Iron and Magnets
posted on 25 Jan 2013 by guy
last changed 8 Jun 2016
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ages: 5 to 99 yrs
budget: $0.00 to $10.00
prep time: 0 to 10 min
class time: 5 to 30 min
This lesson provides general backround on magnets, and suggests several classroom activities involving magnets and iron filings. These include
Some lecture slides are attached to help tell the story.
optional equipment: compass, flexible magnet
keywords: iron, magnet, magnetism, magnetic field, field lines, Faraday, filings, ferromagnetism, lodestone
More than 2400 years ago, ancient scholars in Greece, India and China knew about naturally magnetic materials called lodestones, which attracted bits of iron. Apparently, lodestones were relatively common near the Greek city of Magnesia (now within Turkey), which gave us the origin of the word "magnet".
Lodestones are made of an iron ore called "magnetite" (Fe3O4), the only naturally occurring material that exhibits significant magnetism. By the 12th century, it was known that lodestones carried two points of maximum attraction, called poles, and that these poles aligned with certain directions on the Earth. They were used as compasses for navigation in 12th century China.
Nowadays, the two poles are called "north" and "south" according to how they align with the earth. North magnetic poles point to a location in the Arctic Ocean north of Canada, and south magnetic poles point to a location at the edge of Antarctica.
All magnets have both a north and a south pole,1 and these poles exert forces on other magnets. Two north poles, or two south poles, repel each other, while a north and south pole attract each other. Two magnets that are free to move will pull each other together until the north pole of one collides with the south pole of the other.
Only a few materials, called "ferromagnetic" materials, exhibit magnetic properties of significant strength. These materials include nickel, iron, cobalt, a few rare earth elements such as gadolinium (if it's cold enough), and some of their alloys. These materials, while not naturally magnetized, can become magnetized when exposed to a strong external magnetic field. In this case, magnetic domains within the material (see below under "microscopic origin of magnetism") can become temporarily aligned to create a strong magnetic field of their own. When the external field is removed, they may remain in their aligned state, thereby making a permanent magnet. A subsequent physical shock or high temperature may cause the magnetic domains to revert to a random alignment, thereby destroying the magnetism of the material. Table 1 gives a list of some common ferromagnetic materials, together with typical values of "residual induction" (labeled Br, and measured in Gauss), which is an indication of the maximum possible field strength in the material. Rare earth neodymium magnets make the strongest known permanent magnets.
|Neodymium (NdFeB)||13,000||strongest permanent magnets|
|Samarium Cobalt (SmCo)||10,000||higher temperature ratings than Neodymium|
|Alnico||12,000||family of materials comprised of Aluminum, Nickel, Cobalt|
|Ferrite||4,000||a.k.a. ceramic magnets, principle component is Hematite (Fe2O3)|
|flexible||1,700||magnets made by mixing magnetic powders with rubber|
Faraday lines of force
In the early 1800's, the English scientist Michael Faraday introduced the concept of "magnetic lines of force", which led to the modern concept of magnetic field lines. Faraday noticed that if we scatter iron filings around a magnet, the filings line up to form chains along certain directions (fig. 1). Each iron filing is shaped like a miniature needle, long and thin, and all the filings within a small region tend to point in the same direction. Faraday explained this behavior in terms of the forces on the ends of the iron filings. He knew that a piece of iron becomes magnetic when brought near another strong magnet, and so an iron filing in a magnetic field becomes a tiny bar magnet. (An example of magnetizing iron can be found in A Floating Compass, where we use a strong magnet to magnetize a steel needle.) He correctly realized that the reason the iron filings pivot to point in a particular direction must be because there is a magnetic force pulling the north pole of the filing in one direction and the south pole in the opposite direction. Faraday characterized these forces by "force field lines", which showed the direction of those forces. At any point in space, the field line through that point shows the direction the force would be on a north magnetic pole located at that point (and opposite the direction of the force on a south magnetic pole). By scattering iron filings around a magnet, he could map out the direction of magnetic forces all around the magnet (fig. 2).
the microscopic origin of magnetism (note for geeks)
The magnetism of a material derives from the magnetic properties of its individual atoms, and the way in which they interact with each other. The magnetism of an atom is determined in turn by the magnetic properties of its electrons.
There are two origins of atomic magnetism: one is associated with the orbital motion of electrons around the nucleus, which works much like the current that powers an electromagnet, and the other comes from the intrinsic magnetism of the electron itself, based on quantum spin. In essence, a single electron by itself has a magnetic field like a microscopic bar magnet. This intrinsic electron magnetism from quantum electron spin is by far the stronger of the two sources.
