An electron is a negatively charged subatomic particle. It can be
either free (not attached to any atom), or bound to the nucleus of
an atom. Electrons in atoms exist in spherical shells of various
radii, representing energy levels. The larger the spherical shell,
the higher the energy contained in the electron.
In electrical conductors, current flow results from the movement
of electrons from atom to atom individually, and from negative to
positive electric poles in general. In semiconductor materials,
current also occurs as a movement of electrons. But in some cases,
it is more illustrative to envision the current as a movement of
electron deficiencies from atom to atom. An electron-deficient atom
in a semiconductor is called a hole. Holes "move" from positive to
negative electric poles in general.
The charge on a single electron is considered as the unit
electrical charge. It is assigned negative polarity. The charge on
an electron is equal, but opposite, to the positive charge on a
proton or hole. Electrical charge quantity is not usually measured
in terms of the charge on a single electron, because this is an
extremely small charge. Instead, the standard unit of electrical
charge quantity is the coulomb, symbolized by C, representing about
6.24 x 1018 electrons. The electron charge, symbolized by
e, is about 1.60 x 10-19 C. The mass of an electron at
rest, symbolized me, is approximately 9.11 x 10-31
kilogram (kg). Electrons moving at an appreciable fraction of the
speed of light, for example in a particle accelerator, have greater
mass because of relativistic effects.
Current is a flow of electrical charge carriers,
usually electrons or electron-deficient atoms. The common symbol for
current is the uppercase letter I. The standard unit is the ampere,
symbolized by A. One ampere of current represents one coulomb of
electrical charge (6.24 x 1018 charge carriers) moving
past a specific point in one second. Physicists consider current to
flow from relatively positive points to relatively negative points;
this is called conventional current or Franklin current. Electrons,
the most common charge carriers, are negatively charged. They flow
from relatively negative points to relatively positive points.
Electric current can be either direct
or alternating. Direct current (DC) flows in the same direction at
all points in time, although the instantaneous magnitude of the
current might vary. In an alternating current (AC), the flow of
charge carriers reverses direction periodically. The number of
complete AC cycles per second is the frequency, which is measured in
hertz. An example of pure DC is the current produced by an
electrochemical cell. The output of a power-supply rectifier, prior
to filtering, is an example of pulsating DC. The output of common
utility outlets is AC.
Current per unit cross-sectional area
is known as current density. It is expressed in amperes per
square meter, amperes per square centimeter, or amperes per square
millimeter. Current density can also be expressed in amperes per
circular mil. In general, the greater the current in a conductor,
the higher the current density. However, in some situations, current
density varies in different parts of an electrical conductor. A
classic example is the so-called skin effect, in which
current density is high near the outer surface of a conductor, and
low near the center. This effect occurs with alternating currents at
high frequencies. Another example is the current inside an active
electronic component such as a field-effect transistor (FET).
An electric current always produces a
magnetic field. The stronger the current, the more intense the
magnetic field. A pulsating DC, or an AC, characteristically
electromagnetic field. This is the principle by which
wireless signal propagation occurs.
alternating current (AC) occurs when charge carriers in a conductor
or semiconductor periodically reverse their direction of movement.
Household utility current in most countries is AC with a frequency
of 60 hertz (60 complete cycles per second), although in some
countries it is 50 Hz. The radio-frequency (RF) current in antennas
and transmission lines is another example of AC.
An AC waveform can be
sinusoidal, square, or sawtooth-shaped. Some AC waveforms are
irregular or complicated. An example of sine-wave AC is common
household utility current (in the ideal case). Square or sawtooth
waves are produced by certain types of electronic oscillators, and
by a low-end uninterruptible power supply (UPS) when it is operating
from its battery. Irregular AC waves are produced by audio
amplifiers that deal with analog voice signals and/or music.
The voltage of an AC
power source can be easily changed by means of a power transformer.
