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 said to be neutralized.
Electrostatic fields and potentials around an object are not easy to visualize. One way of doing so is by use of field and equipotential lines. A field line represents the path a small charge would take, if it were free to move under the influence of the force due to the electrostatic field alone. Field lines always leave a conductor at a right angle (90°) to the surface.
In Figure 1.1 a charged spherical conducting object has a potential V. Each point in the surrounding space can also be assigned a potential, according to the work required to move a unit charge to that position. If all the positions of equal potential are marked, an equipotential line (or in three dimensions a surface) is marked out. A system of equipotential surfaces could be marked, forming contours of potential showing the presence of the peak in potential rather like the contours on a map showing the presence of a hill. Equipotential lines are always at a right angle (90°) to the field lines.
Equipotential lines are like contour lines on a map of an area of the earth's surface. Height is a form of potential energy. If a ball is released on a smooth hillside, it will roll down the hill perpendicular to the contour lines. Similarly, if a same polarity charge (e.g. a positive charge, next to a high positive potential) is present in the electrostatic field, it will move away from the peak in potential, in a path perpendicular to the equipotential lines. These paths form lines of electrostatic field. The intensity of the field is given by how close together the field and equipotential lines are.
Figure 1.1 Field lines and equipotential around a charged sphere.
The electric field E (vector, as it has magnitude and direction) is the gradient of voltage V over a distance s. Electric field, therefore, has the units volts per meter (V/m).
In Figure 1.1 if the charged sphere is very small, it is effectively a point of charge. The electrostatic field around the charge Q at a distance r from this point is proportional to the charge present, according to Coulomb's law (Cross 1987)
From this equation, in this case the field strength decreases rapidly