The magnetic properties of a single atom depend on the arrangement of the intrinsic magnetic fields of all its electrons. According to the Pauli Exclusion Principle, atoms with filled electron orbitals have all their electrons aligned in pairs with their magnetic fields in opposite directions. In this way, the electron magnetic fields cancel out in pairs (the orbital magnetic contributions also tend to cancel), and therefore atoms with filled shells have extremely small net magnetic fields. Atoms with unfilled shells tend to have one or more unpaired electrons, and in fact the first electrons in a shell tend to align with their magnetic fields in the same direction,3 which makes the overall magnetic field of the atom stronger.
The magnetism of a bulk material depends not only on the strength of the magnetic field of its atoms, but also how those atoms interact with each other. In most materials, atoms are oriented more or less randomly within the material so that the magnetic fields of many atoms tend to cancel each other out. However, in just a few (ferromagnetic) materials, the neighboring atoms tend to align so that their magnetic fields are pointing in the same direction. In those cases, the atomic magnetic fields combine together to give a very strong magnetic field over large regions of the material.
Most magnetic atoms (i.e. atoms that do not have filled electron shells) prefer to align so that nearest neighbors have magnetic fields pointing in opposite directions, with the south pole of one atom next to the north pole of the neighbor, and the north pole next to the south pole of the neighbor. This magnetic interaction is familiar to us from common bar magnets (see figure 3). When these atoms are put in an external magnetic field, each atom tries to align itself with the external magnetic field (the way a magnetic compass tries to align itself with the Earth's magnetic field), but the magnetic interactions between atomic neighbors limit their ability to do this, and the material becomes only weakly magnetic. These materials are called paramagnetic.
In some (ferromagnetic) materials, there is a competing tendency for each atom to align in the same direction as its neighbors. This effect is a result of the quantum mechanical exchange interaction, based on the Pauli exclusion principle. In these cases, the exchange interaction causes electrons with the same spin (from adjacent atoms) to be pushed farther apart. Moving the atoms farther apart reduces the energy associated with their electrostatic repulsion, thereby making the material more stable than when the spins are opposite and the atoms are closer together. For materials where the exchange interaction overwhelms the magnetic interaction, the atomic fields (which result from the electron spins) will align in the same direction and the material will become strongly magnetic.
In ferromagnetic materials, a region in which all the atomic fields are pointing in the same direction is called a "magnetic domain". However, even for ferromagnetic materials, magnetic domains that grow too large will become unstable, and will then split to become smaller domains with magnetic fields pointing in different directions. However, when the material is put in a strong external magnetic field, the boundaries of the magnetic domains shift to align more of the atoms in the same direction. When the external magnetic field is removed, some materials will remain in the aligned state, giving a residual magnetic field that is permanent.
identifying magnetic materials
Let students use a magnet to identify different magnetic materials in the room. See our lesson on Magnetic Materials for a checklist of common metals to search for.
paper clip battles
Younger students can find great entertainment in simply handling a few magnets. Have them try to steer paper clips around on a table by moving a magnet underneath the table. Be careful to choose magnets accordingly for young children. The larger neodymium magnets will attract each other with great force, and will shatter if they collide. Use something weaker for younger audiences.
find the north pole
Challenge your students to figure out which side of an unmarked magnet is the north pole. (Neodymium magnets can often be found in cubes or spheres, which makes for an added challenge.) There are a couple of possible approaches.
- Since a north magnetic pole is attracted to the north geographic pole of the Earth, allowing the magnet to turn freely like a compass will let it's north pole swing north. Float the magnet on a liquid surface (maybe in a paper boat) or suspend it from a string that can twist.
- Another possibility is to use a compass. The south pole of a compass will point to the north pole of another magnet. Move the compass around the magnet to verify which side is the north pole.
Older students may wish to experiment with magnetic jewelry. You can attach two magnets to either side of an earlobe, for instance. And don't worry, magnetic fields are quite harmless to the human body (as long as you don't swallow the magnet!). The primary risk is getting pinched.
iron filing art
All students should be encouraged to try their hands at making iron filing pictures as in figure 1. Try it with various different shaped magnets. All you need is a magnet, a jar of iron filings and a flat surface like a clipboard or a piece of picture glass. Flexible magnets like those found on refrigerators make particularly interesting patterns. Flexible magnets are made by mixing ferromagnetic powder with rubber. The result is usually many separate macroscopic magnetic domains scattered throughout the material. It's as if the material were made of many individual magnets glued together, all pointing in different directions. Scatter some iron filings and see if you can tell where the poles are located. Folks with an artistic inclination may wish to experiment with cut magnetic shapes from flexible magnetic sheet. Figure 5 shows an example using magnetic letters glued to the back of a piece of white cardboard.
