An electric circuit is a path along which current can flow. This is a rather broad definition because under the right conditions almost any object can conduct electricity. For example, air is usually considered an insulator (that is, a substance that blocks the flow of electricity), but at sufficiently high voltages air experiences dielectric breakdown: the electrons are stripped from its atoms and become mobile charge-carriers. Under these conditions, air readily conducts an electric current, as when lightning strikes.
In the context of man-made devices, an electric circuit is usually a pathway designed to permit the controlled flow of electricity. Simple circuits consist of three of components: (1) a source of electrical power, usually called a voltage source; (2) a load, that is, something which consumes electrical power, either by storing it up temporarily or dissipating it as heat; and (3) conductors, which allow the free passage of electricity through their substance and connect the source and load to each other to form a complete circuit. For example, a battery can provide voltage, wires of copper or aluminum can serve as conductors, and an electric motor or light bulb can serve as the load.
Two fundamental terms related to circuitry are current and voltage. For most electrical circuits and devices, a current is composed of electrons flowing through the circuit. When an ordinary light bulb shines, for instance, there is a current in its filament that consists of a flow of electrons. Voltage is a measure of electrical potential energy per unit charge: just as a gallon of water sitting at the top of a cliff has more potential energy than a gallon at the bottom, energy which can be retrieved by letting the water fall downhill through a suitable mechanism, an electron sitting on the negative side of a given voltage difference has more potential energy than an electron sitting on the positive side, and this energy, too, can be retrieved by letting the electron pass through a suitable device. Circuits and electrical devices are usually rated to operate at or near a particular current (quantity of electrons flowing) and voltage (force causing the electrons to flow); exceeding these ratings is likely to ruin the device.
A personal computer consists mostly of electrical devices built into circuits, including the motor of the fan used to cool the computer, various solid-state electronic devices, and all the wires connecting the computer's parts. The microscopic logic elements on the computer's processor and memory chips are all parts of electrical circuits, with electrons being driven through them by a voltage provided by the power supply (a separate unit). Whenever electricity flows through any of these devices, an electric circuit is achieved. A computer is thus composed of electrical circuits that must operate in unison.
The components of electrical circuits are often referred to as circuit elements. Active circuit elements supply electrical power to the circuit; the power supply in a computer is an example of an active circuit element. Devices such as transistors that modify the flow of electricity through other devices, and require power in order to do so, are also sometimes classified as active elements. Passive circuit elements include resistors, capacitors, and inductors, all of which dissipate or absorb energy. A resistor is a device or that is designed to provide a specified amount of resistance to the flow of electricity. When current passes through a resistor (such as a light bulb filament), a certain portion of the current is converted to pure heat, depending on the strength of the current and the properties of the resistor itself. A capacitor is a device that can store electric charge. A typical capacitor consists of two closely spaced conducting sheets, each carrying an equal and opposite charge and separated by a thin insulating material. Capacitors will allow an alternating current to pass, but will block direct current. (Alternating and direct currents are described below). The amount of charge that a capacitor can hold for a given applied voltage is called its capacitance. Whereas a capacitor stores electrical energy in the form of charge, an inductor stores current in the form of a magnetic field. A solenoid, or long, tightly wound coil of wire, is an example of an inductor. Inductors can block the passage of alternating current while allowing the flow of direct current (the opposite of what capacitors do). An inductor's capacity is called its inductance. Both capacitors and inductors find application in many electrical devices. All electrical circuits, to some degree, exhibit resistance, capacitance, and inductance. The only exception are devices which exhibit the property of superconductivity, where resistance is zero. Superconductivity is only known to occur at extremely low temperatures.
Switches may also be classified as circuit elements. Switches are used to control the flow of electricity in a circuit. A switch can halt the flow of electricity in a circuit, in which case the circuit is "open"; the same switch can allow electrical flow, resulting in a circuit that is "closed". A light switch is a well-known example. Practically all appliances and electrical devices (including computers) incorporate switches.
Circuits may be categorized in many ways. For instance, there are direct current (DC) and alternating current (AC) circuits. A direct current is a current that flows in one direction, like a river. Most of the circuitry in computers and televisions requires direct current. An alternating current changes direction periodically, like a hand-saw. The most familiar example of AC is the electricity supplied to homes and offices by power companies, which in the United States has a frequency of 60 Hertz (i.e., the current reverses direction 60 times per second). Some devices convert one type of current into the other. For example, the power-supply circuits in computers change the 60 Hz AC electricity from the wall plug into direct current for internal use.
Electrical circuits may also be classified as being either analog or digital. An analog circuit is one in which the current varies smoothly with time. By contrast, digital circuits only allow current to flow or not flow, with abrupt transitions between these two states. Digital circuits are composed of gates. A gate is a kind of electronic switch that produces a binary electrical output, called a "bit", which is usually identified as either 0 or 1. Whether the output of a gate is 0 or 1 depends upon two factors: (1) the conditions of all electrical inputs to the gate, and (2) the construction of the gate. Gates are built to implement certain functions described in Boolean logic. There are three basic Boolean logic gates: AND, OR, and NOT. Take, for example, the AND gate: it receives the value of two input bits and tests them to determine whether or not they are both equal to 1. If they are, the output from the AND gate is 1; if either of the inputs is 0, then the output is 0. The output of a digital circuit can provide input to another circuit or signify the final result of some set of operations. Operations of unlimited complexity can be performed using sufficiently elaborate combinations of the AND, OR, and NOT gates.
The gates that make up digital circuits are themselves made from electronic components called diodes and transistors. (Passive elements, especially capacitors and resistors, are also used.) A diode is a device that is much more resistant to the flow of current in one direction than the other. It functions as a one-way turnstile for current. Diodes are usually composed of semiconducting materials like silicon; other elements must be mixed with the silicon in precise layers, like those of a wedding cake, to produce the desired behavior. Transistors are constructed in much the same way. Transistors possess three or more electrodes (electrical connections) and use the voltage at one input to control the current flow between the other two. In this way they act like faucets. Transistors have replaced vacuum tubes for most electronics applications.
An integrated circuit is a device fabricated on an single, solid crystal of semiconducting material and composed of many individual electronic components, especially diodes, capacitors, and transistors. The term "integrated" signifies that all the components of the circuit are part of one solid object (the chip)--integrated into it-- rather than separate devices connected by wires. Integrated circuits are indispensable to most modern electronic devices, including computers. They serve as amplifiers, oscillators, and microprocessors while consuming little power.
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