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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Electric charge | 5/5 | https://en.wikipedia.org/wiki/Electric_charge | reference | science, encyclopedia | 2026-05-05T10:52:17.037385+00:00 | kb-cron |
== 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 sliding ===
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 sliding. 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.
== Role of charge in electric current == Electric current is the flow of electric charge through an object. 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. The direction of the conventional current in most metallic wires is opposite to the drift velocity of the actual charge carriers; i.e., the electrons.
== 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, electric charge is a relativistic invariant. This means that any particle that has electric charge q has the same electric charge regardless of how fast it is travelling. This property has been experimentally verified by showing that the electric 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 that of 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 Positron or antielectron is an antiparticle or antimatter counterpart of the electron
== References ==
== External links == Media related to Electric charge at Wikimedia Commons How fast does a charge decay?