Electrostatics
From Encyclopædia
Electrostatics is the study of phenomena resulting from electrical charge, or
electricity, that is at rest. The study of electromagnetism is greatly simplified by a knowledge of electrostatics.As early as 1600 many of the ideas later incorporated in the electrostatic force law were already known: two types of
electricity exist (later named positive and negative by
Benjamin Franklin); samples of the same kind repel each other, and those of opposite kinds attract; and if the distance between charges is increased, the force of attraction or repulsion decreases.After
Joseph Priestley deduced (1767) that the force between two charged bodies decreases as the
square of the distance, so that doubling the distance decreases the force by a factor of 4, Charles A. de Coulomb verified it by precise experiments. The basic law of electrostatics is therefore called Coulomb's law, which states that the magnitude F of the force between two charges of amounts, p and q, separated by a distance r is F = kpq/rr. The direction of the force is along the line joining the two charges. If p and q is in coulombs (one
electron has 1.60/10,000,000,000,000,000,000 coulombs of charge), r is in meters, and F is in newtons (1 newton = 0.225
pounds), then k, the proportionality
constant, is 8.988 billion.The Electric FieldMichael FARADAY, an English physicist, first suggested the existence of electric and magnetic fields. At the time, the electric field was thought to be the response of the ether (see ETHER,
physics) to the presence of a charge. It is now known that the ether is nonexistent and that electric and magnetic fields can be established in empty space. A charged particle sets up the field E, and another charge elsewhere in space is acted on by the field. For two small charges--p for the first, or source, charge and q for the second, or field, charge--E = kp/rr and F = qE. These two equations can be combined, eliminating E, to reproduce Coulomb's law.The field E is the electrostatic field that results from a single charge. The fields caused by a collection of charges are merely the sum of the individual fields.Field lines are often used to represent the electric field. They are in the same direction as the field, and the
density of lines represents the magnitude. The lines start on positive charges and stop on negative charges.It is difficult to separate charges that attract; work must be performed to do so. On the other
hand, work is obtained from charges that repel. The electric potential energy is the work, positive or negative, that can be retrieved when the charges are allowed to return to their initial positions. The electric field is said to be conservative because all the work done in separating the charge can be recovered (see
conservation, LAWS OF). The electric potential is measured in volts that represent the
number of
joules of work (1
joule equals 2.78 X 0.0000001 kilowatt hours) for each coulomb of charge.Conductors, which are
metals or other materials that permit
electrons to move around rather freely, may have electric fields within them, but only if current is flowing, which indicates a motion of charge. Here the field exerts a force on the charges, causing them to move. Thus, in the electrostatic
case, E = 0 inside a conductor. Furthermore, in electrostatics, all of the unbalanced charge is found on the surface of the conductor.Static ElectricityThe accumulation of electric charge causes static
electricity. It makes the air crackle and hair stand out. This phenomenon is most noticeable when the dryness of the air inhibits the recombining of charges that have become separated by friction or by other means. Static
electricity is not as evident on a humid day, because the resistance to current is reduced by the moisture, and the static charge can leak off the objects.Under ideal conditions, the static charge accumulated when shoes are scuffed on a carpet exceeds 20,000 volts. Although this may cause a painful shock, it is not harmful, since low current is involved.ApplicationElectrostatic force causes the deflection of the
electron beam in a
television tube. The beam moves through two sets of parallel plates, one oriented vertically and one horizontally. An electric field is set up by charging the plates. This imposes a force on the
electrons and they are deflected from their original course. The two sets of plates work together to sweep the beam over the entire face of the tube in only a few milliseconds.The electrostatic precipitator is an important industrial application of static
electricity. Most of the polluting smoke particles rising in a smokestack are electrically charged, a result of the burning process. By placing electrically charged plates in the stack, the particles can be forced into a collector where they are removed.High-voltage electrostatic fields are responsible for natural
lightning. Similar intense fields can be developed in the laboratory by using devices such as the Van de Graaf generator, which can develop a potential of many millions of volts. This field can then be used to accelerate charged particles in a
vacuum, raising them to velocities near that of
light. Beams of such high-speed particles are useful in research and industry.W. Farrell EdwardsBibliography:
cross, J., Electrostatics (1987); Green, Charles, The Electrostatics Handbook (1973); Halliday, David, and Resnick, Robert, Fundamentals of
physics, 3d ed. (1988); Hockey, S. W., Fundamentals of Electrostatics (1973); Moore, A. D., ed., Electrostatics and Its Applications (1973).
