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Electromagnetic fields (EMF) are a combination of invisible electric and magnetic fields of force. They occur both naturally and due to human activity.
Naturally occurring EMF are for example, the earth static magnetic field to which we are constantly exposed, electric fields caused by electric charges in the clouds or by the static electricity produced when two objects are rubbed together as well as sudden electric and magnetic fields caused by lightning, etc.
Man-made electromagnetic fields (EMF) are for example generated by extremely low frequency (ELF) sources, such as power-lines, wiring and appliances (the focus of this study) as well as by higher frequency sources such as radio and television antennas and, more recently, cellular telephones and their antennas (which are not reviewed in this study). More...
All electric flows have an associated electric field and magnetic field. Both electric and magnetic fields are essentially 'invisible lines of force', each associated with a different characteristic of electricity. An electric field is the force created by the attraction and repulsion of electric charges (the cause of electric flow), and is measured in volts per meter (V/m). A magnetic field is a force created as a consequence of the movement of the charges (flow of electricity). The magnitude (intensity) of a magnetic field is usually measured in tesla (T) or sometimes in gauss (G). The intensity of both electric and magnetic fields decreases with distance from the field source. Electric fields are more easily shielded or blocked than magnetic fields.
Both electric and magnetic fields can vary in time. Whereas, direct current (DC) fields have a steady direction, flow rate and strength, alternating current (AC) fields change direction, flow rate and strength over time with a certain frequency.
Most electricity carried in powerlines, wiring and appliances is alternating current (AC). Worldwide, altenating current (AC) moves back and forth (cycles) either 50 or 60 times per second (the latter predominantly in US), that is, at a frequency of 50Hz and 60Hz respectively. Such electromagnetic fields are classified as Extremely Low Frequency (ELF) fields, as their frequency is within the range of 3 to 3000 Hz.
Electromagnetic fields produced by sources other than electricity, such as cellular phone antennas, have higher frequencies and are therefore not classified as Extremely Low Frequency (ELF) fields, nor are they discussed within this study. More...
Figure on frequency ranges from different sources
Further details on the difference of scale of Gauss and Tesla
Because electricity is so much a part of our lives, there are electromagnetic fields around us most of the time. We are exposed to electric and magnetic fields from many sources including high, medium and low tension (voltage) power lines, electric wiring inside buildings and electric appliances such as refrigerators, computer monitors, electric saws and drill presses.
While strong electric fields are produced by high voltage electricity, strong magnetic fields are produced by strong electric currents (high amperage). A general rule is that relatively strong electric and weak magnetic fields are observed below high voltage transmission lines, whereas household appliances and industrial devices and machines may produce weak electric and strong magnetic fields.
Both electric and magnetic fields weaken with distance away from the source. With respect to magnetic fields, field strength weakens more rapidly from point sources, such as appliances, than from line sources, such as power lines and wiring. Magnetic fields are reduced to background levels within about one meter (3-4ft) from appliances, while magnetic fields from low voltage lines delivering electricity to houses, offices and factories and high voltage lines delivering electricity from a powerplant to the power provider disappear within 20 to 60 meters (60-200ft) and 100 to 300 m (300-1000 ft) respectively.
Overlapping fields can interact to strengthen or weaken the total effect. Therefore, the strength of the electric and magnetic fields depends not only on the distance from the source but also the distance from, and location of, other nearby sources. More...
Table showing examples of magnetic fields generated by domestic appliances
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