Table of Contents

What Is Magnetism?

Magnetism is a fundamental force of nature arising from the motion of electric charges. It produces magnetic fields that can attract or repel certain materials, exert forces on moving charges, and interact with electric fields. Along with electricity, magnetism forms one half of electromagnetism — one of the four fundamental forces governing the universe.

You interact with magnetism every second of your life, even if you don’t realize it. The Earth’s magnetic field shields you from solar radiation. The electric motors in your car, your refrigerator, and your phone’s vibration mechanism all use magnets. Your hard drive (if you still have a spinning one) stores data magnetically. MRI machines generate images of your internal organs using powerful magnets. Magnetism is everywhere — and understanding it explains a surprising amount of the technology we take for granted.

Magnetic Fields: The Invisible Force

A magnetic field is the region of space around a magnet (or a moving electric charge) where magnetic forces act. You can’t see magnetic fields directly, but you can visualize them: scatter iron filings near a magnet and they align themselves along the field lines, revealing the characteristic pattern that loops from one pole to the other.

Field Lines and Poles

Every magnet has two poles: north and south. Magnetic field lines emerge from the north pole and loop around to enter the south pole. Outside the magnet, lines go north to south; inside the magnet, they go south to north, forming continuous loops.

A critical point: there are no magnetic monopoles. You can’t have a north pole without a south pole. If you cut a bar magnet in half, you get two complete magnets, each with its own north and south poles. Physicists have searched for magnetic monopoles — hypothetical particles with only one magnetic pole — but none have been found. Their non-existence (as far as we know) is one of the asymmetries between electricity and magnetism.

Measuring Magnetic Fields

The strength of a magnetic field is measured in tesla (T) in SI units, or gauss (G) in the older CGS system. One tesla equals 10,000 gauss.

Some reference points:

Earth’s magnetic field at the surface: about 25-65 microtesla (0.25-0.65 gauss)
Refrigerator magnet: about 5 millitesla (50 gauss)
Neodymium rare-earth magnet: about 1-1.4 tesla
MRI machine: 1.5-3 tesla (some research MRIs reach 7-11 tesla)
Strongest continuous lab magnet: about 45 tesla (National High Magnetic Field Laboratory)
Neutron star surface: roughly 100 million tesla

The range is staggering — twelve orders of magnitude from Earth’s field to a neutron star.

Types of Magnetism

Not all materials respond to magnetic fields the same way. The differences arise from how electrons within atoms are arranged and whether their tiny magnetic moments align cooperatively.

Ferromagnetism: The Strong One

Ferromagnetism is what most people think of when they hear “magnet.” Iron, nickel, cobalt, and certain alloys are ferromagnetic — they can be permanently magnetized and are strongly attracted to magnets.

The secret is magnetic domains. In a ferromagnetic material, groups of atoms align their magnetic moments in the same direction, forming microscopic regions called domains. In an unmagnetized piece of iron, these domains point in random directions, and their effects cancel out. Apply an external magnetic field, and the domains aligned with the field grow at the expense of misaligned ones. Remove the field, and some alignment persists — you’ve made a permanent magnet.

The Curie temperature is the temperature above which a ferromagnetic material loses its permanent magnetism. For iron, this is 770 degrees Celsius (1,418 degrees Fahrenheit). Above this temperature, thermal energy overwhelms the alignment forces between atoms. Heat a permanent magnet enough, and it stops being magnetic. This is why blacksmiths could demagnetize iron tools by heating them to red-hot temperatures.

Paramagnetism: The Weak Attraction

Paramagnetic materials (aluminum, platinum, oxygen) are weakly attracted to magnetic fields. Their atoms have magnetic moments, but unlike ferromagnets, these moments don’t spontaneously align with each other. Apply an external field and they partially align, creating a weak attraction. Remove the field and the alignment disappears immediately.

The effect is typically thousands of times weaker than ferromagnetism. You won’t stick a paramagnetic material to your refrigerator.

