A magnet is a material or object that produces an invisible magnetic field, allowing it to exert force on other magnetic materials like iron, nickel, and cobalt, or to attract and repel other magnets. Understanding how magnets function involves delving into the nature of these fields and the atomic structures that give rise to them.
Understanding Magnetic Fields
Every magnet is surrounded by a magnetic field. This field transmits magnetic forces, allowing magnets to attract or repel objects without direct contact. Magnetic fields are shown with lines of force, depicting their direction and strength. These lines emerge from the magnet’s north pole and enter its south pole, forming continuous, closed loops both outside and within the magnet.
The density of these field lines indicates the strength of the magnetic field; where the lines are closer together, the field is stronger. The magnetic effect is most pronounced at the poles of a magnet, where the field lines are most concentrated. A small compass placed within a magnetic field will align its needle with these lines, pointing in the direction of the field at that specific location.
The Atomic Origins of Magnetism
Magnetism originates at the atomic level, stemming from the behavior of electrons within a material. Electrons possess both orbital motion around the nucleus and an intrinsic property called spin, both of which generate tiny magnetic moments. These individual electron spins act like miniature magnets, each having a north and south pole. In most materials, the magnetic moments of electrons are paired and oriented in opposing directions, effectively canceling out their magnetic effects.
However, in certain materials, particularly ferromagnetic ones, some electrons remain unpaired, and their magnetic moments do not cancel. Within these materials, groups of atoms form microscopic regions called magnetic domains. Each domain acts like a tiny magnet, with the magnetic moments of its constituent atoms aligned in a single direction. In an unmagnetized material, these domains are randomly oriented, resulting in no net external magnetic field. When exposed to an external magnetic field, these domains can rotate and align, causing the material to become magnetized and produce its own macroscopic magnetic field.
Different Kinds of Magnets
Magnets can be categorized based on how their magnetic properties are established and sustained. Permanent magnets, like those found on refrigerators, retain their magnetism even after being removed from an external magnetic field. They are typically made from “hard” ferromagnetic materials, such as alnico or ferrite, which undergo special processing to align their internal microcrystalline structures. This causes the magnetic domains within the material to remain aligned, making them difficult to demagnetize.
Electromagnets, in contrast, generate magnetism through the flow of electric current and are temporary in nature. They commonly consist of a wire coil wrapped around a ferromagnetic core, such as iron. When electric current passes through the coil, it creates a magnetic field, turning the assembly into a magnet. The strength of this magnetic field can be precisely controlled by adjusting the amount of current or the number of turns in the coil, and the magnetism disappears when the current is turned off.
Temporary magnets, distinct from electromagnets, are materials that can be magnetized when exposed to a strong magnetic field but lose their magnetic properties once the external field is removed. Soft iron is a common example; its magnetic domains align easily in the presence of an external field but quickly return to a random orientation when the field is gone. Objects like paper clips and iron nails exhibit temporary magnetism when brought near a strong permanent magnet.
Magnetic Interactions and Materials
Magnets interact with each other and with various materials based on the alignment of their magnetic fields. A fundamental rule of magnetism states that opposite poles attract each other, while like poles repel. This interaction occurs because the magnetic field lines from opposing poles combine and strengthen, leading to attraction, whereas field lines from similar poles push against each other, causing repulsion. The strength of this force depends on the distance between the magnets and their individual magnetic strengths.
Materials respond differently when placed in a magnetic field, classifying them into three main types.
Ferromagnetic Materials
Ferromagnetic materials, including iron, nickel, cobalt, and their alloys, are strongly attracted to magnets and can be permanently magnetized. They contain unpaired electrons and exhibit strong interactions between their magnetic domains, allowing for significant alignment in an external field.
Paramagnetic Materials
Paramagnetic materials, such as aluminum, platinum, and oxygen, are weakly attracted to magnets. They possess some unpaired electrons, but their magnetic moments only weakly align with an external field and lose their magnetism once the field is removed.
Diamagnetic Materials
Diamagnetic materials, including water, copper, and bismuth, are weakly repelled by magnetic fields. These materials have all their electrons paired, and an external magnetic field induces a small, opposing magnetic field within them.