How Magnetism Works and Why Earth Has a Magnetic Field

01 The origin of magnetic force

Every magnetic field traces back to motion. A current-carrying wire generates a circular field. An electron orbiting a nucleus creates a tiny magnetic moment. Even the electron itself, as a point particle with intrinsic angular momentum—spin—carries a magnetic dipole moment. The question "what is a magnet?" reduces to "what is moving, and how?"

As the minutephysics video makes vivid, the connection between electricity and magnetism is not a curiosity but a structural identity. Special relativity links the two: what one observer measures as a pure electric field, another moving relative to the charges measures as a magnetic field. The distinction between electric and magnetic is frame-dependent, while the electromagnetic field itself is invariant.

02 Ferromagnetism: why iron sticks

In most materials, the magnetic moments of individual atoms are randomly oriented, canceling to zero. Ferromagnetic materials—iron, nickel, cobalt, and some alloys—are different. Their unpaired electrons in partially filled d-orbitals create atomic moments, and quantum exchange interactions align neighboring moments parallel. This produces the strong, persistent magnetism of a bar magnet or a compass needle.

Above the Curie temperature, thermal energy overwhelms the exchange interaction and ferromagnetism vanishes. For iron, this threshold is 770°C (1043 K). Below it, a ferromagnet organizes into domains—regions of aligned moment separated by walls. Applying an external field moves the walls and rotates the domains, a process that can be permanent (hard magnets) or reversible (soft magnets).

03 Earth's field: a planetary dynamo

Earth is not a bar magnet. Its field is generated by the geodynamo: convection of molten iron and nickel in the liquid outer core, 2,890 to 5,150 km below the surface. As this conductive fluid moves through the existing magnetic field, it induces electric currents. Those currents generate their own magnetic field, reinforcing the original—a self-sustaining feedback loop.

The field at Earth's surface has a strength of approximately 25 to 65 microteslas (0.25 to 0.65 gauss), strongest near the poles and weakest near the equator. It extends thousands of kilometers into space, forming the magnetosphere—the cavity carved out of the solar wind by the field's pressure.

04 The magnetosphere and solar wind

The Sun continuously ejects charged particles at 300 to 800 km/s. This solar wind compresses Earth's field on the day side to about 10 Earth radii and stretches it into a long magnetotail on the night side. Without the magnetic shield, this stream of plasma would strip the atmosphere, as it did on Mars when that planet lost its dynamo and its field died.

Trapped within the magnetosphere are the Van Allen radiation belts—zones of energetic protons and electrons held in spiraling paths along field lines. When solar storms intensify, charged particles spiral down field lines into the polar regions, exciting atoms in the upper atmosphere. The resulting emission of light is the aurora borealis and aurora australis.

05 Reversals: the field that flips

Earth's magnetic field is not stable. Paleomagnetic records in volcanic rocks reveal that the dipole reverses, on average, every 200,000 to 300,000 years—though the timing is irregular. The last full reversal, the Brunhes-Matuyama event, occurred approximately 780,000 years ago. During a reversal, the field does not vanish; it weakens and reorganizes, with multiple poles emerging temporarily.

The process takes thousands of years. Current models from the European Space Agency's Swarm mission show the field has weakened by about 9% over the past 200 years, and the South Atlantic Anomaly—a region of unusually weak field over South America—is growing. Some scientists interpret this as a precursor to a future excursion or reversal, but the evidence is inconclusive.

06 From compass to quantum sensing

Magnetism's practical reach extends from the medieval compass to modern spintronic devices. Electromagnetic induction—Faraday's discovery in 1831—underpins every electric motor, generator, and transformer. Magnetic resonance imaging uses nuclear magnetic resonance to map tissue. Hard disk drives encode data in magnetic domains on a spinning platter. Quantum sensors now measure magnetic fields at the femtotesla level, detecting neural activity and mapping mineral deposits.

The strongest known magnetic fields in the universe belong to magnetars: neutron stars with fields of 10⁹ to 10¹¹ tesla. At these intensities, the field's energy density exceeds that of normal matter. A magnetar's field would distort atoms into cigar-shaped configurations from a thousand kilometers away.

import math # Dipole field at equator: B = mu_0 * m / (4*pi * r^3)
# Earth dipole moment ~ 8e22 A*m^2 mu_0 = 4e-7 * math.pi # T*m/A
m_earth = 8e22 # A*m^2
r_earth = 6.371e6 # meters B = (mu_0 * m_earth) / (4 * math.pi * r_earth**3)
print(f"Equatorial surface field: {B*1e6:.1f} uT")

07 Why it matters

Earth's magnetic field is not merely a geological curiosity. It is the boundary condition for life on this planet. Without the magnetosphere, the solar wind would gradually strip the atmosphere of its lighter molecules—hydrogen, then helium, then others—as it did on Mars. The geomagnetic shield is the difference between a planet that can support complex life and one that cannot.

Watch the minutephysics video for a crisp explanation of how relativity makes magnets work, then dig into the dynamo. The field you cannot see is one of the reasons you are here to wonder about it.