Diamagnetism: The Universal Repulsion

Diamagnetic materials (copper, gold, water, bismuth, most organic compounds) are weakly repelled by magnetic fields. This is actually the most fundamental type of magnetism — all materials are diamagnetic, but in ferromagnetic and paramagnetic materials, the stronger effects dominate.

Diamagnetism occurs because an external magnetic field induces tiny currents in the electron orbits, and those currents create a field opposing the applied field (per Lenz’s law). The effect is extremely weak — except in superconductors, where diamagnetism is perfect and the material completely expels magnetic fields (the Meissner effect). This is why superconductors can levitate above magnets.

Speaking of levitation: diamagnetic levitation of living things is possible. In 2000, researchers at the University of Nijmegen levitated a frog in a 16-tesla magnetic field. The water in the frog’s body is diamagnetic, and in a strong enough field, the repulsive force exceeds gravity. The frog was unharmed.

Antiferromagnetism and Ferrimagnetism

Antiferromagnets (like chromium and manganese oxide) have neighboring atoms with magnetic moments that point in opposite directions, canceling out. The material shows no net magnetism but has interesting properties useful in specialized applications.

Ferrimagnets (like magnetite, Fe3O4) also have opposing atomic moments, but the opposing moments are unequal, producing a net magnetic field. Magnetite — the first magnetic material known to humans — is a ferrimagnet. It’s the “lodestone” that ancient navigators used for compasses.

Electromagnetism: Electricity and Magnetism United

One of the greatest discoveries in physics: electricity and magnetism are two aspects of the same force.

Oersted’s Discovery

In 1820, Hans Christian Oersted noticed that a compass needle deflected when placed near a wire carrying electric current. This was revolutionary — it showed that electric currents create magnetic fields. Within months, Andre-Marie Ampere had worked out the mathematical relationship.

Ampere’s law: A current-carrying wire produces a circular magnetic field around it. The field strength is proportional to the current and inversely proportional to the distance from the wire. Wrap the wire into a coil (a solenoid) and the field inside becomes uniform and strong — you’ve made an electromagnet.

Faraday’s Induction

In 1831, Michael Faraday discovered the reverse: a changing magnetic field induces an electric current in a conductor. Move a magnet through a coil of wire, and current flows. Stop moving it, and the current stops. Change it faster, and more current flows.

Faraday’s law of induction is the principle behind electric generators, transformers, and induction cooking. Every power plant in the world — coal, nuclear, hydroelectric, wind — works by spinning magnets near coils of wire to generate electricity. The details differ, but the underlying physics is Faraday’s law.

Maxwell’s Equations

James Clerk Maxwell unified electricity and magnetism into a single theoretical framework in the 1860s. His four equations describe how electric and magnetic fields are generated, how they interact, and how they propagate through space.

Maxwell’s equations predicted that changing electric and magnetic fields could sustain each other in a self-propagating wave — and calculated that this wave would travel at the speed of light. His conclusion: light itself is an electromagnetic wave. This was confirmed experimentally by Heinrich Hertz in 1887.

The implications were staggering. Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays are all electromagnetic waves — magnetic and electric fields oscillating together, differing only in frequency. Maxwell’s equations unified optics with electricity and magnetism in one of the most elegant theoretical achievements in the history of physics.

Earth’s Magnetic Field

Our planet is a giant magnet — but an odd one. Earth’s magnetic field is generated not by a permanent magnet at the core but by the geodynamo: convection currents of molten iron in the outer core, driven by heat from the inner core and radioactive decay.

The field extends far into space, forming the magnetosphere — a protective bubble that deflects most of the solar wind (a stream of charged particles from the Sun). Without the magnetosphere, the solar wind would gradually strip away Earth’s atmosphere, as likely happened to Mars (which lost its global magnetic field billions of years ago).

Magnetic Reversals

Earth’s magnetic poles have flipped hundreds of times over geological history. The north magnetic pole becomes the south, and vice versa. The last reversal occurred about 780,000 years ago (the Brunhes-Matuyama reversal). Reversals take a few thousand years to complete and are recorded in volcanic rocks — as lava cools, iron minerals align with the current field direction, locking in a permanent record.

