electromagnetism

History

Until the beginning of the 19th century, electricity and magnetism were considered independent. In 1820, Hans Christian Ørsted discovered that a current deflects a magnetic needle, and Michael Faraday in 1831 showed that a moving magnet creates a current in a coil. In the 1860s, the Scottish physicist James Clerk Maxwell generalized these laws into a system of equations that not only described all known phenomena but also predicted electromagnetic waves.

How it works

When electrons in an antenna oscillate, they create changing electric and magnetic fields around them, which, like ripples on water, spread through space as an electromagnetic wave. The eye perceives high-frequency waves as light, and an antenna picks up radio frequencies as a Wi-Fi signal.

History

Until the beginning of the 19th century, electricity and magnetism were considered independent. In 1820, Hans Christian Ørsted discovered that a current deflects a magnetic needle, and Michael Faraday in 1831 showed that a moving magnet creates a current in a coil. In the 1860s, the Scottish physicist James Clerk Maxwell generalized these laws into a system of equations that not only described all known phenomena but also predicted electromagnetic waves.

How it works

When electrons in an antenna oscillate, they create changing electric and magnetic fields around them, which, like ripples on water, spread through space as an electromagnetic wave. The eye perceives high-frequency waves as light, and an antenna picks up radio frequencies as a Wi-Fi signal.

History

The recognition of the connection between electricity and magnetism began with the experiment of H.C. Ørsted (1820), showing that a current causes a magnetic needle to deflect. M. Faraday in 1831 discovered electromagnetic induction—the emergence of current in a circuit when the magnetic flux changes. A brilliant synthesis was accomplished by James Clerk Maxwell: in 1861–64 he formulated a system of equations, adding to Ampère's law the 'displacement current'—a term with the time derivative of the electric field—which ensured consistency and predicted waves. The existence of waves was experimentally confirmed by H. Hertz in 1887. Hendrik Lorentz in 1895 introduced the Lorentz force and created the electron theory, explaining the behavior of charges at the microlevel. The quantum stage was completed in the mid-20th century by the works of R. Feynman, J. Schwinger and S. Tomonaga, who built renormalizable quantum electrodynamics.

How it works

Electrodynamics obeys the principle of relativity and is invariant under Lorentz transformations. From Maxwell's equations follows the wave equation for the fields. In quantum electrodynamics, the state of a system is described by a vector in Fock space, creation and annihilation operators for photons are built from the potentials, and the interaction with matter (fermions) is given by the interaction Lagrangian eψ̄γ^μψA_μ (where e is the charge, ψ is the Dirac field, A_μ is the electromagnetic potential). Scattering processes are calculated perturbatively in the fine-structure constant α ≈ 1/137 using Feynman diagrams. At small distances, the effective charge increases (vacuum polarization effect), which has been verified with high accuracy. The classical limit gives the Lorentz force F = q(E + v × B), describing the motion of charged bodies in fields.

Open problems & fun facts

The absence of observed magnetic monopoles, although Maxwell's equations allow them.; The reason for electric charge quantization: why charge is always a multiple of the elementary charge.; Combining quantum electrodynamics with general relativity to describe strong gravitational fields.

💡 Modern experiments test Coulomb's law down to distances of 10⁻¹⁸ m, and it has not been violated so far—the inverse-square dependence holds with fantastic accuracy. This is one of the most precisely tested laws of physics.