1/r2. This is also indicated by the spreading of the field lines with distance from the charge. Field lines can be considered to begin and end on electrostatic charges, and so a high density of field lines at a surface implies a high charge density as well as high electrostatic field.
For other shapes of charge patterns, the equipotentials will not in general be spherical, and field lines will in general be curved rather than straight lines. Field lines are always perpendicular to the equipotentials and are always perpendicular to conducting surfaces as these are also equipotentials.
1.2.6 Gauss's Law
In Figure 1.1, eight field lines cut the equipotential line. Each of these lines in principle originates on a charge. So, the field lines cutting a surface is related to the net charge within it. Gauss's law generalizes this to state that the component of electric field perpendicular to a surface is proportional to the charge enclosed by the surface. For further information, the reader should refer to more academic texts such as Cross (1987).
1.2.7 Electrostatic Attraction (ESA)
A charge in an electrostatic field experiences a force, as described in Section 1.2.2. So, a charged particle or object will also experience a force according to the charge it carries. This causes charged particles and objects to be attracted or repelled by other objects, some of which may be product or items that are required to be kept clean. This effect is known as ESA.
A lesser known phenomenon that contributes to electrostatic attraction or repulsion is dielectrophoresis (Cross 1987). In this case, uncharged particles can be attracted or repulsed in a divergent or convergent electrostatic field due to differences in the permittivity of the particle and the material in which it is immersed.
1.2.8 Permittivity
Coulomb's law shows that the field due to a point charge is proportional to the charge and inversely proportional to the distance from it squared (Cross 1987).
Permittivity (dielectric constant), ɛ, was defined to give a convenient constant of proportionality in this relation.
For air, the permittivity is very close to the permittivity of free space ɛ0 (vacuum) ɛ = ɛ0 = 8.8 × 10−12 Cm−1. Other materials have different permittivity and affect field strengths correspondingly. In general, a dielectric material has a permittivity greater than air. This is conveniently expressed as a relative permittivity ɛr, and the permittivity is given by
Polymers often have relative permittivity in the range 2–3 and many other materials in the range 2–10. Materials such as ceramics can have far higher permittivity.
1.3 Electric Current
Moving charges form electrical currents. One coulomb of charge has passed if 1 ampere has flowed for one second.
or for a varying current
and so
1.4 Electrostatic Discharge (ESD)
IEC 61340‐1:2012 defines an electrostatic discharge as “transfer of charge by direct contact or by breakdown from a material or object at a different electrical potential to its immediate surroundings.” IEC 61340‐5‐1:2016a gives a slightly different definition of “Rapid transfer of charge between bodies that are at different electrostatic potentials.”
There are various types of electrostatic discharge that are important in different fields. In ESD in electronics handling, the main types of concern are
Spark discharges between conducting objects or materials
Brush discharges between a conducting object and an insulating material
Corona discharges from sharp conducting objects and materials
Electrostatic discharges are discussed further in Chapter 2.
1.4.1 ESD Models
ESD from different sources produces very different discharge current waveforms. These can be modeled and simulated by simple electronic circuits. Three standard ESD source circuit models, human‐body model (HBM), machine model (MM), and charged device model (CDM), have been developed and standardized for testing ESD susceptibility of electronic components. This is discussed further in Chapter 3.
1.4.2 Electromagnetic Interference (EMI)
An ESD event can produce very large and fast‐changing currents and voltages. These produce fast‐changing electromagnetic fields with strong and fast‐changing magnetic and electric field components and a broad frequency spectrum, sometimes extending to over GHz frequencies. This can be radiated and conducted to be picked up by nearby electronic circuits and can cause temporary malfunction. This is known as electromagnetic interference.
1.5 Earthing, Grounding, and Equipotential Bonding
Electrostatic discharges occur because of voltage differences between the objects between which the discharge occurs. If there were no voltage difference, then no ESD could occur.
So, one way to prevent ESD from occurring is to eliminate voltage differences between objects. If the two objects are conductors, connecting them electrically ensures that they are eventually at the same voltage. This must be so, as if any voltage difference were to arise, charge (current) would flow due to the voltage difference, until there is no voltage difference. The practice of connecting conductors together to eliminate voltage differences is known as equipotential bonding.
If two conductors at two different voltages are brought into contact, an electrostatic discharge will occur as part of the voltage equalization process. If one of the conductors is susceptible to ESD damage, it could risk being damaged as a result. So, ESD‐susceptible parts must only make contact with other conductors, including grounded conductors, in circumstances designed to protect against damage.
In many practical cases, one of the conductors concerned may already be electrically connected to an electrical earth or can be conveniently connected to earth. The earth is often defined as our 0 V reference point in electricity power
