Technical note 2

Fields and waves

Leakage of power via fields is the cause of interaction between circuits and surroundings (EMI)

Electric fields are induced by charges in high impedance circuits, which are also susceptible to these fields.
The so-called 'hand effect' (human hand influences a function) is proof of the presence of an electric field:
when approaching the input of an LF amplifier, hum is induced by the electric field of the mains voltage.
Electric fields can be suppressed (shielded) with grounded metal plates, which capture the electric field.

Magnetic fields are induced by currents in low-impedance circuits, which are also susceptible to these fields.
Magnetic fields can be suppressed with metal boxes that completely enclose the noisy or susceptible circuit.

Electromagnetic waves (plane waves) induce or are induced by accelerating charges (alternating currents) flowing via wiring bundles, metal strips, brackets or groundplanes, which act as antennas.
Plane waves consist of an electric field and a magnetic field; these two fields are directed perpendicular to each other and move with the speed of light.
Electromagnetic waves can thus be suppressed (shielded) with grounded and well-closed metal boxes.