This allows the voltage to be stepped up (increased) for
transmission and distribution. High-voltage transmission is more
efficient than low-voltage transmission over long distances, because
the loss caused by conductor resistance decreases as the voltage
The voltage of an AC
power source changes from instant to instant in time. The
effective voltage of an AC utility power source is usually
considered to be the DC voltage that would produce the same power
dissipation as heat assuming a pure resistance. The effective
voltage for a sine wave is not the same as the peak voltage.
To obtain effective voltage from peak voltage, multiply by 0.707. To
obtain peak voltage from effective voltage, multiply by 1.414. For
example, if an AC power source has an effective voltage of 117 V,
typical of a household in the United States, the peak voltage is 165
DC (Direct current) is the unidirectional flow or
movement of electric charge carriers, usually electron. The
intensity of the current can vary with time, but the general
direction of movement stays the same at all times. As an adjective,
the term DC is used in reference to voltage whose polarity never
In a DC circuit, electrons emerge from the negative,
or minus, pole and move towards the positive, or plus, pole.
Nevertheless, physicists define DC as traveling from plus to minus.
Direct current is produced by electrochemical and
photovoltaic cells and batteries. In contrast, the electricity
available from utility mains in most countries is AC (alternating
current). Utility AC can be converted to DC by means of a power
supply consisting of a transformer, a rectifier (which prevents the
flow of current from reversing), and a filter (which eliminates
current pulsations in the output of the rectifier).
Virtually all electronic and computer hardware needs
DC to function. Most solid-state equipment requires between 1.5 and
13.5 volts. Current demands can range from practically zero for an
electronic wristwatch to more than 100 amperes for a radio
communications power amplifier. Equipment using vacuum tubes, such
as a high-power radio or television broadcast transmitter or a CRT
(cathode-ray tube) display, require from about 150 volts to several
thousand volts DC.
An ampere is a unit of measure of the rate of
electron flow or current in an electrical conductor. One ampere of
current represents one coulomb of electrical charge (6.24 x 1018
charge carriers) moving past a specific point in one second.
Physicists consider current to flow from relatively positive points
to relatively negative points; this is called conventional current
or Franklin current. The ampere is named after Andre Marie Ampere,
French physicist (1775-1836).
The abampere (symbolized abA) is the unit of current
in the cgs (centimeter/gram/second) system of electromagnetic units.
It is the equivalent of one abcoulomb (1 abC) of charge carriers
moving past a specific point in one second.
The abampere is a moderately large unit of
current, equivalent to 10 amperes (A). In most applications, the
ampere, which is the unit of current in the International System of
Units (SI), is preferred.
An ampere hour (abbreviated Ah, or sometimes amp
hour) is the amount of energy charge in a battery that will allow
one ampere of current to flow for one hour. A milliampere hour
(mAh) is 1,000th of an Ah, and is commonly used as a measure of
charge in portable computer batteries. The mAh provides an
indication of how long the PC will operate on its battery without
having to recharge it.
ampere per meter
The ampere per meter (symbolized A/m) is the
International Unit of magnetic field strength. It is derived from
basic standard units, but is expressed directly in base units and
cannot be further reduced.
Consider the interior of a long, cylindrical coil
with a single winding and an air core. Suppose that the linear
current density in this coil is 1 ampere per meter of displacement
as measured along the coil axis. (This expression differs from
current density per unit area, which is expressed in
amperes per meter squared.) Then the magnetic field
strength in the interior of the coil is defined as 1 A/m.
For a given coil, the magnetic field strength is
directly proportional to the linear current density. Thus, if the
linear current density doubles, so does the magnetic field strength;
if the linear current density becomes 1/10 as great, the magnetic
field strength also diminishes by a factor of 10.
Sometimes, magnetic field strength is expressed in
units called oersteds (symbolized Oe). The oersted is a larger unit
than the ampere per meter. Approximate conversions are:
1 Oe = 79.578 A/m
1 A/m = 0.012566 Oe