What Is Magnetism

The Nature of Magnetism

Magnetism is a fundamental force of nature that arises from the motion of electric charges. Every moving charged particle generates a magnetic field around it, and every magnetic field exerts forces on moving charges and magnetic materials. This deep connection between electricity and magnetism is why physicists treat them as two aspects of a single phenomenon called electromagnetism.

The most familiar magnetic objects are permanent magnets, materials that produce persistent magnetic fields without requiring an external energy source. These magnets always have two poles, designated north and south. Like poles repel each other and opposite poles attract, analogous to the behavior of electric charges. However, unlike electric charges, magnetic poles cannot be isolated: cutting a magnet in half produces two smaller magnets, each with its own north and south pole. No experiment has ever detected a magnetic monopole, a particle with only one magnetic pole.

The magnetic field surrounding a magnet can be visualized using iron filings sprinkled on a sheet of paper placed over the magnet. The filings align along the field lines, revealing the characteristic pattern: lines emerge from the north pole, arc through the surrounding space, and enter the south pole, continuing through the interior of the magnet to form closed loops. This closed-loop structure is a fundamental property of all magnetic fields and is captured mathematically by Gauss's law for magnetism.

Where Magnetism Comes From

At the atomic level, magnetism originates from two sources: the orbital motion of electrons around the nucleus and the intrinsic spin of electrons. Each electron acts as a tiny magnet because its motion constitutes a circulating current, and circulating currents create magnetic fields. In most materials, the magnetic contributions of individual electrons cancel out due to random orientations, resulting in no net magnetism.

In ferromagnetic materials like iron, cobalt, and nickel, quantum mechanical interactions cause neighboring electron spins to align parallel to each other within small regions called magnetic domains. Each domain acts as a miniature magnet. In an unmagnetized piece of iron, these domains point in random directions, and their fields cancel. When an external magnetic field is applied, the domains aligned with the field grow at the expense of misaligned domains, and the material becomes magnetized. In permanent magnets, this alignment persists after the external field is removed.

Electromagnets produce magnetic fields using electric current flowing through a coil of wire. The strength of an electromagnet can be controlled by adjusting the current, and the field can be turned on and off instantly. Wrapping the coil around a core of ferromagnetic material like iron dramatically increases the field strength, because the core's domains align with the coil's field. Electromagnets are used in electric motors, generators, MRI machines, particle accelerators, and industrial lifting equipment.

Earth's Magnetic Field

Earth itself is a giant magnet, with a magnetic field generated by convection currents of molten iron and nickel in its liquid outer core. This geomagnetic field extends far into space, forming a protective bubble called the magnetosphere that shields life on Earth from harmful solar wind and cosmic radiation. Without this magnetic shield, the solar wind would gradually strip away Earth's atmosphere, as is believed to have happened on Mars.

Earth's magnetic poles do not coincide exactly with its geographic poles. The magnetic north pole is currently located in the Canadian Arctic, roughly 500 kilometers from the geographic North Pole, and it drifts several kilometers per year. Compasses work by aligning with Earth's magnetic field, pointing approximately toward geographic north but with a deviation called magnetic declination that varies by location.

The geomagnetic field has reversed its polarity hundreds of times throughout geological history, with the most recent reversal occurring about 780,000 years ago. During a reversal, the field weakens substantially before reestablishing itself with opposite polarity. These reversals are recorded in volcanic rocks, whose magnetic minerals freeze in the field direction at the time of cooling, providing a geological record of Earth's magnetic history.

Types of Magnetic Behavior

Materials exhibit several distinct types of magnetic behavior depending on their atomic structure. Diamagnetic materials, including copper, gold, water, and most organic compounds, are weakly repelled by magnetic fields. Diamagnetism arises from induced currents in electron orbits that oppose the applied field. The effect is extremely weak and is present in all materials, though it is usually overwhelmed by stronger magnetic effects.

Paramagnetic materials, such as aluminum, platinum, and oxygen, are weakly attracted to magnetic fields. In these materials, individual atoms have permanent magnetic moments that tend to align with an external field, but thermal energy randomizes their orientations, resulting in only a slight net magnetization. Paramagnetism is stronger than diamagnetism but still relatively weak.

Ferromagnetic materials, including iron, cobalt, nickel, and certain alloys, exhibit strong magnetic behavior and can retain magnetization after the external field is removed. This is the type of magnetism we encounter in everyday permanent magnets. The Curie temperature is the temperature above which a ferromagnetic material loses its permanent magnetism and becomes paramagnetic. For iron, this temperature is approximately 770 degrees Celsius.

Antiferromagnetic materials have atomic moments that align in alternating directions, canceling each other out and producing no net external field. Ferrimagnetic materials, like the iron oxides used in magnetic recording tape, have alternating moments of unequal strength, producing a net magnetization that is weaker than ferromagnetism but still practically useful.

Measuring and Describing Magnetic Fields

The strength of a magnetic field is measured in tesla (T) in the SI system. One tesla is a very strong field. Earth's magnetic field at the surface is approximately 25 to 65 microtesla (millionths of a tesla). A typical refrigerator magnet produces about 5 millitesla. Medical MRI machines operate at 1.5 to 7 tesla. The strongest continuous magnetic fields produced in laboratories reach about 45 tesla, while pulsed magnets have briefly achieved over 1,200 tesla.

Magnetic flux, measured in webers (Wb), quantifies the total magnetic field passing through a surface. One weber equals one tesla times one square meter. Magnetic flux is a crucial concept in electromagnetic induction: changes in flux through a conducting loop induce voltage in that loop, which is the basis of electric generators and transformers.

The magnetic field is a vector quantity, meaning it has both magnitude and direction at every point in space. The direction of the field at any point is defined as the direction a compass needle would point if placed there. The field lines used to visualize magnetic fields represent the direction of the field, while their density indicates the field's strength.

Magnetism in Technology and Nature

Magnetic phenomena are essential to countless technologies. Electric motors and generators rely on the interaction between magnetic fields and current-carrying conductors. Magnetic storage media record data by magnetizing tiny regions on a disk or tape surface. Magnetic resonance imaging produces detailed medical images using strong magnetic fields and radio waves. Magnetic levitation (maglev) trains float above their tracks using magnetic repulsion, eliminating friction and enabling speeds exceeding 600 km/h.

In nature, many organisms can sense Earth's magnetic field and use it for navigation. Migratory birds, sea turtles, salmon, and certain bacteria all possess magnetoreception abilities. The exact biological mechanism remains an active area of research, with leading hypotheses involving magnetite crystals in cells or quantum chemical reactions in cryptochrome proteins in the eyes.