   #copyright

Electric charge

2007 Schools Wikipedia Selection. Related subjects: Electricity and
Electronics

   Electromagnetism
   Electricity · Magnetism
           Electrostatics
   Electric charge
   Coulomb's law
   Electric field
   Gauss's law
   Electric potential
   Electric dipole moment
           Magnetostatics
   Ampère's law
   Magnetic field
   Magnetic dipole moment
          Electrodynamics
   Electric current
   Lorentz force law
   Electromotive force
   (EM) Electromagnetic induction
   Faraday-Lenz law
   Displacement current
   Maxwell's equations
   (EMF) Electromagnetic field
   (EM) Electromagnetic radiation
         Electrical Network
   Electrical conduction
   Electrical resistance
   Capacitance
   Inductance
   Impedance
   Resonant cavities
   Waveguides
                       Flavour in particle physics
   Flavour quantum numbers:
     * Lepton number: L
     * Baryon number: B
     * Strangeness: S
     * Charm: C
     * Bottomness: B'
     * Topness: T
     * Isospin: I[z] or I
     * Weak isospin: T[z]
     * Electric charge: Q
     __________________________________________________________________

   Combinations:
     * Hypercharge: Y
          + Y=2(Q-I[z])
          + Y=B+S+C+B'+T
     * Weak hypercharge: Y[W]
          + Y[W]=2(Q-T[z])
          + Y[W]= B−L
     __________________________________________________________________

   Related topics:
     * CPT symmetry
     * CKM matrix
     * CP symmetry
     * Chirality

   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, and is
   quantized when expressed as a multiple of the so-called elementary
   charge e. 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 have the opposite
   charge. There are other charged particles.

   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 the magnitude of the
   repelling force is proportional to the product of the two charges, and
   weakens proportionately to the square of the distance.

   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, since naturally the number of electrons in every atom is equal to
   the number of the protons, so their charges cancel out. Situations in
   which the net charge is non-zero are often referred to as static
   electricity. Furthermore, even when the net charge is zero, it can be
   distributed non-uniformly (e.g., due to an external electric field),
   and then the material is said to be polarized, and the charge related
   to the polarization is known as bound charge (while the excess charge
   brought from outside is called free charge). An ordered motion of
   charged particles in a particular direction (in metals, these are the
   electrons) is known as electric current. The discrete nature of
   electric charge was proposed by Michael Faraday in his electrolysis
   experiments, then directly demonstrated by Robert Millikan in his
   oil-drop experiment.

   The SI unit for quantity of electricity or electric charge is the
   coulomb, which represents approximately 6.24 × 10^18 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 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.

   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.

History

   Coulomb's torsion balance
   Coulomb's torsion balance

   As reported by the Ancient Greek philosopher Thales of Miletus around
   600 BC, charge (or electricity) could be accumulated by rubbing fur on
   various substances, such as amber. The Greeks noted that the charged
   amber buttons could attract light objects such as hair. They 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.

   In 1600 the English scientist William Gilbert returned to the subject
   in De Magnete, and coined the New Latin word electricus from ηλεκτρον
   (elektron), the Greek word for "amber", which soon gave rise to the
   English words "electric" and "electricity." He was followed in 1660 by
   Otto von Guericke, who invented what was probably the first
   electrostatic generator. Other European pioneers were Robert Boyle, who
   in 1675 stated that electric attraction and repulsion can act across a
   vacuum; Stephen Gray, who in 1729 classified materials as conductors
   and insulators; and C. F. du Fay, who proposed in 1733 that electricity
   came in two varieties which cancelled each other, and expressed this in
   terms of a two-fluid theory. When glass was rubbed with silk, du Fay
   said that the glass was charged with vitreous electricity, and when
   amber was rubbed with fur, the amber was said to be charged with
   resinous electricity. In 1839 Michael Faraday showed that the apparent
   division between static electricity, current electricity and
   bioelectricity was incorrect, and all were a consequence of the
   behaviour of a single kind of electricity appearing in opposite
   polarities.

   One of the foremost experts on electricity in the 18th century was
   Benjamin Franklin, who argued in favour 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. William Watson arrived at the
   same explanation at about the same time.

   We now know that the Franklin/Watson model was close, but too simple.
   Matter is actually composed of several kinds of electricity (several
   kinds of electrically charged particles,) the most common being 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 simplifies
   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. The true direction of
   electric current is a source of confusion for beginners in electronics,
   and a minority of educators ignore the standard and instead assume that
   moving charges are exclusively negative.

Properties

   Aside from the properties described in articles about electromagnetism,
   charge is a relativistic invariant. This means 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 high 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).

Conservation of charge

   The total electric charge of an isolated system remains constant
   regardless of changes within the system itself. This law is inherent to
   all processes known to physics and can be derived in a local form from
   gauge invariance of the wave function. The conservation of charge
   results in the charge-current continuity equation. More generally, the
   net change in charge density ρ within a volume of integration V is
   equal to the area integral over the current density J on the surface of
   the area S, which is in turn equal to the net current I:

          - \frac{\partial}{\partial t} \int_V \rho\, \mathrm{d}V = \int_S
          \mathbf{J} \cdot \mathrm{d}\mathbf{S} = \int J S \cos\theta = I

   Thus, the conservation of electric charge, as expressed by the
   continuity equation, gives the result:

          I = -\frac{dQ}{dt}

   where I is the net outward current through a closed surface and Q is
   the electric charge contained within the volume defined by the surface.

   Retrieved from " http://en.wikipedia.org/wiki/Electric_charge"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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