externfält

Externfält refers to a field that is imposed on a system from outside and is not generated by the system’s own dynamics. It acts as an external influence that can alter the behavior of the system, such as its energy levels, trajectories, or statistical properties. External fields are distinguished from self-generated or self-consistent fields, which arise from the system’s charges, currents, or mass distribution and are determined by internal equations.

In physics, common examples include external electromagnetic fields, gravitational fields, or mechanical constraints. They can be static or time-varying, and in many analyses are treated as fixed backgrounds. In some theories, the external field can be coupled to the system and respond (back-action), but the simplest approach uses a prescribed external field.

In quantum mechanics and materials science, the external field is represented by an external potential V_ext(r, t) added to the system’s Hamiltonian: H = T + V_int + V_ext. It models interactions with applied fields, substrates, impurities, or optical fields. The external field is often considered classical and spatially dependent; many response calculations use linear response theory to relate field strength to induced observables.

In mathematics and theoretical physics, an external field Q(x) may modify an energy functional, for example in weighted potential theory: E_Q(μ) = ∫∫ log 1/|x−y| dμ(x) dμ(y) + ∫ Q(x) dμ(x). Such external fields influence equilibrium measures and spectral distributions in problems like random matrix theory.

See also external potential, field theory, and potential theory.