Electric fields

Electric fields are produced by voltage. Voltage is the pressure behind the flow of electricity. It can be likened to the pressure of water in a hose, most noticeable when the nozzle is closed.

Electric-field strengths are measured in:

• volts per metre (V/m)
• kilovolts per metre (kV/m) 1 kV/m = 1000 V/m

More on units

The atmospheric electric field at ground level is normally about 100 V/m in fine weather and may rise to many thousands of volts per metre during thunderstorms.

Electricity in homes is at a voltage of 230 V (volts) but outside homes it is distributed at higher voltages - from 11,000 V (usually written 11 kV) up to 400,000 V (400 kV). Generally, the higher the voltage, the higher the electric field.

Anything which uses or carries electricity is potentially a source of power-frequency electric fields. They modulate the Earth's steady natural fields. The strength (or amplitude) of the electric-field modulation depends on the voltage of the equipment, which remains more or less constant as long as the equipment is energised.

Electric fields are readily screened by most building materials and by trees, hedges etc (unlike magnetic fields). Therefore, the electric field produced by a power line inside a house is much less than the field outside. Electric fields next to trees, fences etc are usually less than away from such objects, though they can also be greater directly above a fence. More on screening fields.

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