Academy of EMC

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How Electrostatic Charges Are Produced?
When two or more non-conductive materials with different affinities for electrons make and break contact, electrostatic voltage potentials up to several ten thousand volts may result. The reason why this happens is that electrons can be exchanged between materials on contact. Materials with weakly bound electrons tend to lose them, while materials with sparsely filled outer shells tend to gain them. Every time a material with weakly bound electrons and a material with the tendency to gain electrons make contact, electrons spend more time on the material with the higher affinity for electrons. When conductive materials are separated, electrons quickly return to the material they initially came from. However, when the materials are not conductive, the separation of the materials may occur before the electrons are back to the material where they initially came from. This electron exchange leads to a positively charged material (the one that lost electrons) and a negatively charged material.
In practice, the magnitude of charge depends heavily on the relative humidity rH [%] of air. In a humid environment, the slight amount of moisture on the surface of a non-conductive material can lead to a rise of conductivity high enough to prevent significant charge separation. Therefore, the electrical charges are larger, the lower the relative humidity. At an optimal relative humidity of 50% to 55%, the conductivity of the material surfaces is increased to such an extent that electrical charges can be dissipated without any problems [15.1]. However, bear in mind that the often prevailing opinion that moist air conducts electricity better than dry air does not apply. It is the moisture on the surface of materials, which prevents static electricity.