Portal:Electronics/Selected article

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Portal:Electronics/Selected article/1

Ohm's law states that, in an electrical circuit, the current passing through a conductor, from one terminal point to another, is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two terminal points and inversely proportional to the resistance of the conductor between the two terminal points. The SI unit of current is the ampere; that of potential difference is the volt; and that of resistance is the ohm, equal to one volt per ampere.

In mathematical terms, this is written as:

,

where I is the current, V is the potential difference, and R is a constant called the resistance.

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Portal:Electronics/Selected article/2
Joule's laws are a set of two laws concerning the heat produced by a current and the energy dependence of an ideal gas to that of pressure, volume, and temperature, respecetively. Joule's first law, also known as the Joule effect, is a physical law expressing the relationship between the heat generated by the current flowing through a conductor. The heating effect of conductors carrying currents is known as Joule heating, named for James Prescott Joule. It is expressed as:

Where Q is the heat generated by a constant current I flowing through a conductor of electrical resistance R, for a time t.

Joule's second law states that the internal energy of an ideal gas is independent of its volume and pressure, depending only its temperature.

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Portal:Electronics/Selected article/3

Electrical resistance is a measure of the degree to which an object opposes the passage of an electric current. The SI unit of electrical resistance is the ohm. Its reciprocal quantity is electrical conductance measured in siemens. The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied to the circuit.

where;R is the resistance of the object, usually measured in ohms, equivalent to J·s/C², V is the potential difference across the object, usually measured in volts, I is the current passing through the object, usually measured in amperes. For a wide variety of materials and conditions, the electrical resistance does not depend on the amount of current flowing or the amount of applied voltage. V can either be measured directly across the object or calculated from a subtraction of voltages relative to a reference point.

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Portal:Electronics/Selected article/4

Mainframes are computers used mainly by large organizations for critical applications, typically bulk data processing such as census, industry/consumer statistics, ERP, and financial transaction processing.

The term originated during the early years of computing and referred to the large mechanical assembly that held the central processor and input/output complex. Later the term was used to distinguish high-end commercial machines from less powerful units which were often contained in smaller packages. Today, this term refers primarily to IBM System z9 mainframes, the lineal descendants of the System/360, but it is also used for the lineal descendents of the Burroughs large systems and the UNIVAC 1100/2200 series mainframes.

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A bipolar junction transistor (BJT) is a type of transistor. It is a three-terminal device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes. Although a small part of the base–emitter current is carried by the majority carriers, the main current is carried by minority carriers in the base, and so BJTs are classified as 'minority-carrier' devices.

The bipolar (point-contact) transistor was invented in December 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley. The junction version was invented by Shockley in 1951.

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Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide. Semiconductor devices have replaced thermionic devices (vacuum tubes) in most applications. They use electronic conduction in the solid state as opposed to the gaseous state or thermionic emission in a high vacuum. The main reason semiconductor materials are so useful is that the behaviour of a semiconductor can be easily manipulated by the addition of impurities, known as doping. Semiconductor conductivity can be controlled by introduction of an electric field, by exposure to light, and even pressure and heat; thus, semiconductors can make excellent sensors.

Semiconductor devices are manufactured both as single discrete devices and as integrated circuits (ICs), which consist of a number—from a few to millions—of devices manufactured and interconnected on a single semiconductor substrate.

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Portal:Electronics/Selected article/7

Electromagnetism is the physics of the electromagnetic field; a field encompassing all of space which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles. The magnetic field is produced by the motion of electric charges, i.e. electric current. The magnetic field causes the magnetic force associated with magnets.

The term "electromagnetism" comes from the fact that electrical and magnetic forces are involved simultaneously. A changing magnetic field produces an electric current (this is the phenomenon of electromagnetic induction, which provides for the operation of electrical generators, induction motors, and transformers). Similarly, a changing electric field generates a magnetic field. Because of this interdependence of the electric and magnetic fields, it makes sense to consider them as a single coherent entity — the electromagnetic field.

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Portal:Electronics/Selected article/8
Capacitance is a measure of the amount of electric charge stored (or separated) for a given electric potential. The capacitance of the majority of capacitors used in electronic circuits is several orders of magnitude smaller than the farad. The energy (measured in joules) stored in a capacitor is equal to the work done to charge it.

In a capacitor, there are two conducting electrodes which are insulated from one another. The charge on the electrodes is +Q and -Q, and V represents the potential difference between the electrodes. The SI unit of capacitance is the farad; 1 farad = 1 coulomb per volt.

The capacitance can be calculated if the geometry of the conductors and the dielectric properties of the insulator between the conductors are known, such as above, where; C is the capacitance in farads, ε is the permittivity of the insulator used (or ε₀ for a vacuum), A is the area of each plane electrode in square metres, d is the separation between the electrodes in metres. The equation is a good approximation if d is small compared to the other dimensions of the electrodes.

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Portal:Electronics/Selected article/9
Inductance is a measure of the amount of magnetic flux produced for a given electric current. The term was coined by Oliver Heaviside in February 1886. The SI unit of inductance is the henry (symbol: H), in honour of Joseph Henry. The symbol L is used for inductance, possibly in honour of the physicist Heinrich Lenz.

The inductance has the following relationship:

where; L is the inductance in henrys, i is the current in amperes, Φ is the magnetic flux in webers. Strictly speaking, the quantity just defined is called self-inductance, because the magnetic field is created solely by the conductor that carries the current.

When a conductor is coiled upon itself N number of times around the same axis (forming a solenoid), the current required to produce a given amount of flux is reduced by a factor of N compared to a single turn of wire. Thus, the inductance of a coil of wire of N turns is given by:

where, λ is the total 'flux linkage'.

