|
Electric charge is a fundamental property of some subatomic particles, which determines their electromagnetic
interactions. It is one of the quantum numbers. Matter that possesses a
charge is influenced by, and produces, electromagnetic fields. The
interaction between charge and an electromagnetic field is the source of one of the four fundamental forces, the electromagnetic force.
Electrons have a charge, by convention, of -1. Protons similarly have a charge of +1. Quarks have charge of ±1/3 or ±2/3.
The antiparticle equivalents of these have the opposite charge. There are
other charged particles.
In quantum mechanics, most physicists believe that hadrons contain quarks which have charges which are
multiples of one-third the elementary charge, but cannot be observed except in combinations which have charge that is a multiple
of the elementary charge.
Q is a measurement of the charge held by an object. The unit of measurement is the coulomb, which represents approximately 6.24 x 1018 elementary charges (the charge on a single electron or proton). The coulomb is defined as the quantity of charge that has passed through the cross-section of a conductor carrying one ampere within one second. (see Ampere)
Electric charge can be directly measured with an electrometer. The
discrete nature of electric charge was demonstrated by Robert
Millikan in his oil-drop experiment.
Formally, a measure of charge should be a multiple of the elementary charge e, but since it is an average, macroscopic quantity, many orders of magnitude larger than a single elementary charge, it can effectively take
on any real value.
History
Charge was discovered by the Ancient Greeks who found that rubbing
fur on various substances, such as amber, would build
up an electric charge imbalance. The Greeks noted that the charged amber buttons could attract light objects such as hair. The Greeks also noted that if they rubbed the amber for long enough, they could even get a
spark to jump. This property derives from the triboelectric
effect. The word electricity derives from ηλεκτρον, the Greek word
for amber.
By the 18th century, the study of electricity had become popular. One of the foremost experts was a man named Benjamin Franklin. Franklin imagined electricity as being a type of invisible fluid present in all
matter. He posited that rubbing insulating surfaces together caused this fluid to change location, and that a flow of this fluid
constitutes an electric current. He also posited that when matter contained too little of the fluid it was "negatively" charged,
and when it had an excess it was "positively" charged. Arbitrarily (or for a reason that was not recorded) he identified the term
"positive" with the type of charge acquired by a glass rod rubbed with silk, and "negative"
with that acquired by an amber rod rubbed with fur.
We now know that Franklin's model was close, but too simple. Matter is actually composed of two kinds of electrically charged
particles: particles called protons which carry a charge of positive electricity, and
particles called electrons which carry a charge of negative electricity. Rather
than one possible electric current there are many: a flow of
negative particles, or a flow of positive particles, or a flow of both negative and positive particles in opposite directions. To
reduce this complexity, electrical workers still use Franklin's convention and they imagine that electric current (known as
conventional current) is a flow of exclusively positive particles. The conventional current simplifes electrical
concepts and calculations, but it ignores the fact that within some conductors (electrolytes, semiconductors, and plasma,) two or
more species of electric charges flow in opposite directions. The flow direction for conventional current is also
backwards compared to the actual electron drift taking place during electric currents in metals, the typical conductor of
electricity, which is a source of confusion for beginners in electronics.
Properties
Aside from the properties described in articles about electromagnetism, it is worth noting that charge is a relativistic invariant. What this means is that any particle that has charge q, no matter how fast it goes, always
has charge q. This property has been experimentally verified by showing that the charge of one helium nucleus (two protons and two neutrons bound together in a
nucleus and moving around at incredible speeds) is the same as two deuterium nuclei (one proton and one neutron bound together, but moving much more slowly than they would if they
were in a helium nucleus).
|