Electric charge is the physical property of
matter that causes it to experience a force
when placed in an electromagnetic field. There
are two types of electric charges; positive
and negative (commonly carried by protons
and electrons respectively). Like charges
repel and unlike attract. An object with an
absence of net charge is referred to as neutral.
Early knowledge of how charged substances
interact is now called classical electrodynamics,
and is still accurate for problems that do
not require consideration of quantum effects.
Electric charge is a conserved property; the
net charge of an isolated system, the amount
of positive charge minus the amount of negative
charge, cannot change. Electric charge is
carried by subatomic particles. In ordinary
matter, negative charge is carried by electrons,
and positive charge is carried by the protons
in the nuclei of atoms. If there are more
electrons than protons in a piece of matter,
it will have a negative charge, if there are
fewer it will have a positive charge, and
if there are equal numbers it will be neutral.
Charge is quantized; it comes in integer multiples
of individual small units called the elementary
charge, e, about 1.602×10−19 coulombs,
which is the smallest charge which can exist
free (particles called quarks have smaller
charges, multiples of 1/3e, but they are only
found in combination, and always combine to
form particles with integer charge). The proton
has a charge of +e, and the electron has a
charge of −e.
Electric charges create an electric field,
if they are moving they also generate a magnetic
field. The combination of the electric and
magnetic field is called the electromagnetic
field, and its interaction with charges is
the source of the electromagnetic force, which
is one of the four fundamental forces in physics.
The study of charged particles, and how their
interactions are mediated by photons, is called
quantum electrodynamics.
The SI derived unit of electric charge is
the coulomb (C) named after French physicist
Charles-Augustin de Coulomb. In electrical
engineering, it is also common to use the
ampere-hour (Ah); in physics and chemistry,
it is common to use the elementary charge
(e as a unit). Chemistry also uses the Faraday
constant as the charge on a mole of electrons.
The symbol Q often denotes charge.
== Overview ==
Charge is the fundamental property of forms
of matter that exhibit electrostatic attraction
or repulsion in the presence of other matter.
Electric charge is a characteristic property
of many subatomic particles. The charges of
free-standing particles are integer multiples
of the elementary charge e; we say that electric
charge is quantized. Michael Faraday, in his
electrolysis experiments, was the first to
note the discrete nature of electric charge.
Robert Millikan's oil drop experiment demonstrated
this fact directly, and measured the elementary
charge. It has been discovered that one type
of particle, quarks, have fractional charges
of either −1/3 or +2/3, but it is believed
they always occur in multiples of integral
charge; free-standing quarks have never been
observed.
By convention, the charge of an electron is
negative, −e, while that of a proton is
positive, +e. Charged particles whose charges
have the same sign repel one another, and
particles whose charges have different signs
attract. Coulomb's law quantifies the electrostatic
force between two particles by asserting that
the force is proportional to the product of
their charges, and inversely proportional
to the square of the distance between them.
The charge of an antiparticle equals that
of the corresponding particle, but with opposite
sign.
The electric charge of a macroscopic object
is the sum of the electric charges of the
particles that make it up. This charge is
often small, because matter is made of atoms,
and atoms typically have equal numbers of
protons and electrons, in which case their
charges cancel out, yielding a net charge
of zero, thus making the atom neutral.
An ion is an atom (or group of atoms) that
has lost one or more electrons, giving it
a net positive charge (cation), or that has
gained one or more electrons, giving it a
net negative charge (anion). Monatomic ions
are formed from single atoms, while polyatomic
ions are formed from two or more atoms that
have been bonded together, in each case yielding
an ion with a positive or negative net charge.
During formation of macroscopic objects, constituent
atoms and ions usually combine to form structures
composed of neutral ionic compounds electrically
bound to neutral atoms. Thus macroscopic objects
tend toward being neutral overall, but macroscopic
objects are rarely perfectly net neutral.
Sometimes macroscopic objects contain ions
distributed throughout the material, rigidly
bound in place, giving an overall net positive
or negative charge to the object. Also, macroscopic
objects made of conductive elements, can more
or less easily (depending on the element)
take on or give off electrons, and then maintain
a net negative or positive charge indefinitely.
When the net electric charge of an object
is non-zero and motionless, the phenomenon
is known as static electricity. This can easily
be produced by rubbing two dissimilar materials
together, such as rubbing amber with fur or
glass with silk. In this way non-conductive
materials can be charged to a significant
degree, either positively or negatively. Charge
taken from one material is moved to the other
material, leaving an opposite charge of the
same magnitude behind. The law of conservation
of charge always applies, giving the object
from which a negative charge is taken a positive
charge of the same magnitude, and vice versa.
