Magnets from Mini to Mighty
Table of Contents
Permanent Magnets: A Study in Sticktoitiveness
There are two basic kinds of magnets – permanent and temporary. We’ll take on permanent first – which is appropriate, because before humans could invent and exploit temporary magnets, they had to discover the permanent kind first.
People first happened upon magnets sometime before 600 BC with the discovery of the mineral magnetite, a naturally occurring magnet. One type of magnetite, lodestone, has the additional property of polarity – it aligns itself with the Earth’s magnetic field. The lodestone made possible the invention of the compass, which was used in navigation as early as 1086.
Unlike temporary magnets, the permanent ones (such as those on your fridge) stick around for awhile (no pun intended). While they may not last forever, you often have to go to some effort to demagnetize them. You can’t turn them on and off with the power switch. Permanent magnets all belong to a class of materials referred to as ferromagnetic.
The other major difference between permanent and temporary magnets is what the magnetic fields look like on an atomic level. These are two different phenomena entirely.
All magnetism comes down to electrons. In the electromagnets we’ll discuss in a minute, magnetic fields result from electron flow through a conductor. In the case of permanent magnets, it’s the spinning of the electrons that creates magnetism, not their movement through a conducting material.
You know that the Earth is a great big magnet. Well, electrons are teeny tiny ones. They have a north and a south pole, too, and spin around an axis. This spinning results in a very tiny but extremely significant magnetic field. Every electron has one of two possible orientations for its axis.
Most materials are either naturally magnetized, or can be induced to be magnetic. In most materials, atoms are arranged in such a way that the magnetic orientation of one electron cancels out the orientation of another.
Iron and other ferromagnetic substances, though, are different. Their atomic makeup is such that smaller groups of atoms band together into areas called domains, in which all the electrons have the same magnetic orientation. Inside each of these domains, the electrons are oriented in the same direction. But the iron is not magnetic because these domains themselves are not aligned.
If you put that piece of iron within a magnetic field, though, the domains all begin to point in the same direction. The result? A magnet! A permanent magnet is nothing more than such an object in which all the domains are aligned in the same direction. There are only four elements in the world that are ferromagnetic at room temperature and can become permanently magnetized: iron, nickel, cobalt and gadolinium. (A fifth element, dysprosium, becomes ferromagnetic at low temperatures.)
Open our interactive Java tutorial and try this yourself!
PHYSICS FACTOID: Some people refer to the two magnetic poles of the Earth by their geographic locations. The magnetic pole near the Earth's geographic North Pole is therefore often called the Earth's "Magnetic North Pole." However, that's actually where the planet's south magnetic pole is located. That's why the north pole of a magnetic needle points there … opposites attract!
Your new magnet could remain magnetized for a good while, depending on what it is made of. Pure iron, for example, will stay magnetized just a few minutes, although magnetization of many iron alloys will be much more “permanent.” Ceramic and alnico – a combination of aluminum, nickel and cobalt – are two common types of permanent magnets. But even more “permanent” magnets could lose their magnetism with, say, a good swift blow from a hammer to knock the aligned domains out of alignment. When it comes to magnets, “permanent” doesn’t necessarily mean “forever”!
Next Page Temporary Magnets: When is a Magnet not a Magnet?
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