An electric circuit is an interconnection of electric components such that electric
charge is made to flow along a closed path (a circuit), usually to perform some
useful task.[54]
The components in an electric circuit can take many forms, which can include
elements such as resistors, capacitors, switches, transformers and electronics.
Electronic circuits contain active components, usually semiconductors, and
typically exhibit non-linear behaviour, requiring complex analysis. The simplest
electric components are those that are termed passive and linear: while they may
temporarily store energy, they contain no sources of it, and exhibit linear
responses to stimuli.[55]: 15–16
The resistor is perhaps the simplest of passive circuit elements: as its name
suggests, it resists the current through it, dissipating its energy as heat. The
resistance is a consequence of the motion of charge through a conductor: in metals,
for example, resistance is primarily due to collisions between electrons and ions.
Ohm's law is a basic law of circuit theory, stating that the current passing
through a resistance is directly proportional to the potential difference across
it. The resistance of most materials is relatively constant over a range of
temperatures and currents; materials under these conditions are known as 'ohmic'.
The ohm, the unit of resistance, was named in honour of Georg Ohm, and is
symbolised by the Greek letter Ω. 1 Ω is the resistance that will produce a
potential difference of one volt in response to a current of one amp.[55]: 30–35
The capacitor is a development of the Leyden jar and is a device that can store
charge, and thereby storing electrical energy in the resulting field. It consists
of two conducting plates separated by a thin insulating dielectric layer; in
practice, thin metal foils are coiled together, increasing the surface area per
unit volume and therefore the capacitance. The unit of capacitance is the farad,
named after Michael Faraday, and given the symbol F: one farad is the capacitance
that develops a potential difference of one volt when it stores a charge of one
coulomb. A capacitor connected to a voltage supply initially causes a current as it
accumulates charge; this current will however decay in time as the capacitor fills,
eventually falling to zero. A capacitor will therefore not permit a steady state
current, but instead blocks it.[55]: 216–20
The inductor is a conductor, usually a coil of wire, that stores energy in a
magnetic field in response to the current through it. When the current changes, the
magnetic field does too, inducing a voltage between the ends of the conductor. The
induced voltage is proportional to the time rate of change of the current. The
constant of proportionality is termed the inductance. The unit of inductance is the
henry, named after Joseph Henry, a contemporary of Faraday. One henry is the
inductance that will induce a potential difference of one volt if the current
through it changes at a rate of one ampere per second. The inductor's behaviour is
in some regards converse to that of the capacitor: it will freely allow an
unchanging current, but opposes a rapidly changing one.[55]: 226–29
Electric power
Main article: electric power
Electric power is the rate at which electric energy is transferred by an electric
circuit. The SI unit of power is the watt, one joule per second.
Electric power, like mechanical power, is the rate of doing work, measured in
watts, and represented by the letter P. The term wattage is used colloquially to
mean "electric power in watts." The electric power in watts produced by an electric
current I consisting of a charge of Q coulombs every t seconds passing through an
electric potential (voltage) difference of V is
P
=
work done per unit time
=
Q
V
t
=
I
V
{\displaystyle P={\text{work done per unit time}}={\frac {QV}{t}}=IV\,}
where
Q is electric charge in coulombs
t is time in seconds
I is electric current in amperes
V is electric potential or voltage in volts
Electric power is generally supplied to businesses and homes by the electric power
industry. Electricity is usually sold by the kilowatt hour (3.6 MJ) which is the
product of power in kilowatts multiplied by running time in hours. Electric
utilities measure power using electricity meters, which keep a running total of the
electric energy delivered to a customer. Unlike fossil fuels, electricity is a low
entropy form of energy and can be converted into motion or many other forms of
energy with high efficiency.[56]
Electronics
Main article: electronics
Surface-mount electronic components
Electronics deals with electrical circuits that involve active electrical
components such as vacuum tubes, transistors, diodes, sensors and integrated
circuits, and associated passive interconnection technologies.[57]: 1–5, 71 The
nonlinear behaviour of active components and their ability to control electron
flows makes digital switching possible,[57]: 75 and electronics is widely used in
information processing, telecommunications, and signal processing. Interconnection
technologies such as circuit boards, electronics packaging technology, and other
varied forms of communication infrastructure complete circuit functionality and
transform the mixed components into a regular working system.
Today, most electronic devices use semiconductor components to perform electron
control. The underlying principles that explain how semiconductors work are studied
in solid state physics,[58] whereas the design and construction of electronic
circuits to solve practical problems are part of electronics engineering.[59]
Electromagnetic wave
Main article: Electromagnetic wave
Faraday's and Ampère's work showed that a time-varying magnetic field created an
electric field, and a time-varying electric field created a magnetic field. Thus,
when either field is changing in time, a field of the other is always induced.
[23]: 696–700 These variations are an electromagnetic wave. Electromagnetic waves
were analysed theoretically by James Clerk Maxwell in 1864. Maxwell developed a set
of equations that could unambiguously describe the interrelationship between
electric field, magnetic field, electric charge, and electric current. He could
moreover prove that in a vacuum such a wave would travel at the speed of light, and
thus light itself was a form of electromagnetic radiation. Maxwell's equations,
which unify light, fields, and charge are one of the great milestones of
theoretical physics.[23]: 696–700
The work of many researchers enabled the use of electronics to convert signals into
high frequency oscillating currents and, via suitably shaped conductors,
electricity permits the transmission and reception of these signals via radio waves
over very long distances.[60]
