Everything about Electrical Charge totally explained
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.
Overview
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 (for example, 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.
Units
The
SI unit of
quantity of electricity or electric charge is the
coulomb, which is equivalent to about 6.24 × 10
18 elementary charges (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.
Formally, a measure of charge should be a multiple of the elementary charge
e (charge is
quantized), but since it's 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's meaningful to speak of fractions of a charge; for example in the charging of a
capacitor.
History
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 (External Link
) 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 behavior of a single kind of electricity appearing in opposite polarities. It is arbitrary which polarity you call positive and which you call 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. Arbitrarily (or for a reason that wasn't 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 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 isn't 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 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's carried by positive charges moving in the direction of the conventional current and/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" which 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, for example the electrons. This is a source of confusion for beginners.
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'd 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 within a volume of integration
is equal to the area integral over the
current density on the surface of the area
, which is in turn equal to the net
current :
»
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.
Further Information
Get more info on 'Electrical Charge'.
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