We know the field is currently weakening — it’s declined about 9% over the past 200 years. Whether this signals an impending reversal is debated. If a reversal occurs, the transition period would likely see a weakened and chaotic field, potentially increasing radiation exposure and disrupting electronics systems that rely on magnetic navigation.

Many animals use Earth’s magnetic field for navigation. Migratory birds, sea turtles, salmon, lobsters, and even bacteria (magnetotactic bacteria contain tiny magnetite crystals) orient themselves using geomagnetic cues. How exactly they sense the field remains an active area of research — leading hypotheses involve either magnetite-based receptors or quantum effects in cryptochrome molecules in the eyes.

Humans have used magnetic compasses for navigation since at least the 11th century CE in China and the 12th century in Europe. The compass — a magnetized needle free to rotate — aligns with Earth’s field, pointing roughly north. This simple technology enabled the Age of Exploration and the expansion of global trade.

Applications of Magnetism

Electric Motors and Generators

Electric motors convert electrical energy into mechanical motion using magnetic forces. A current-carrying coil in a magnetic field experiences a torque that makes it rotate. This principle powers everything from industrial machinery to electric vehicles to the tiny motors in your phone’s vibration mechanism.

Generators do the reverse: mechanical motion (from turbines, windmills, or engines) rotates coils in magnetic fields, generating electricity through Faraday’s law. This is how nearly all electricity is produced worldwide.

The global electric motor market exceeds $200 billion annually. Motors consume roughly 45% of all electricity generated worldwide. Even modest efficiency improvements in motor design have massive economic and environmental implications.

Data Storage

Magnetic data storage records information by magnetizing tiny regions on a disk or tape surface. A region magnetized in one direction represents a 1; the other direction represents a 0. Hard disk drives (HDDs) use spinning platters with billions of such regions, read and written by a magnetic head floating nanometers above the surface.

A modern HDD stores data at densities exceeding 1 terabit per square inch — meaning each magnetized region is only about 10 nanometers across. This extraordinary density is achieved through technologies like perpendicular recording and heat-assisted magnetic recording (HAMR).

Magnetic tape storage, despite seeming old-fashioned, remains the most cost-effective medium for archival data. Major cloud providers and enterprises store exabytes of cold data on tape.

Medical Imaging (MRI)

Magnetic Resonance Imaging uses powerful magnets (typically 1.5 or 3 tesla) to align hydrogen atoms in your body, then uses radio waves to disturb that alignment. As atoms return to their aligned state, they emit signals that vary by tissue type. A computer reconstructs these signals into detailed images.

MRI produces extraordinary soft tissue contrast without ionizing radiation — making it invaluable for brain imaging, joint injuries, cancer detection, and more. About 40 million MRI scans are performed annually in the United States alone.

The superconducting magnets in MRI machines require cooling with liquid helium to near absolute zero (-269 degrees Celsius). This makes helium — a non-renewable resource — strategically important for medical imaging. Helium supply concerns have driven research into helium-free MRI designs.

Particle Accelerators

The Large Hadron Collider at CERN uses 1,232 superconducting dipole magnets (each 15 meters long, generating 8.3 tesla) to bend proton beams around its 27-kilometer ring. These magnets, cooled to 1.9 kelvin (colder than outer space), are among the most sophisticated engineering achievements in history.

Smaller particle accelerators used in hospitals for radiation therapy also rely on magnets to steer and focus particle beams with millimeter precision.

Magnetic Levitation

Maglev trains use magnetic levitation to eliminate friction between train and track. The Shanghai Maglev reaches 431 km/h (268 mph) in regular service — the world’s fastest commercial train. Japan’s SCMaglev has reached 603 km/h (375 mph) in testing.

Two main technologies exist: electromagnetic suspension (EMS), using conventional electromagnets attracted to a ferromagnetic rail, and electrodynamic suspension (EDS), using superconducting magnets that induce currents in the guideway. Both eliminate mechanical contact and the associated friction, noise, and wear.