There are a several options for obtaining iron filings. You can make your own using a file and some iron nails, but this technique can be pretty tedious. Iron can also be recovered from beach sand by trolling a strong magnet through it, or from iron-fortified cereal, but quantities are limited. Fortunately, iron filings can also be purchased from a number of sources (check the iron filings equipment link at the top of the page).
questions to ponder
Fig. 3: Forces on a small magnet in the vicinity of a large magnet. The force on the south pole is stronger since the magnetic field is stronger at that location.
Fig. 4: Forces on the small magnet are to the outside and down in this case. Field lines are drawn for reference.
- Iron is always attracted to magnets. Why isn't it ever repelled by magnets?
When a lump of iron encounters an external magnetic field, the domains line up in the same direction as the external magnetic field so that the lump turns into a bar magnet with the north pole facing in the direction of the magnetic field lines. The north pole is pulled in the direction of the field lines and the south pole is pulled in the opposite direction. (Remember, that's the definition of the field lines — they describe the force on a magnetic pole in an external magnetic field.) If the lump is arranged like the small magnet in figure 3, its south pole will be pulled towards the north pole of the large magnet, and its north pole will be repelled from it, but with a weaker force since it is further away. The net effect is for the lump to be attracted to the magnet. In any other location, the same thing happens. In figure 4, the small magnet is once again oriented in the direction of the magnetic field lines of the large magnet, and the force on the small south pole is to the right and slightly down, while the force on the small north pole is left and slightly down. The net effect is that the small magnet is attracted downwards towards the large magnet.
- When we scatter iron filings near a big magnet, why don't they all fly towards the big magnet like a large lump of iron does?
We know that a large(ish) lump of iron will move towards the magnet for the reasons outlined above: the force on the near pole of the lump is stronger than the force on the far pole. For a tiny filing of iron, the two poles of the filing are so close together that the strength of the external magnetic field at those two points is nearly the same. Therefore, the forces on the two ends of the filing are nearly equal, and in opposite directions. The forces nearly balance, and the difference is not big enough to overcome friction and move the filing.
- If nickel is supposed to be ferromagnetic, why can't I pick up a 5-cent piece (a "nickel") with a magnet the way I can pick up a piece of iron?
You may have to contact the U.S. Treasury department to complain about this. The modern U.S. nickel coin is only 25% nickel metal; the rest is copper.4 The magnetic properties of nickel and its alloys very widely depending on the chemical composition and crystal structure of the alloy. U.S. nickel coins do not exhibit magnetic attraction. Interestingly, most Canadian nickels are magnetic. Prior to 1982 they were primarily nickel metal, except during WWII when a copper-zinc version was produced. Since 2000 they have mostly been made of nickel plated steel.5
Students should be encouraged to try their own hands at making iron filing pictures as in figure 1. Try it with various different shaped magnets. All you need is a magnet, a jar of iron filings (you can make your own using a file and some iron nails, which is pretty tedious, or you can buy a jar from any of a number of sources), and a flat surface like a clipboard or a piece of picture glass.
Folks with an artistic inclination may wish to experiment with cut magnetic shapes from flexible magnetic sheet. Figure 5 shows an example using magnetic letters glued to the back of a piece of white cardboard.
The eloquent and entertaining Walter Lowen at MIT has posted his introductory lecture on magnetism at http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring... .
Michael Fowler at the University of Virginia offers a beautiful summary of the historical discoveries in electricity and magnetism at http://galileoandeinstein.physics.virginia.edu/more_stuff/E&M_Hist.html .
Markus Ehrenfried offers an excellent introduction to the complicated topics of quantum spin and the Pauli Exclusion Principle at http://web.archive.org/web/20150904045145/http://www.markusehrenfried.de... .
- 1. You cannot isolate the two poles even if you break apart a bar magnet; you will end up with two bar magnets, each with its own pair of poles.
- 2. http://www.intemag.com/faqs.html#properties
- 3. This tendency for electrons in unfilled shells to align in the same direction is summarized in Friedrich Hund's rules for atomic electrons.
- 4. http://en.wikipedia.org/wiki/Nickel_%28United_States_coin%29
- 5. See http://en.wikipedia.org/wiki/Nickel_%28Canadian_coin%29 on the history of the Canadian nickel.