Dictionary of electricity. The meanings of the electricity terms. Electricity Glossary Dictionary
Alternating Current (ac).—A flow of electric charge that reverses its direction periodically. For example, the alternating current used in a home has a frequency of 60 cycles per second (hertz).
Ammeter.—An instrument for measuring electric current. Basically, it consists of a low resistance in parallel with a galvanometer coil.
Ampere.—A unit of current, defined in terms of the force on either of two parallel current-carrying conductors. The current in each wire is defined as 1 ampere when the wires are separated by a distance of 1 meter and there is a force of 2 x 10^-7 newton on each section of wire 1 meter long.
Battery.—A group of two or more electrochemical cells connected together. It converts chemical energy into electrical energy.
Capacitance.—The property by which electric charge is stored on conductors separated by an insulator. It is defined as the charge on one of two parallel conductive plates divided by the potential difference between the plates.
Capacitive Reactance.—The opposition offered to a changing current by the capacitance of a device or circuit. A purely capacitive reactance causes a sine-wave current to lead the applied alternating voltage by 90°, or one quarter of a cycle.
Capacitor.—A device that provides capacitance in an electric circuit. Essentially, it consists of two metal plates separated by an insulating material such as air or paper.
Cell.—A device that generates direct current by converting chemical energy into electrical energy. There are two main types: primary cells, such as the dry cells used in a flashlight; and secondary cells, such as the cells of an automobile battery.
Charged Body.—An object having an excess of electrons or an insufficiency of electrons. The former is a negatively charged body; the latter is a positively charged body.
Circuit.—A system having one or more paths for electric current. The system consists of conductors, an energy source, and one or more devices such as resistors, capacitors, inductors, electron tubes, or transistors.
Circuit Breaker.—A device for opening or closing an electric circuit, chiefly to protect electrical equipment by automatically disconnecting it when the current rises above safe values.
Conductor.—A material, such as copper or aluminum, that has very low resistance to electric current.
Coulomb.—A unit of electric charge, defined as the quantity of electricity passing through a cross section of a conductor in 1 second when the current is 1 ampere. A coulomb is equal to the charge of 6.24 X I018 electrons.
Current.—The flow of electric charge. Strictly, the net transfer of electric charge per unit time.
Cycle.—The passage of an alternating current or voltage through a complete sequence of positive and negative values, including a rise to a maximum in the positive direction, a return to zero, a drop to a maximum value in the negative direction, and another return to zero.
Dielectric.—An electrical insulator in which an electric field can be sustained with minimum loss of energy.
Direct Current (dc).—The flow of electric charge in one direction only, as opposed to alternating current. Direct current may be steady or fluctuating, but it never reverses its direction.
Electric Field.—The condition existing at a point if a force of electrical origin is exerted on a charged body at that point. An electric field may be thought of as imaginary lines of electrical force between points of positive and negative charge.
Electricity.—A form of energy associated with two atomic particles: the electron and the proton. At rest, electrons or protons manifest electricity, or electrical energy, through an electric field. In motion, these same particles have both electric and magnetic fields. However, electrons are of primary interest in electrical engineering because electrons flow through a wire, whereas protons do not.
Electromagnet.—A device consisting of an iron core and a surrounding coil of wire. It becomes magnetized when current flows through the coil, and it loses its magnetism when the current ceases.
Electromagnetic Induction.—The production of an electromotive force (emf) by a change in the magnetic flux passing through a conductor, the change being caused by moving the conductor through a magnetic field or by varying the magnetic field.
Electromotive Force (emf).—The work per unit charge required to move a small positively charged body around a closed path in an electric field. The term “emf” refers to a source of electrical energy such as a battery or generator. An emf is measured in volts.
Farad.—A unit of capacitance. A capacitor has a capacitance of 1 farad when there is a potential difference of 1 volt between its plates and the charge on one of the plates is 1 coulomb.
Free Electrons.—Electrons loosely bound to their parent atoms and therefore able to move about. Metals are rich in free electrons and therefore are good conductors.
Frequency.—The number of cycles of alternating current that occur in a given length of time. One cycle per second is called a hertz (Hz).
Fuse.—A device for protecting electrical circuits from damage by excessive current.
Galvanometer.—An instrument for measuring very small currents. The current to be measured passes through a coil of wire free to rotate in the field of a permanent magnet. The interaction of current and field rotates the coil by an amount proportional to the current. A pointer attached to the coil indicates the magnitude of the current on a scale.
Generator.—A machine that converts mechanical energy into electrical energy. Essentially, the mechanical energy moves conductors through a magnetic field, which induces electric currents in them.
Ground.—A conducting path between an electrical apparatus and the earth. A ground usually is provided to prevent accumulation of static charges on the apparatus and to give an escape route for power accidentally applied to the apparatus frame.
Henry.—A unit of inductance. One henry is the self-inductance of a circuit—or the mutual inductance between two circuits—when there is an induced emf of 1 volt and the current is changing at the rate of 1 ampere per second.
Impedance.—The total opposition presented by a circuit or device to an alternating current. Impedance includes resistance, inductive reactance, and capacitive reactance.
Inductance.—The property by which an emf is induced in a conductor by a changing current. In one coil of wire, the property is called self-inductance, which is the induced emf divided by the rate of change of the current in the coil.
Inductive Reactance.—The opposition to a changing current created by the inductance of a device or circuit. A purely inductive reactance causes a sine-wave alternating current to lag the applied alternating voltage by 90°.
Inductor.—A device that provides inductance in an electric circuit. It usually is a tightly wound coil of wire, often wound on an iron core to intensify the magnetic field.
