The ESD Control Program Handbook. Jeremy M. Smallwood
1.2 Charge, Electrostatic Fields, and Voltage
1.2.1 Charge
The charge is a property of elementary particles – electrons and protons – that make up the atoms of all materials. All materials are made up of positively charged atomic nuclei and negative electrons (Cross 1987). The charge on a proton in the nucleus is labeled positive, and the charge on an electron is labeled negative. The charge effects of a proton are equal and opposite to those of an electron, so if a proton and electron are together in an atom, their effects cancel exactly. In this case, the atom is neutral. Atoms of different elements have many protons and electrons, depending on the particular element. For example, a hydrogen atom has 1 proton and 1 electron, and a carbon atom has 12 protons in the nucleus and 12 electrons surrounding it. An object or material is made of huge numbers of atoms and so extraordinary numbers of electrical charges.
When talking of static electricity, it is often said that charge is “generated” in certain circumstances. That is not so – all that happens is that a small number of negative charges become separated from their positive companions in a material and end up in a different place. For every negative charge appearing somewhere, there must be a positive charge appearing somewhere else. An object is described as charged if it has a net imbalance of the number of positive and negative charges that it contains. The electrical effects of the charges are no longer balanced, and a net static electrical charge exists at that location. It is this net charge imbalance that we are referring to when we talk about the charge on an object or material.
The unit of charge is the coulomb (C). In practice, the coulomb is a rather large amount of charge and microcoulombs (μC, 10−6C), nanocoulomb (nC, 10−9C), or even picocoulombs (pC, 10−12C) are more usual. A single electron or proton has a charge of 1.6 × 10−19C. So, an object having even 1nC of net charge has a large number, 6.2 × 109, of unneutralized electrons or protons.
1.2.2 Ions
Ions are very small particles having a small electrostatic charge. They are naturally present in the air but may also be generated deliberately or accidentally around objects at high voltage.
Charges are naturally present in atoms. Protons in the atomic nucleus have positive charge, and electrons in the atom have negative charge. A negative ion is formed when a particle gains one or more electrons. A positive ion is formed when a particle loses one or more electrons. Ions may consist of free electrons, single atoms, many atoms, or molecules (Wikipedia 2018). Sometimes these ions may become attached to larger particles.
1.2.3 Dissipation and Neutralization of Electrostatic Charge
Where there is an imbalance of charges present, there will usually be voltage differences.
Charges exert forces on each other and create an electrostatic field in which various effects occur. Like charges repel each other, and unlike charges attract each other.
If like charges have built up in a region, they repel each other, and if they are able to move, they will move apart and gradually spread out and dissipate. Unlike charges will be attracted and move together.
When opposite polarity charges are sufficiently close together, their effects cancel, and the charge is said to be neutralized.
1.2.4 Voltage (Potential)
Electric potential is defined in terms of the work done in moving a charge from one place to another in an electric field (Cross 1987). If a charge Q is moved a distance s against a uniform electric field E, the potential difference V between the start and end positions is
The energy taken to move a charge between two points is the same, no matter what route is taken between the points. Potential difference is measured in volts (V) and is often referred to as voltage. The unit volt (V) is equivalent to joules per coulomb. Voltage is a measure of the potential energy at a point and is perhaps analogous to pressure in a fluid system or height in a gravitational system.
Engineers often talk about the potential of (for example) a conductor (see Section 1.7.3 for a discussion of conductors and insulators), as a synonym to voltage. This is not strictly correct as potential is strictly the work done in bringing a charge from infinity to the place of measurement (Jonassen 1998).
A voltage or potential difference at a place of measurement must always be referred to another place. In practice, the potential difference is usually quoted with reference to the potential of the earth (also referred to as ground; see Section 1.5), which is defined for convenience as zero volts. If this other place is not specifically stated, it is usually ground (the earth).
All points in space surrounding a charge have a voltage (potential) – typically this voltage will be different from its neighboring points. For a conducting surface, if it is not initially an equipotential, voltage differences cause charge (current) to flow until the voltage around the surface is eventually equal. So, an electrically conducting surface in equilibrium is an equipotential surface.
1.2.5 Electric or Electrostatic Field
Any charge has a region of influence around it, in which various electrostatic effects are noticed – this region is the electric (electrostatic) field due to the charge. Charge is the fundamental source of static electricity, and the electrostatic field shows the effect of the charge in the world around the charge source. In this field, we find that
Like polarity charges are repelled.
Opposite polarity charges are attracted.
Conductors (e.g. metals) redistribute their charges and experience a change of potential (voltage) in response to the field.
Particles of many materials may be attracted or repelled within the field.
Static electricity phenomena are due to these basic effects.
Dust particles, and small objects, are attracted or repelled by a field, especially if they are themselves charged (e.g. ionized particles in the air). The force F experienced by a charge q in an electrostatic field E is (Cross 1987)
If equal positive and negative charge are sufficiently close together, from a distance their electric field effects cancel, and no external field is noticed. The charges are