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Electric current
Electric current is the flow (movement) of electric charge. The SI unit of
electric current is the Ampere (A), which is equal to a flow of one Coulomb of
charge per second. Electric current is measured using an ammeter.
In solid conductive metal, a large population of electrons is mobile or free
electrons. These electrons are bound to the metal lattice but not to any individual
atom. Even without an external electric field applied, these electrons move about
randomly due to thermal energy but on average, there is zero net current within the metal. Given an imaginary plane through which the wire passes, the number of electrons moving from one side to the other in any period of time is exactly equal to the number passing in the opposite direction.
A typical metal wire for electrical conduction is the stranded copper wire.
When a metal wire is connected across the two terminals of a IX? voltage
source such as a battery, the source pieces an electric field across the conductor.
The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field. The free electron is therefore the current carrier in a typical solid conductor.
We know that electrical signals are electromagnetic waves which propagate
at very high speed outside the surface of the conductor (moving at the speed of
light, as can be deduced from Maxwell's Equations). For example, in AC power
lines, the waves of electromagnetic energy propagate through the space between
the wires which is usually filled with insulating material, moving from a source to
a distant load, even though the electrons in the wires only move back and forth
over a tiny distance. The velocity of the flowing charges is quite low. The
associated electromagnetic energy travels at a speed which is much faster. The
velocity factor is a measure of the speed of electromagnetic propagation compared to the speed of light in a vacuum. The velocity factor is affected by the nature of the insulating medium surrounding the conductor, and also the magnetic properties of the materials of the conductor and its surroundings.
The nature of these three velocities can be clarified by analogy with the three
similar velocities associated with gases. The low drift velocity of charge carriers is
analogous to air motions; to wind. The large signal velocity is roughly analogous
to the rapid propagation of sound waves, while the large random motion of charges is analogous to heat; to the high thermal velocity of randomly vibrating gas panicles.
Conventional current was defined early in the history of electrical science as
a flow of positive charge. In solid metals, like wires, the positive charge carriers
are immobile, and only the negatively charged electrons flow. Because the electron carries negative charge, the electron current is in the direction opposite to that of conventional (or electric) current.
Diagram showing conventional current notation. Electric charge moves from the
positive side of the power source to the negative.
In other conductive materials, the electric current is due to the flow of
charged particles in both directions at the same time. Electric currents in
electrolytes arc flows of electrically charged atoms (ions), which exist in both
positive and negative varieties. For example, an electrochemical cell may be
constructed with salt water (a solution of sodium chloride) on one side of a
membrane and pure water on the other. The membrane lets the positive sodium
ions pass, but not the negative chloride ions, so a net current results. Electric
currents in plasma arc flows of electrons as well as positive and negative ions. In
ice and in certain solid electrolytes, flowing protons constitute the electric current. To simplify this situation, the original definition of conventional current still stands.
There are also materials where the electric current is due to the flow of
electrons and yet it is conceptually easier to think of the current as due to the flow
of positive "holes" (the spots that should have an electron to make the conductor
neutral). This is the case in a p-type semiconductor.
Natural examples include lightning and the solar wind, the source of the
polar auroras (the aurora borealis and aurora australis). The artificial form of
electric current is the flow of conduction electrons in metal wires, such as the
overhead power lines that deliver electrical energy across long distances and the
smaller wires within electrical and electronic equipment. In electronics, other
forms of electric current include the flow of electrons through resistors or through
the vacuum in a vacuum tube, the flow of ions inside a battery, and the flow of
holes within a semiconductor.