An electromagnetic field (also EMF or EM field) is a physical field produced by the movement of electrically charged objects.
Illustration of electric and magnetic field. The electric field is present when the lamp is plugged in, due to the voltage difference between the connectors. The magnetic field is only present when the lamp is switched on, because that's when current flows in the cable. Source: Hydro-Québec
All around the world, people are constantly exposed to electromagnetic waves.
Examples of man-made sources:
cellular telephones
radio-, and television transmissions
WiFi networks
satellite communications
unintended emissions and stray fields arising from electronic circuits
electric motors
cables and power transmission networks
Examples of natural sources:
local build-up of electric charges in the atmosphere associated with thunderstorms
light from the sun carries infrared and ultraviolet radiation
ionizing radiation from the Earth and space
the Earth's magnetic field
Electric and magnetic fields are also present in electromagnetic radiation. Electromagnetic radiation has a magnitude (size) and a frequency (time-dependent periodic variation). The frequencies of electromagnetic radiation ranges from static electric and magnetic fields, through radiofrequency and infrared radiation, to x-rays. The European power frequency is 50 Hz.
Published June 28, 2011
Quantities and units
B and H both describe the magnetic field. The quantities B and H are related to each other by the magnetic permeability (µ) of the medium that they exist in.
B=µH,
where µ is the magnetic permeability. For air and human tissues µ is approximately 4π x 10-7 H/m.