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 the
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 socalled 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, samesign charged
particles repel one another, while differentsign 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 nonzero
are often referred to as static electricity. Furthermore, even when the net
charge is zero, it can be distributed nonuniformly (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 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 crosssection 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. The discrete nature of electric
charge was proposed by Michael Faraday in his electrolysis experiments,
then directly demonstrated by Robert Millikan in his oildrop experiment.
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.
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