Everyday Magnets

Beyond these high-tech applications, magnets are in countless everyday items: refrigerator door seals (flexible magnetic strips), phone speakers, headphones, credit card strips, magnetic clasps, whiteboard magnets, and compass needles. The humble refrigerator magnet — made of powdered ferrite in a flexible polymer — is probably the most common magnet most people encounter daily.

Quantum Origins of Magnetism

At the deepest level, magnetism is a quantum mechanical phenomenon. Here’s why.

Electrons have an intrinsic property called spin — a form of angular momentum with no classical analogue. This spin gives each electron a tiny magnetic moment, making it a miniature magnet. In most materials, electron spins point in random directions and cancel out. In ferromagnetic materials, quantum mechanical interactions (the exchange interaction) cause neighboring spins to align parallel, producing bulk magnetism.

The exchange interaction has no classical explanation — it arises from the Pauli exclusion principle and the quantum mechanical nature of electron wavefunctions. This is why magnetism puzzled physicists for centuries: there is simply no way to explain permanent magnets using classical physics alone. As Niels Bohr and J.H. van Leeuwen proved in the 1920s (the Bohr-van Leeuwen theorem), classical statistical mechanics predicts that no material should exhibit any net magnetization. Magnetism is fundamentally quantum.

Orbital angular momentum — the motion of electrons around nuclei — also contributes to magnetic moments, though in most solids, spin dominates.

A Brief History

Magnetism has been known since antiquity. The ancient Greeks knew that lodestone (magnetite) attracted iron — Thales of Miletus wrote about it around 600 BCE. The word “magnet” likely derives from Magnesia, a region in Greece where lodestone was found.

Chinese navigators developed the magnetic compass by the 11th century CE, and it reached Europe by the 12th century. William Gilbert published “De Magnete” in 1600 — the first systematic study of magnetism — establishing that Earth itself is a giant magnet.

The 19th century brought the revolution: Oersted’s discovery linking electricity and magnetism (1820), Faraday’s induction (1831), and Maxwell’s theoretical unification (1860s) transformed magnetism from a curiosity into the foundation of electrical engineering and modern technology.

The 20th century added quantum mechanical understanding (explaining why magnets work at all), superconducting magnets, magnetic resonance, and the extraordinary data storage densities that power the information age.

Key Takeaways

Magnetism is a fundamental force arising from the motion of electric charges and the quantum mechanical spin of electrons. It manifests in several forms — ferromagnetism, paramagnetism, diamagnetism — depending on how a material’s electrons interact. United with electricity through Maxwell’s equations, magnetism underlies electric motors, generators, data storage, medical imaging, and countless other technologies.

What makes magnetism remarkable is the gap between its everyday familiarity (fridge magnets, compasses) and its deep strangeness (no classical explanation exists for permanent magnetism — it’s pure quantum mechanics). It’s a force you’ve known since childhood that turns out to be far weirder and more wonderful than you probably imagined.

Frequently Asked Questions

Why do magnets attract and repel?

Magnets have north and south poles. Opposite poles (north-south) attract because their magnetic field lines connect smoothly between them, creating a lower energy state. Like poles (north-north or south-south) repel because their field lines push against each other. The fundamental cause is the electromagnetic force acting between moving charges.

Can you make a magnet stronger?

You can strengthen an electromagnet by increasing the electric current, adding more coil turns, or inserting a ferromagnetic core. Permanent magnets can be re-magnetized by exposing them to a strong external magnetic field. However, every material has a saturation point beyond which it cannot be magnetized further.

Why does Earth have a magnetic field?

Earth's magnetic field is generated by the geodynamo — convection currents of molten iron and nickel in the outer core, driven by heat from the inner core. These moving conductive fluids generate electric currents, which in turn produce the magnetic field. The field has flipped polarity hundreds of times over Earth's history.

Do magnets lose their magnetism over time?

Yes, but slowly under normal conditions. Permanent magnets gradually weaken due to thermal vibrations that randomly reorient magnetic domains. Heat, physical shock, and exposure to opposing magnetic fields accelerate demagnetization. Neodymium magnets lose about 1% of their strength per decade at room temperature.