Electric charge

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Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields. The interaction between a moving charge and an electromagnetic field is the source of the electromagnetic force, which is one of the four fundamental forces.
Electric charge is a characteristic of some subatomic particles. It is quantized in that, when expressed in units of the so-called elementary charge e, it takes integer or fractional values. Electrons by convention have a charge of −1, while protons have the opposite charge of +1. Quarks have a fractional charge of −1⁄3 or +2⁄3. The antiparticle equivalents of these (positrons, antiprotons, and antiquarks, respectively) have the opposite charge. There are other charged particles. The discrete nature of electric charge was proposed by Michael Faraday in his electrolysis experiments, and then directly demonstrated by Robert Millikan in his oil-drop experiment.
In general, same-sign charged particles repel one another, while different-sign charged particles attract. This is expressed quantitatively in Coulomb's law, which states that the magnitude of the electrostatic repelling force between two particles is proportional to the product of their charges and the inverse square of the distance between them.
The electric charge of a macroscopic object is the sum of the electric charges of its constituent particles. Often, the net electric charge is zero, because it is favorable for the number of electrons in every atom to equal the number of protons (or, more generally, for the number of anions, or negatively charged atoms, in every molecule to equal the number of cations, or positively charged atoms). When the net electric charge is non-zero and motionless, one has the phenomenon known as static electricity. Even when the net charge is zero, it can be distributed non-uniformly (e.g., due to an external electric field, or due to molecular motion), in which case the material is said to be polarized. The charge due to the polarization is known as bound charge, while the excess charge brought from outside is called free charge. The motion of charged particles (e.g., of electrons in metals) in a particular direction is known as electric current.

Units


The SI unit of quantity of electric charge is the coulomb, which is equivalent to about 6.25 × 1018 e (the charge on a single electron or proton). Hence, the charge of an electron is approximately −1.602 x 10−19 C. The coulomb is defined as the quantity of charge that has passed through the cross-section of an electrical conductor carrying one ampere within one second. The symbol Q is often used to denote a quantity of electricity or charge. The quantity of electric charge can be directly measured with an electrometer, or indirectly measured with a ballistic galvanometer.
After finding the quantized character of charge, in 1891 Stoney proposed the unit 'electron' for this fundamental unit of electrical charge. This was before the discovery of the particle by J.J. Thomson in 1897. Today, the name "electron" for the unit of charge is no longer widely used except in the derived unit "electronvolt". This is quite surprising considering the wide use of this unit in the fields of physics and chemistry. The unit is today treated as nameless, refered to as "fundamental unit of charge" or simply as "e".
Formally, a measure of charge should be a multiple of the elementary charge e (charge is quantized), 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. Furthermore, in some contexts it is meaningful to speak of fractions of a charge; e.g. in the charging of a capacitor.