtesy of NASA http://liftoff.msfc.nasa.gov/academy/space/mag_field.html

In further experiments, using instruments similar to the one pictured below, he was able to determine that the magnetic influence surrounded the wire in a circle.

Courtesy of Gabinete de Fisica of the University of Coimbra.

 A magnetic needle balances on the central rod. The two end posts support a metal wire. Each end of the wire extends down through the wooden posts and is connected to a small metal post in the base. When one metal post was connected to the positive pole of a battery and the other metal post was connected to the negative pole of a battery, current would flow in the wire. The needle would then swing until it was at right angles to the wire.

The rule that has emerged from Oersted's work is as follows: If you hold a wire in the palm of your right hand so that the thumb points in the direction of the current, your fingers circle in the same direction as the magnetic field.

This drawing illustrates this rule (sometimes called the "right-hand rule"). The red line is a segment of a wire and the arrow designates the direction of the current, I. The blue circles represent the magnetic field lines, B, and the arrow heads signify the direction of the magnetic field (remember, physicists agree that the magnetic field goes from north to south).

Oersted's discovery that a current creates a magnetic field was very important.

Michael Faraday, a research physicist and lecturer at the Royal Institution in England, read Oersted's paper describing his discovery and his conclusions. Since Oersted proved that an electric current could create a magnetic field, Faraday became determined to use a magnetic field to create an electric current. In 1831 he succeeded in creating an electric current with a changing magnetic field. The emphasis is on the changing magnetic field. For example, if a magnetic is moved in the region of a current carrier connected in a closed circuit or vice versa, current will begin to flow in the carrier. If the magnet remains still, no current will flow.

This is the basic principle of an electric generator.
Create a cylindrical coil of wire with a hollow middle, connect the coil to a meter that measures current, and move a strong bar magnet in and out of the coil. Current will flow one way when you move the magnet in and the other way when you pull the magnet out. No current will flow when the magnet is still.
+ Website Link to see an animation

Alternately, you could turn a cylindrical coil of wire between opposite poles of stationary magnets. The force turning the coils of wire could be your hand, or the wind or falling water. This electric generator could be small and light a bulb or large and light a city.

The connection between electricity and magnetism was made complete with Faraday's work. An electric current - any moving charge, really - creates a magnetic field. A changing magnetic field causes current to flow in a conductor in a closed circuit. (More precisely, a changing magnetic field creates an electric field and an electric field causes charged particles to move.)

Now imagine, for a moment, that a wire carrying a current is placed in a magnetic field caused by a strong magnet in such a way that the magnetic field is perpendicular to the current. In the drawing below the vertical arrows represent the magnetic field, the thick red line represents the wire and the red arrow shows the direction of the current.

Experiment shows that the wire experiences a force that is perpendicular to both the external magnetic field and to the wire. (This force can be large enough to cause a wire to leap out of the U of a horseshoe magnet!) In the case pictured above the force would always push the wire toward you - out of your computer screen. A helpful tool is an extension of the right hand rule discussed above. Open your right hand flat and allow your thumb to point in the direction of the current, and cause your fingers to point in the direction of the external magnetic field. Your palm would then push the wire in the direction of the force.

Remember that a current is simply a movement of charge. Imagine that there is a small, positively charged particle (red circle) moving in a magnetic field (blue lines). The effect of a magnetic field on a charged particle can be described by that on a current carrying wire.

Looking at this from above, we would see the magnetic field lines pointing toward us. The convention for drawing lines is to show the point of the arrow when the line is coming toward you and an x when the line is going away.

The magnetic field lines come out of the screen and the charged particle is moving right, and the force is perpendicular to the movement. The Force perpendicular to the motion will cause the charge to move in a circular path. (use a pop-up for "Centripetal Force" and also use "Circular Path from Magnetic Field") Negatively charged particles circle in the opposite direction. The size of the magnetic field and the size of the charge, mass and velocity of the particle determine the curvature of the circle.

There are very many important applications of this principle, from aiming a beam of charged particles in a television picture tube, to particle accelerators in atomic physics to instruments flown on satellites to identify charged particles.

One very interesting and important application of this relationship is the self-excited dynamo machine.

With a very weak upward magnetic field (solid arrows), the electrons in a rotating disk of conducting material will move inward (if the circuit is complete) because of the magnetic field. The inward movement of electrons is equivalent to a current (dotted line) directed outward toward the edge of the disk. A conductor that touches the disk and then winds around the shaft counter-clockwise completes the circuit. As current flows (dotted lines) counter-clockwise around the wire, it generates a magnetic field that is directed upward everywhere inside the loop. (Convince yourself of this by grasping a hoop in your right hand with your thumb pointing in a counter-clockwise direction tangent to the hoop. Your fingers will be curled upwards inside the hoop.) The magnetic field due to the current in the loop adds to the original upward weak field to make it stronger. A stronger upward magnetic field makes the current stronger which makes the magnetic field even stronger.

This is one of the proposed mechanisms for the Earth's magnetic field. In the 20th century scientists determined that the outer portion of the Earth's core was mostly molten iron. Convection creates flows of molten metal. Replace the rotating disk in our model with the molten outer core of the Earth. The presence of any weak magnetic field, even one from the Sun or the moving molten iron in the Earth, can create a ring of current in the molten outer core. This would increase the strength of the weak magnetic field. As the field gets stronger, the current gets greater making the field even stronger. This process accounts for about 90% of the Earth's magnetic field. The dynamo process explains why the Earth's magnetic field approximates a bar magnetic through the center of the Earth. (For more detail visit The Dynamo Process and Origin of the Earth's Magnetism.)

The connection between electricity and magnetism proved to have profound implications -- and applications. In addition to the ideas already discussed, Faraday was able to develop the connection into a theory of electromagnetism and link it to visible light. Other scientists have broadened his work until electromagnetism has become the cornerstone of physics that it is today. More of this can be explored in The Electromagnetic Spectrum, a critical tool in astronomy.More on the Earth's magnetic field is explored in A Magnet in Space and The Earth's Magnetosphere. In addition, the electromagnetic theory proved essential to understanding the dynamic processes of the Sun, which is developed in Why Do Sunspots and CME's Occur? and The Sun's Magnet.

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Content provided by David P. Stern, NASA
http://www-spof.gsfc.nasa.gov/Education/Imagnet.html
http://www-spof.gsfc.nasa.gov/Education/MagTeach.htm
http://www.phy6.org/earthmag/inducemg.htm
http://www.phy6.org/earthmag/oersted.htm
http://www.phy6.org/earthmag/lodeston.htm
http://www.phy6.org/earthmag/to1820.htm>
and
Energy Information Administration
and
http://stargazers.gsfc.nasa.gov/students/electromagnetism.htm

Magnet icon from http://eetd.lbl.gov/ECS/aerogels/images/magnet.html

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created May 19, 2007