Charge
Glossary Definition
for 16-19
Description
Electric charge is an intrinsic property of matter carried by some fundamental particles. Charge can take a positive or negative value.
Charge is usually represented by the symbol q or Q.
The magnitude of the charge carried by a single electron or proton is usually considered to be the basic natural unit of charge and is given the symbol e.
Discussion
Often but not always, q is used for the charge of a single particle while Q is used for the overall charge of a larger object.
Both static electricity (an imbalance of charge that does not readily move) and current electricity can be explained, on an atomic scale, in terms of charged particles – usually electrons. When objects are charged by frictional contact, there is always a transfer of electrons. For example, if you charge a balloon by rubbing it against your sweater, electrons move from the sweater (to which they are less strongly bound) to the balloon, giving the balloon an overall negative charge and your sweater an equal amount of positive charge. In a plasma (ionised gas) and in an electrolyte (solution containing ions) electric current involves the motion of both positive and negative ions.
All charged particles experience a force in an electric field; all moving charged particles experience a force in a magnetic field. The size of a particle’s charge influences the size of force that it experiences in both these fields. The sign of its charge influences the direction of the force.
Charge is always conserved; it can neither be created nor destroyed. The charge carried by an object is the algebraic sum of all of its constituent positive and negative charges. An object with no overall positive or negative charge is said to be electrically neutral. In any closed system, the total amount of charge remains constant. In any chemical or nuclear reaction, and any reaction involving fundamental particles, the algebraic sum of the charges remains constant.
Observations of electric charge date back to the ancient Greeks, who noticed that if amber (‘elektron’ in Greek) is rubbed it can attract small objects. In the 17th century, objects such as rubbed amber were said to be ‘charged’ (filled) with ‘electricity’; experiments showed that two objects charged in the same way (e.g. amber rubbed with fur) always repel each other, but objects charged in different ways can attract each other (e.g. amber rubbed with fur attracts glass rubbed with silk, but repels ebonite (a type of hard rubber) rubbed with wool. It was found that charged objects can be classified into just two types – amber rubbed with fur and ebonite rubbed with wool, both have the same type of charge, which historically, came to be known as negative, while glass rubbed with silk has what is now known as a positive charge.
When the electron was discovered in the late 19th century, it was found to have negative charge.
SI unit
coulomb, C
Expressed in SI base units
A s
Mathematical expressions
F= qE
where Fis the force exerted on a particle of charge q in an electric field E.
F = qvB sinθ
where F is the magnitude of the force exerted on a particle of charge q travelling at speed v in a magnetic field of magnitude B and θ is the angle between the magnetic field and the particle’s velocity.
Q = It
where Q is the total charge that passes a point in a circuit in time t and I is the current.
Related entries
- Electric current
- Electric field
- Magnetic field
- Magnetic flux
- Electrostatic potential
In context
The charge carried by a proton is e = 1.60 × 10–19 C. An electron carries charge –e = –1.60 × 10–19 C.
An electron is a fundamental particle – it is not composed of anything else. A proton is composed of two up-quarks (u) each with a charge +2e/3, and a down-quark (d) with charge –e/3. Quarks are fundamental particles. A neutron is composed of two d-quarks and one u-quark, giving it an overall charge of zero.
A coulomb of positive (or negative) charge is equivalent to the charge carried by 6.25 × 1018 protons (or electrons). The charge on the dome of a van de Graff generator used in bench-top demonstrations is of the order ~ 10–6 C. In a single cloud-to-ground lightning strike, the charge transferred is typically 5–350 C.