Insulator.—A material that does not conduct any significant amount of electric current.
Inverter.—A machine or electronic circuit for converting direct current to alternating current.
Kilowatt-Hour.—A unit of electrical energy, equal to 1,000 watts of power consumed continuously for 1 hour, as by devices such as air conditioners, radios, refrigerators, TV sets, and toasters.
Line of Force.—An imaginary line used to represent an electric or magnetic field. The density of the lines of force varies in proportion to the strength of the field.
Load.—A device that consumes electrical power.
Magnetic Circuit.—A closed path for magnetic flux, such as the iron parts of a motor or the iron core of a transformer.
Magnetic Field.—A condition existing when a magnetic material or a moving charged body experiences a magnetic force. A magnetic field can be thought of as lines of force between the north and south poles of a magnet or as lines of force surrounding the path of electric charges in motion.
Magnetic Flux.—The total number of lines of magnetic force through a given cross-sectional area perpendicular to the lines of force.
Magnetic Flux Density.—The magnetic flux per unit area, usually expressed in webers per square meter.
Magnetomotive Force (mmf).—The force that produces the magnetic flux in a magnetic circuit. It is analogous to the emf that produces the flow of electrons in an electric circuit. Strictly, it is the work per unit magnetic pole required to carry the pole once around a magnetic circuit. The unit of mmf is the ampere-turn.
Ohm.—A unit of electrical resistance. When the applied potential difference is 1 volt, the current in a 1-ohm resistor is 1 ampere.
Ohmmeter.—An instrument for measuring resistance by comparing the current through an unknown resistance with that through a known resistance.
Open Circuit.—A condition in which there is no path for electric current between two points.
Parallel Circuit.—A circuit in which all paths are connected between two given points. Each path has the same potential difference across it as any of the other paths. However, the currents through each path can be different, depending on the resistors or other components in each path.
Permanent Magnet.—A device that, once magnetized, retains its magnetism after the magnetizing force has been removed. Also, the magnet resists any force that tends to demagnetize it.
Permeability.—A characteristic of a material proportional to B/H where B is the magnetic flux density produced in the material by a magnetic field, and H is the intensity of the field. The relative permeability of a material is the ratio of its permeability to the permeability of space.
Phase.—The difference between the times at which alternating quantities (currents or voltages) pass in the same direction (positive or negative) through the same value of their wave forms. For example, if two quantities having the same frequency pass a zero value simultaneously in the same direction, they are said to be in phase. If one of these quantities passes through zero in the same direction before or after the other, the two quantities are said to be out of phase.
Phase Angle.—A measure of phase in degrees or radians. One cycle of an alternating quantity is equal to 360°, or 27t radians. A phase angle of zero degrees, or zero radians, means that the two alternating quantities are in phase. A phase angle of 180°, or n radians, means that one quantity leads or lags the other by that amount.
Potential.—The electric potential energy per unit charge on a small charged body at a given point, where the electric potential energy is the work required to bring the charged body from infinity to the point.
Potential Difference.—The difference between the potentials at two points in an electric field. It is synonymous with ‘”voltage.” Potential difference can be thought of as the force that causes charge to flow between two points in a conductor.
Power.—The work done per unit time. In a dc circuit, the power is the product of the applied potential difference V and the current I. In an ac circuit, the power is VI times the cosine of the phase angle between V and I.
Power Factor.—The cosine of the phase angle between voltage and current in an ac circuit, where both are sine waves that have the same frequency.
Power Loss.—The ratio of the power absorbed by the input circuit of a device to the power delivered to a load.
Reactance.—The opposition offered to electric current by the inductance or capacitance in a circuit.
Rectifier.—A device that conducts current much more readily in one direction than in the other and therefore can be used to convert alternating current to direct current.
Relay.—An electromechanical or semiconductor device that, under the control of an input signal, closes one or more circuits connected to its output.
Reluctance.—The opposition presented to magnetic flux in a magnetic circuit. It is equal to the magnetomotive force divided by the magnetic flux, and is analogous to resistance in an electric circuit.
Resistance.—A property of a material that opposes electric current, thereby causing part of the current to be dissipated as heat.
Resistor.—A device used to introduce resistance in an electric circuit, usually for the purpose of limiting the current or reducing the voltage in the circuit.
Series Circuit.—An electric circuit in which circuit devices are connected in sequence so that the same current flows through each device.
Short Circuit.—An electrical path of very low impedance, usually provided by a fault that forms a circuit with less length and impedance than the normal circuit.
Shunt.—An electrical (or magnetic) path connected in parallel with another path.
Sine Wave.—A wave whose amplitude varies with time according to the equation A — B sincut, where A is the instantaneous value of the wave, B is the amplitude of a peak or valley of the wave, co is the angular frequency, and t is the elapsed time. A graph of a sine wave voltage or current is an undulating line, as described under cycle.
Switch.—A device that opens, closes, or changes the connections in an electric circuit by manual, mechanical, or electrical means.
Transducer.—A device that converts energy from one form to another. For example, a microphone converts sound energy into electrical energy.
Volt.—A unit of potential, potential difference, voltage, or electromotive force. One volt sends a current of 1 ampere through a resistance of 1 ohm.
Voltage.—A synonym for potential difference.
Voltmeter.—An instrument for measuring the potential difference between two points. It consists of a very high resistance in series with a galvanometer.
Watt.—A unit of power, equal to 1 joule of energy developed or expended per second. One watt is the power in a circuit when the current is 1 ampere and the applied voltage is 1 volt.