alternatingfield

An alternating field is a field whose intensity and/or direction varies periodically with time. In physics, this most commonly means an electric field E(t) or a magnetic field B(t) that oscillates, typically sinusoidally, with angular frequency ω: E(t) = E0 cos(ωt), B(t) = B0 cos(ωt). Such fields are contrasted with static (time-invariant) fields and are central to the description of alternating-current and electromagnetic wave phenomena.

In linear media, the response to an alternating field can be analyzed in the frequency domain using complex amplitudes or phasors. The field's frequency, amplitude, and phase relative to a driving source determine quantities such as power absorption, impedance, and the induced dipole moment. Real systems may exhibit dispersion and loss, making the response depend on frequency.

Alternating fields are used in numerous technologies and research areas, including AC power distribution, radio communications, RF and microwave engineering, magnetic resonance imaging and spectroscopy, and nondestructive testing. They enable resonant excitation, inductive coupling, and information transfer, while controlling energy transfer through frequency and amplitude tuning.

A notable specialized application is alternating-field demagnetization, used in paleomagnetism and archaeology. By applying a decreasing-amplitude alternating magnetic field to a sample, remanent magnetization is gradually removed, revealing a primary magnetization signal. The method relies on the gradual overcoming of magnetic coercivity by the diminishing field.

See also: alternating current, electric field, magnetic field, demagnetization, resonance.