The Fundamentals of ElectrostaticsJanuary 15, 2021 by Lorenzo Mari
Learn about the basics of static electricity, what it is, and its generation processes.
A fundamental fact of electricity is that every proton and electron in an atom have a unique, fascinating trait called an electric charge. This property of protons and electrons to have an electric charge causes mysterious forces to appear between them – the electric forces.
Figure 1. Electrostatic lamp. Image courtesy of Niek Verlaan.
Transferring electrons between bodies creates a familiar electric phenomenon called electrostatics, or static electricity.
What is Static Electricity?
Static electricity, or electrostatics, studies the interaction between two charged particles at rest or moving with a minimal velocity. The protons and electrons in atoms have an electric charge – more specifically, an electrostatic charge. In the 19th Century, atomic physics designated positive charge for protons and negative charge for electrons.
An intriguing property of charged particles is that like charges repel and unlike charges attract, exerting forces on one another. Figure 2 is an illustration of this statement.
Figure 2. Electric interactions between like and unlike charges. Image based on Alonso-Finn, 1970.
Protons and electrons have the same amount of charge and, when in close proximity, they nullify each other’s charge, making their combination electrically neutral. A force field extends into space around a charged particle, but a neutral material does not exert electrostatic force.
Most bodies are composed of equal amounts of positive and negative charges so that the net electrical interaction between any two macroscopic bodies is small or zero. For this reason, we overlook the electrostatic forces but notice them when generating static electricity because the presence of electrical charge produces forces between the charged bodies.
In 1767, Joseph Priestley published one of the fundamental physics laws known as the Inverse Square Law of Electrostatics. This law describes how positive and negative charges exert forces on each other. Priestley found that the force between two charged particles is proportional to their charges and the inverse of the square of their separation. This law only applies to charged bodies whose dimensions are small compared to the distance.
In 1785, French engineer and physicist Charles-Augustin de Coulomb measured the inverse square dependence directly – 18 years after Priestley’s discovery. Nowadays, the law is known as Coulomb’s law.
The Charging Mechanism
The following three processes can charge an object:
The Triboelectrification Process
The triboelectrification process generates charges when two different objects are brought into contact or rubbed together. The triboelectric series is a list of materials arranged such that a material develops a positive charge when it contacts any material lower in the series. Table 1 shows a typical triboelectric series.
Table 1. Typical Triboelectric Series
It is vital to understand the mechanism of charging by contact and separation. When two bodies of unlike materials touch (at a distance smaller than 10 nm) electrons may drift from one body to the other through the contact surface.
If the bodies are suddenly separated, the electrons return to their parent substance when the parents are good conductors. However, the electrons cannot return if one or both parent substances are insulators, leaving the electrons trapped on the surface of the material to which they migrated. Then, the surfaces get a positive or negative charge depending on the deficiency or surplus of electrons. By convention, a shortage of electrons indicates a region of positive polarity and an excess of electrons causes an area of negative polarity.
The amount of charge depends on the contact area, regardless of their being stationary or in motion. When in motion, the friction increases the contact area and the amount of charge.
Sometimes, in the presence of impurities like dust or unexpected ions, identical materials may develop a charge. This may also happen when the surfaces in contact have different areas.
A familiar way of generating static electricity is by combing hair. While combing the hair with plastic or rubber combs in a dry environment, the comb attracts some hair strands, and the combed strands repel themselves trying to protrude from the head. This attraction and repulsion is due to electrostatic forces.
While combing the hair, electrons from the hair’s surface relocate to the comb’s surface. The comb surface has an excess of electrons and becomes negatively charged, while the hair strands have an electron shortage and become positively charged. For this reason, the comb and the hair attract while the hair strands repel each other. The comb and the hair acquired static electricity.
The Inductive Process
The inductive process generates charges in the presence of an electric field. Electric fields induce charges at the surfaces of nearby conducting objects. Uncharged conductive objects have the same amount of positive and negative charges, mutually bound. These charges can be partially disarranged when influenced by an external electrical field.
Figure 3 shows a negatively charged plastic rod close to a metallic body. The rod repels the electrons in the metal so that the side facing the rod accumulates a positive charge and the opposite side gets a negative charge due to electric induction. The metal returns to its neutral state after removing the plastic rod.
Figure 3. Distribution of charges under the influence of an electric field. Image courtesy of R. Kurtus, 2009.
The Conductive Process
In the conductive process, a neutral object charges when contacting a charged object. If we touch a cork ball with an electrified amber rod, the cork ball will become electrified.
Reviewing Electrostatics Essentials
Electrostatic charge denotes an excess or deficiency of electrons on an object’s surface.
Electric charges exert forces on one another. Unlike charges attract, and like charges repel. A typical atom has a neutral charge.
The main processes to charge an object are the triboelectrification process, the inductive process, and the conductive process.
- In the triboelectrification process, electrostatic charge is generated by transferring electrons between surfaces when contacting or rubbing two different substances.
- In the inductive process, electrostatic charge is generated in the presence of an electric field.
- In the conductive process, electrostatic charge is generated from contact with a charged object.