Static Fields

Static electric and magnetic fields are constant fields that do not change in intensity or direction over time.  Hence, they have a frequency of 0 Hz. There are both natural and man-made sources of static fields. Static electric fields, also known as electrostatic fields, are created by charges that are fixed in space. The best known and most powerful display of electric static fields occur in nature as lightning. Static shocks are also common, but trivial, events caused by electrostatic fields.

Static magnetic fields are created by magnets or charges that move at a steady flow, such as in direct current (DC) electricity. They exert an attracting force on metallic objects, and so magnets are commonly used for this purpose. In nature, the geomagnetic field of the Earth exerts a force from south to north that allows, for example, the operation of a compass or the location of the Earth’s magnetic Poles. Much stronger fields are generated by some types of industrial and medical equipment, such as magnetic resonance imaging (MRI) devices, used for medical diagnosis. The table below includes the levels associated with some common sources of static electric and magnetic fields.

 
FieldSourceMagnitudeExplanation/Examples

 

 

Electric



Thunderclouds/Lighting Up to 3 kV/m Distribution of charges within stormy clouds
Non-condutive materials Up to 500 kV/m Friction (e.g. clothes)
Direct current (DC) power cables Up to 20 kV/m Charges moving at a steady flow
DC rail systems Up to 300 V/m Inside the train

 

 

Magnetic



Geomagnetic field 30 to 70 μT Molten iron & Earth rotation
DC rail systems Up to 2 mT DC power electricity
Household magnets Around 10 mT Naturally ocurring different distribution of charges
Magnetic resonance imaging (MRI)* 1.5-10 T Use of electromagnets and superconducting materials

Note: Electric and magnetic field estimates from GreenFacts. The current reference level for public exposure to static magnetic fields is 40 mT. There is no specific reference level for public exposure to static electric fields. However, the EC Recommendation 1999 states that “for most people, the annoying perception of surface electric charges will not occur at field strengths less than 25 kV/m.” *The ICNIRP guidelines for static magnetic fields (ICNIRP, 1994) state that "For specific work applications, exposure up to 8 T can be justified, if the environment is controlled and appropriate work practices are implemented to control movement-induced effects."