Even when an object's net charge is zero,
charge can be distributed non-uniformly in
the object (e.g., due to an external electromagnetic
field, or bound polar molecules). In such
cases the object is said to be polarized.
The charge due to polarization is known as
bound charge, while charge on an object produced
by electrons gained or lost from outside the
object is called free charge. The motion of
electrons in conductive metals in a specific
direction is known as electric current.
== Units ==
The SI derived unit of quantity of electric
charge is the coulomb (symbol: C). The coulomb
is defined as the quantity of charge that
passes through the cross section of an electrical
conductor carrying one ampere for one second.
This unit was proposed in 1946 and ratified
in 1948. In modern practice, the phrase "amount
of charge" is used instead of "quantity of
charge". The amount of charge in 1 electron
(elementary charge) is approximately 1.6×10−19
C, and 1 coulomb corresponds to the amount
of charge for about 6.24×1018 electrons.
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 George 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. The unit is today
treated as nameless, referred to as elementary
charge, fundamental unit of charge, or simply
as e. A measure of charge should be a multiple
of the elementary charge e, even if at large
scales charge seems to behave as a real quantity.
In some contexts it is meaningful to speak
of fractions of a charge; for example in the
charging of a capacitor, or in the fractional
quantum Hall effect.
The unit faraday is sometimes used in electrochemistry.
One faraday of charge is the magnitude of
the charge of one mole of electrons, i.e.
96485.33289(59) C.
In systems of units other than SI such as
cgs, electric charge is expressed as combination
of only three fundamental quantities (length,
mass, and time), and not four, as in SI, where
electric charge is a combination of length,
mass, time, and electric current.
== History ==
From ancient times, persons were familiar
with four types of phenomena that today would
all be explained using the concept of electric
charge: (a) lightning, (b) the torpedo fish
(or electric ray), (c) St Elmo's Fire, and
(d) that amber rubbed with fur would attract
small, light objects. The first account of
the amber effect is often attributed to the
ancient Greek mathematician Thales of Miletus,
who lived from c. 624 – c. 546 BC, but there
are doubts about whether Thales left any writings;
his account about amber is known from an account
from early 200s. This account can be taken
as evidence that the phenomenon was known
since at least c. 600 BC, but Thales explained
this phenomenon as evidence for inanimate
objects having a soul. In other words, there
was no indication of any conception of electric
charge. More generally, the ancient Greeks
did not understand the connections among these
four kinds of phenomena. The Greeks observed
that the charged amber buttons could attract
light objects such as hair. They also found
that if they rubbed the amber for long enough,
they could even get an electric spark to jump,
but there is also a claim that no mention
of electric sparks appeared until late 17th
century. This property derives from the triboelectric
effect.
In late 1100s, the substance jet, a compacted
form of coal, was noted to have an amber effect,
and in the middle of the 1500s, Girolamo Fracastoro,
discovered that diamond also showed this effect.
Some efforts were made by Fracastoro and others,
especially Gerolamo Cardano to develop explanations
for this phenomenon.In contrast to astronomy,
mechanics, and optics, which had been studied
quantitatively since antiquity, the start
of ongoing qualitative and quantitative research
into electrical phenomena can be marked with
the publication of De Magnete by the English
scientist William Gilbert in 1600. In this
book, there was a small section where Gilbert
returned to the amber effect (as he called
it) in addressing many of the earlier theories,
and coined the New Latin word electrica (from
ἤλεκτρον (ēlektron), the Greek word
for amber). The Latin word was translated
into English as electrics. Gilbert is also
credited with the term electrical, while the
term electricity came later, first attributed
to Sir Thomas Browne in his Pseudodoxia Epidemica
from 1646. (For more linguistic details see
Etymology of electricity.) Gilbert 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. In 1733 Charles
François de Cisternay du Fay, inspired by
Gray's work, made a series of experiments
(reported in Mémoires de l'Académie Royale
des Sciences), showing that more or less all
substances could be 'electrified' by rubbing,
except for metals and fluids and proposed
that electricity comes in two varieties that
cancel each other, which he expressed 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 charged
with resinous electricity. Another important
two-fluid theory from this time was proposed
by Jean-Antoine Nollet (1745). 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 behavior of a single
kind of electricity appearing in opposite
polarities. It is arbitrary which polarity
is called positive and which is called negative.
Positive charge can be defined as the charge
left on a glass rod after being rubbed with
silk.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. For a reason that was not recorded,
he identified the term positive with vitreous
electricity and negative with resinous electricity.
William Watson independently arrived at the
same explanation at about the same time (1746).
It is now known that the Franklin–Watson
model was fundamentally correct. There is
only one kind of electrical charge, and only
one variable is required to keep track of
the amount of charge. On the other hand, just
knowing the charge is not a complete description
of the situation. Matter is composed of several
kinds of electrically charged particles, and
these particles have many properties, not
just charge.
