Magnets

Permanent Magnets and the Earth

Everybody knows how magnets work. You can stick them on your refrigerator to hold up notes, drag them through sand to collect iron grains and push other magnets to amaze yourself. Of course, if you look harder, it gets more complex. All magnets have at least 2 poles, places where the magnetic force is the greatest. These poles are called north or south depending on which way the magnet points when freely suspended like a compass needle. The north pole of a magnet points to the north pole of the earth. The basic rule of magnets is like poles repel and unlike poles attract. If you bring magnets together with the north poles facing each other, they will push away. If a north pole faces a south pole, they will pull together.

A bar magnet is shown with imaginary force lines looping from one pole to the other. If you place this magnet under a sheet of paper and sprinkle iron dust on top, the particles will align with the force lines and trace a similar pattern. This pattern is often called a field, perhaps because it looks like plowed furrows in a corn field. The iron particles respond to the magnetic field by temporarily becoming little magnets themselves. That's why iron is attracted to the magnet. So when iron dust was sprinkled on top of the magnet, the iron bits became magnetized with all of north pieces facing the south pole of the original magnet. If the dust were shaken off and the magnet removed, the iron particles would lose most of their acquired magnetism, retaining only a small residual amount that would fade in time.

In the iron dust experiment, the iron bits became magnetized and strongly attracted to the magnet. But the paper seemed unaffected, as was the surrounding air. Different materials respond to magnets differently and may be categorized in 3 groups: ferromagnetic, diamagnetic and paramagnetic. These are just big words that describe simple ideas. Ferromagnetic material contains iron (or similar metal) that becomes strongly magnetized when placed near a magnet. Pure iron is considered a soft ferromagnetic material since the magnetic properties are largely temporary. If iron is alloyed with other elements, hard ferromagnetic material may be produced which can be more permanently magnetized. Diamagnetic or paramagnetic material never becomes magnetized but may be very weakly repulsed or attracted by magnets. The emphasis is on weakly; it takes a very strong magnet to demonstrate this. In the real world, only iron and steel make much of a difference to magnetics. Magnetic fields pass through most other objects, including human bodies, without much apparent effect.

So, why do magnets point to the north pole of the Earth? The earth itself is a very big magnet. In fact, the magnetic field lines that surround our planet look very much as if a giant bar magnet were stuck in the center. At this point, things get a bit confusing. Since the north pole of a compass magnet points north, then the magnet inside our planet must have it's south pole on top. Remember, only unlike poles attract. This is shown by the cartoon drawing of the Earth containing a bar magnet with the south pole up. To make matters worse, many health advocates call magnetic south poles by a new name, bio-north poles. If I weren't so confused, I'd say this makes sense. Things get even stranger when you consider that the magnetic poles of the Earth wander over tens of thousands of years and can flip in a field reversal that may cause planetary mayhem. Compared to a typical bar magnet that you might hold in your hand, our planet's magnetic field is much weaker but extends further. Magnetic field strength is measureed in units called gauss and the Earth's magnetic field measures about 1/2 gauss. The strength of a bar magnet might be 1,000 gauss but only very near the poles. A few inches away, the strength could easily drop tenfold. At a few feet, the tiny field of the Earth would be stronger than the bar magnet. At laboratory strengths greater than 10,000 gauss, a larger measure called tesla is preferred. One tesla is equal to 10,000 gauss.  These magnetic units are named after the famous mathematician Carl Friedrich Gauss and the quirky electrical pioneer, Nikola Tesla.

Permanent magnets can be made out of ordinary iron or steel but they make relatively poor magnets. Nickel alloys are also ferromagnetic and can be turned into an Alnico magnet that is more powerful and longer lasting than an iron magnet. They were very popular until ceramic and rare earth magnets were developed. Ceramic magnets made of iron oxide are not very powerful but they are cheap and can be molded into many shapes. The magnets on your refrigerator are probably ceramic. Rare earth magnets are more expensive, less rugged but very strong. The most popular of such magnets are called "Neo" because they are a brittle alloy of iron, boron and neodymium. Such magnets are usually plated with nickel or gold to protect them from damage. A neodymium magnet shaped like a cube 1 inch on each side can require nearly 100 pounds of force to pull it from a steel plate. Permanent magnet specifications are a bit confusing and many sales websites make things worse by hyping misleading numbers. Hopefully, the explanation below will help.

Permanent magnets are created by placing ferromagnetic alloys in a magnetizing field. The magnetizing field is represented by the symbol H. Ferromagnetic material amplifies the original field to create a more intense induced field represented by the symbol B. In air, these two fields are equal. But in iron, the induced field may be hundreds of times stronger than the magnetizing field. When creating a magnet, the magnetizing field must eventually be turned off leaving us with just the residual field of the new permanent magnet. This residual field is somewhat less than the original amplified field and is denoted by the symbol Br. The residual magnetism sounds like it should be the field strength at the poles of our magnet but this is not the case. The residual field number describes something deep inside the magnet. It can be measured by connecting the poles of the magnet with a large piece of iron and sliding a thin probe into the gap between one pole and the iron. If you just hold a magnet in your hand, the actual magnetic field at the pole face or some distance away is much less than the strength of the residual field, Br. It depends strongly on the shape and size of the magnet, your distance from the surface of the magnet and the value of the residual field.

Another potentially misleading number is the Maximum Energy Product, BHmax. Remember, B represents the total induced field while H represents the driving field during manufacturing. As the driving field is increased, ferromagnetic material amplifies the strength until the magnetism becomes so great that the material is saturated. At this point, all the little magnetic domains in the iron have become aligned and there is no mechanism for amplification left over. Increasing the driving field has no additional effect on the iron. This is the maximum in the Maximum Energy Product. Just multiply B times H at saturation and you wind up with a pretty big number. What does it mean? The Maximum Energy Product is just a figure of merit. The Br and BHmax of a magnet may be interesting numbers. But again, they don't answer the question "How strong is my magnet one inch from the north pole?"  For that information, you have to know the geometry of the magnet and do some calculating.