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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 a charge of −1/3 or +2/3. The antiparticle equivalents of these have the opposite charge. There are other charged particles.
Q is a measurement of the charge held by an object. The SI unit of electric charge 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.
It was noticed that electricity seemed to come in two varieties, that cancel each other, and this was expressed in terms of a two-fluid theory. When glass is rubbed with silk, the glass was said to be charged with vitreous electricity, and when amber was rubbed with fur, the amber was said to be charged with resinous electricity.
By the 18th century, the study of electricity had become popular. One of the foremost experts was Benjamin Franklin, who argued in favor of a one-fluid theory of electricity. Franklin imagined electricity as being a type of invisible fluid present in all matter, for example he believed that it was the glass in a Leyden jar that held the accumulated charge. 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 vitreous electricity and "negative" with resinous electricity.
We now know that Franklin's model was close, but too simple. Matter is actually composed of several kinds of electrically charged particles, most common are the positively charged proton and the negatively charged electron. Rather than one possible electric current there are many: a flow of electrons, a flow of electron "holes" which act like positive particles, or in electrolytic solutions, a flow of both negative and positive particles called ions moving 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).
SI electricity units
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