## What Is Static Electricity ?

When a material or object holds a net electrical charge, either positive or negative, it is said to have a static charge. The term static is a relative one as in many cases static charges will slowly decrease over a period of time.

The length of time that this takes is dependent on the resistance of the material. For practical purposes the two extremes can be taken as plastics and metal. Plastics generally have very high resistivities. This allows them to maintain static charges for long periods of time; on the other hand metals have very low resistances and an earthed metal object will hold its charge for an
imperceptibly small period of time.
Static electricity is usually measured in volts. Whilst mains voltages of 220 volts AC are considered dangerous, levels of static electricity of 100 kV are common. The voltage present on a material is dependent on two factors; the amount of charge on the material and the capacitance of the material. The simple relationship is Q=CV where Q is the charge, V the voltage and C the capacitance of the material. It can be seen that for a given charge on a material, the lower the capacitance the higher the voltage and vice versa. Plastics generally have very low capacitive values and hence a small charge can produce very high voltages. Conversely metals tend to have high capacitive values and therefore a relatively high charge will produce low voltages. This is why, in practice, problems with static electricity are most noticeable when working with plastic, as it is the voltage level which causes the attraction of dust, operator shock and misbehaviour of
materials.
There are two main types of static electricity, volumetric and surface. Volumetric static charges are charge imbalances within the body of a material whereas surface static electricity is only present on the very outer surface of a material. In practice nearly all the static electricity problems found in industry relate to surface charges. Whilst there is no way of neutralising volumetric static charges they rarely cause a problem and their effects are normally minimal when compared to surface static charges.