Physics · Chapter 19
Study notes aligned to the official NEB syllabus.
Electrostatics is the branch of physics that deals with electric charges at rest, that is, with the forces, fields, and potentials produced by stationary charges. Almost every electrical effect we meet, from the crackle of a woollen sweater to the working of a capacitor, begins with the simple idea of electric charge.
This chapter introduces electric charge and its carriers, explains how bodies become charged by friction (and, as related standard scope, by induction), shows why the charge on a spherical conductor can be treated as though it sits as a point charge at the centre, states Coulomb's law for the force between two point charges, and applies the principle of superposition to find the net force on a charge due to several other charges.
Electric charge is the fundamental physical property of matter that causes it to experience a force when placed in an electric or magnetic field. It is charge that makes one object attract or repel another electrically.
There are two kinds of charge, named by Benjamin Franklin:
A neutral atom has equal numbers of protons and electrons, so its net charge is zero. A body becomes charged only when this balance is disturbed by the transfer of electrons (protons, being locked in the nucleus, are not transferred in ordinary processes):
The SI unit of charge is the coulomb (C).
The charge carriers are the mobile charged particles that actually move to transport charge:
Like charges repel, unlike charges attract. Two positive or two negative charges push each other apart, while a positive and a negative charge pull together.
Charge is quantised. Charge always occurs in whole-number multiples of a basic unit, the electronic charge $e$: $$q = ne, \qquad n = \pm 1, \pm 2, \pm 3, \dots$$ where $e = 1.6 \times 10^{-19}\ \text{C}$ is the magnitude of the charge on one electron (or proton). No isolated charge smaller than $e$ is ever observed in ordinary matter.
Charge is conserved. The total charge of an isolated system remains constant. Charge is never created or destroyed, only transferred from one body to another, so in every process the algebraic sum of charges before equals that after.
Charge is a scalar quantity. It has magnitude and sign but no direction (although the force it produces is a vector).
Charge is invariant. The magnitude of the charge on a body does not depend on its speed; it is the same whether the body is at rest or moving.
Charge is additive. The total charge on a body is the algebraic sum of all the individual charges on it, adding positive and negative charges with their signs.
Charging by friction is the production of equal and opposite charges on two different bodies when they are rubbed together. It is the oldest known way of producing charge (the ancient Greeks noticed that amber rubbed with fur attracted light objects).
Mechanism. When two suitable materials are rubbed together, the intimate contact allows electrons to transfer from one surface to the other. The material with the weaker hold on its electrons loses them and becomes positively charged; the material that grabs those electrons becomes negatively charged. Because electrons are only transferred and not destroyed, the two bodies always acquire equal and opposite charges, in keeping with conservation of charge.
Standard examples:
Materials can be arranged in a triboelectric series in the order in which they tend to give up or gain electrons; a material higher in the series charges positive when rubbed against one lower down.
Electrostatics is the branch of physics that deals with electric charges at rest, that is, with the forces, fields, and potentials produced by stationary charges. Almost every electrical effect we meet, from the crackle of a woollen sweater to the working of a capacitor, begins with the simple idea of electric charge.
This chapter introduces electric charge and its carriers, explains how bodies become charged by friction (and, as related standard scope, by induction), shows why the charge on a spherical conductor can be treated as though it sits as a point charge at the centre, states Coulomb's law for the force between two point charges, and applies the principle of superposition to find the net force on a charge due to several other charges.
Electric charge is the fundamental physical property of matter that causes it to experience a force when placed in an electric or magnetic field. It is charge that makes one object attract or repel another electrically.
There are two kinds of charge, named by Benjamin Franklin:
A neutral atom has equal numbers of protons and electrons, so its net charge is zero. A body becomes charged only when this balance is disturbed by the transfer of electrons (protons, being locked in the nucleus, are not transferred in ordinary processes):
The SI unit of charge is the coulomb (C).
The charge carriers are the mobile charged particles that actually move to transport charge:
Like charges repel, unlike charges attract. Two positive or two negative charges push each other apart, while a positive and a negative charge pull together.
Charge is quantised. Charge always occurs in whole-number multiples of a basic unit, the electronic charge : where is the magnitude of the charge on one electron (or proton). No isolated charge smaller than is ever observed in ordinary matter.
Charge is conserved. The total charge of an isolated system remains constant. Charge is never created or destroyed, only transferred from one body to another, so in every process the algebraic sum of charges before equals that after.
Charge is a scalar quantity. It has magnitude and sign but no direction (although the force it produces is a vector).
Charge is invariant. The magnitude of the charge on a body does not depend on its speed; it is the same whether the body is at rest or moving.
Charge is additive. The total charge on a body is the algebraic sum of all the individual charges on it, adding positive and negative charges with their signs.
Charging by friction is the production of equal and opposite charges on two different bodies when they are rubbed together. It is the oldest known way of producing charge (the ancient Greeks noticed that amber rubbed with fur attracted light objects).
Mechanism. When two suitable materials are rubbed together, the intimate contact allows electrons to transfer from one surface to the other. The material with the weaker hold on its electrons loses them and becomes positively charged; the material that grabs those electrons becomes negatively charged. Because electrons are only transferred and not destroyed, the two bodies always acquire equal and opposite charges, in keeping with conservation of charge.
Standard examples:
Materials can be arranged in a triboelectric series in the order in which they tend to give up or gain electrons; a material higher in the series charges positive when rubbed against one lower down.