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Portal:Electronics/Selected article/10

In electromagnetics and communications engineering, the term waveguide may refer to any linear structure that guides electromagnetic waves. However, the original and most common meaning is a hollow metal pipe used for this purpose.

A dielectric waveguide employs a solid dielectric rod rather than a hollow pipe. An optical fibre is a dielectric guide designed to work at optical frequencies. Transmission lines such as microstrip, coplanar waveguide, stripline or coax may also be considered to be waveguides.

The electromagnetic waves in (metal-pipe) waveguide may be imagined as travelling down the guide in a zig-zag path, being repeatedly reflected between opposite walls of the guide. For the particular case of rectangular waveguide, it is possible to base an exact analysis on this view. Propagation in dielectric waveguide may be viewed in the same way, with the waves confined to the dielectric by total internal reflection at its surface.

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Portal:Electronics/Selected article/11

In electronics, a vacuum tube or thermionic valve, is a device generally used to amplify, switch or otherwise modify, a signal by controlling the movement of electrons in an evacuated space.

For most purposes, the vacuum tube has been replaced by the much smaller, less power-hungry, and less expensive transistor, either as a discrete device or in an integrated circuit. However, tubes are still used in specialized applications, such as in high-end audio systems and high power RF transmitters. Cathode ray tubes are still used as a display device in television sets and computer monitors (although they face serious competition from LCD and plasma displays), and magnetrons are the source of microwaves in microwave ovens.

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Portal:Electronics/Selected article/12

In science and technology, a battery is a device that stores chemical energy and makes it available in an electrical form. Batteries consist of electrochemical devices such as two or more galvanic cells, fuel cells or flow cells. The modern development of batteries started with the Voltaic pile, announced by the Italian physicist Alessandro Volta in 1800.

Formally, an electrical "battery" is an interconnected array of similar voltaic cells ("cells"). However, in many contexts (other than the expression dry cell) it is common to call a single cell used on its own a battery.

A battery is a device in which chemical energy is directly converted to electrical energy. It consists of one or more voltaic cells, each of which is composed of two half cells connected in series by the conductive electrolyte.

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Portal:Electronics/Selected article/13
In physics, Coulomb's law is an inverse-square law indicating the magnitude and direction of electrostatic force that one stationary, electrically charged object of small dimensions exerts on another. It is named after Charles-Augustin de Coulomb who used a torsion balance to establish it.

The magnitude of the electrostatic force between two point charges is directly proportional to the magnitudes of each charge and inversely proportional to the square of the distance between the charges.

For calculating the direction and magnitude of the force simultaneously, one will wish to consult the full vector version of the Law

where is the electrostatic force vector, q₁ is the charge on which the force acts, q₂ is the acting charge, is the distance vector between the two charges, is position vector of q₁, is position vector of q₂, is a unit vector pointing in the direction of , and ε₀ is a constant called the permittivity of free space.

This vector equation indicates that opposite charges attract, and like charges repel. When is negative, the force is attractive. When positive, the force is repulsive.

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Portal:Electronics/Selected article/14

An antenna or aerial is an arrangement of aerial electrical conductors designed to transmit or receive radio waves which is a class of electromagnetic waves. In other words, antennas basically convert radio frequency electrical currents into electromagnetic waves and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, radar, and space exploration. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances.

Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Some antenna devices (parabola, horn antenna) just adapt the free space to another type of antenna.

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Portal:Electronics/Selected article/15

A LASER (acronym for Light Amplification by Stimulated Emission of Radiation) is an optical source that emits photons in a coherent beam. The term has since entered the English language as a standard word, laser, losing the capitalization in the process. The back-formed verb lase means "to produce laser light" or "to apply laser light to".

Laser light is typically near-monochromatic, i.e., consisting of a single wavelength or color, and emitted in a narrow beam. This contrasts with common light sources, such as the incandescent light bulb, which emit incoherent photons in almost all directions, usually over a wide spectrum of wavelengths. Laser action is explained by the theories of quantum mechanics and thermodynamics. Many materials have been found to have the required characteristics to form the laser gain medium needed to power a laser, and these have led to the invention of many types of lasers with different characteristics suitable for different applications.

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Portal:Electronics/Selected article/16

An electronic amplifier is a device for increasing the power of a signal. An idealized amplifier can be said to be "a piece of wire with gain", as the output is an exact replica of the input, but larger. It does this by taking power from a power supply and controlling the output to match the input signal shape but with a larger amplitude, in this sense an amplifier may be considered as modulating the output of the power supply.

Real world amplifiers are not ideal and this control is thus imperfect. One consequence is that the power supply itself may influence the output, and must itself be considered when designing the amplifier. The amplifier circuit has an "open loop" performance, that can be described by various parameters. The majority of modern amplifiers apply some negative feedback to form a control loop surrounding the gain stage itself.

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Portal:Electronics/Selected article/17

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Magnetic circuit breakers are implemented using a solenoid (electromagnet) whose pulling force increases with the current. The circuit breaker's contacts are held closed by a latch and, as the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action.

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Electrical engineering is a professional engineering discipline that deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century with the commercialization of the electric telegraph and electrical power supply. The field now covers a range of sub-disciplines including those that deal with power, optoelectronics, digital electronics, analog electronics, computer science, artificial intelligence, control systems, electronics, signal processing and telecommunications.

The term electrical engineering may or may not encompass electronic engineering. Where a distinction is made, electrical engineering is considered to deal with the problems associated with large-scale electrical systems such as power transmission and motor control, whereas electronic engineering deals with the study of small-scale electronic systems including computers and integrated circuits.

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