Nærfelteffekter

Nærfelteffekter refers to phenomena that occur in the immediate vicinity of an oscillating electric or magnetic dipole. These effects are characterized by a strong dependence on distance and a rapid decrease in strength as one moves away from the source. Unlike far-field effects, which involve propagating electromagnetic waves, near-field effects are dominated by reactive fields that store and release energy.

In the electric near-field, the electric field lines are not straight and parallel but curve significantly, and their strength falls off as the inverse cube of the distance from the source. This is in contrast to the inverse square law seen in the far-field. Similarly, the magnetic near-field exhibits a similar rapid decay, often described as inverse cube dependence. The interaction between the electric and magnetic fields in the near-field is also different from the far-field, where they are coupled and propagate as a wave.

These effects are crucial in understanding the behavior of antennas at very close range, magnetic resonance imaging (MRI) systems, and inductive coupling for wireless power transfer. The principles of near-field effects are also relevant in microscopy techniques that utilize evanescent waves, which are a form of near-field radiation. The spatial resolution in such techniques is often limited by the wavelength of light, but near-field microscopy can overcome this limitation by operating in the near-field region. Understanding and manipulating nærfelteffekter is fundamental for designing and optimizing various electronic and electromagnetic devices and applications.