== The role of charge in electrostatics ==
All bodies are electrified, but may appear
not electrified because of the relatively
similar charge of neighboring objects in the
environment. An object further electrified
+ or – creates an equivalent or opposite
charge by default in neighboring objects,
until those charges can equalize. The effects
of attraction can be observed in high-voltage
experiments, while lower voltage effects are
merely weaker and therefore less obvious.
Coulomb's law has a corollary for acceleration
in a gravitational field. See also Casimir
effect.
== The role of charge in static electricity
==
Static electricity refers to the electric
charge of an object and the related electrostatic
discharge when two objects are brought together
that are not at equilibrium. An electrostatic
discharge creates a change in the charge of
each of the two objects.
=== Electrification by friction ===
When a piece of glass and a piece of resin—neither
of which exhibit any electrical properties—are
rubbed together and left with the rubbed surfaces
in contact, they still exhibit no electrical
properties. When separated, they attract each
other.
A second piece of glass rubbed with a second
piece of resin, then separated and suspended
near the former pieces of glass and resin
causes these phenomena:
The two pieces of glass repel each other.
Each piece of glass attracts each piece of
resin.
The two pieces of resin repel each other.This
attraction and repulsion is an electrical
phenomenon, and the bodies that exhibit them
are said to be electrified, or electrically
charged. Bodies may be electrified in many
other ways, as well as by friction. The electrical
properties of the two pieces of glass are
similar to each other but opposite to those
of the two pieces of resin: The glass attracts
what the resin repels and repels what the
resin attracts.
If a body electrified in any manner whatsoever
behaves as the glass does, that is, if it
repels the glass and attracts the resin, the
body is said to be vitreously electrified,
and if it attracts the glass and repels the
resin it is said to be resinously electrified.
All electrified bodies are either vitreously
or resinously electrified.
An established convention in the scientific
community defines vitreous electrification
as positive, and resinous electrification
as negative. The exactly opposite properties
of the two kinds of electrification justify
our indicating them by opposite signs, but
the application of the positive sign to one
rather than to the other kind must be considered
as a matter of arbitrary convention—just
as it is a matter of convention in mathematical
diagram to reckon positive distances towards
the right hand.
No force, either of attraction or of repulsion,
can be observed between an electrified body
and a body not electrified.
== The role of charge in electric current
==
Electric current is the flow of electric charge
through an object, which produces no net loss
or gain of electric charge. The most common
charge carriers are the positively charged
proton and the negatively charged electron.
The movement of any of these charged particles
constitutes an electric current. In many situations,
it suffices to speak of the conventional current
without regard to whether it is carried by
positive charges moving in the direction of
the conventional current or by negative charges
moving in the opposite direction. This macroscopic
viewpoint is an approximation that simplifies
electromagnetic concepts and calculations.
At the opposite extreme, if one looks at the
microscopic situation, one sees there are
many ways of carrying an electric current,
including: a flow of electrons; a flow of
electron holes that act like positive particles;
and both negative and positive particles (ions
or other charged particles) flowing in opposite
directions in an electrolytic solution or
a plasma.
Beware that, in the common and important case
of metallic wires, the direction of the conventional
current is opposite to the drift velocity
of the actual charge carriers; i.e., the electrons.
This is a source of confusion for beginners.
== Conservation of electric 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 rate of change
in charge density ρ within a volume of integration
V is equal to the area integral over the current
density J through the closed surface S = ∂V,
which is in turn equal to the net current
I:
−
d
d
t
∫
V
ρ
d
V
=
{\displaystyle -{\frac {d}{dt}}\int _{V}\rho
\,\mathrm {d} V=}
∂
V
{\displaystyle \scriptstyle \partial V}
J
⋅
d
S
=
∫
J
d
S
cos
⁡
θ
=
I
.
{\displaystyle \mathbf {J} \cdot \mathrm {d}
\mathbf {S} =\int J\mathrm {d} S\cos \theta
=I.}
Thus, the conservation of electric charge,
as expressed by the continuity equation, gives
the result:
I
=
−
d
Q
d
t
.
{\displaystyle I=-{\frac {\mathrm {d} Q}{\mathrm
{d} t}}.}
The charge transferred between times
t
i
{\displaystyle t_{\mathrm {i} }}
and
t
f
{\displaystyle t_{\mathrm {f} }}
is obtained by integrating both sides:
Q
=
∫
t
i
t
f
I
d
t
{\displaystyle Q=\int _{t_{\mathrm {i} }}^{t_{\mathrm
{f} }}I\,\mathrm {d} t}
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.
== Relativistic invariance ==
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).
== See also ==
SI electromagnetism units
Color charge
Partial charge
