Junior Physics Resources

A Glossary of Electrical Terms

© Ian Sefton, School of Physics, The University of Sydney

March 1995, February 1996

Cross-references to other entries are indicated in bold type.

A:         symbol for ampere, the SI unit of current.

AC: literally, alternating current. Often used loosely to refer to other things which involve alternating current; for example AC voltage probably means either alternating emf or alternating potential difference.

active device: a device which does something. It is nearly always a source of emf, but its important property could be that it is sending a signal of some kind. (See also passive device.)

active terminal: of a power point is the "live" or high potential (high voltage) terminal. The neutral terminal is supposed to remain close to earth potential, but it may vary from that. See also earth.

alternating current:    (AC) a current which is continually changing its value and direction in a regular fashion. Usually it means a current which can be described by the equation: i = io sin(2p f t) (a "sinusoidal" current) in which i is current, io is a constant called the amplitude of the current, t is time and f is the frequency of the current. In Australia the frequency of commercially generated AC power is 50 Hz, which means that the current changes direction 100 times per second (twice for each cycle).

ammeter: current meter; a contraction of ampere-meter.

amp: colloquial name for ampere.

amp hour: colloquial for ampere hour.

ampere: the SI unit of electric current. Its symbol is A, so a current of 5 amperes is written as 5 A. A typical domestic appliance such as a toaster will carry a current of several amperes.

Ampère, André-Marie: (1775-1836) French physicist who gave his name to the unit of current.

ampere hour: a unit for describing the life of a battery which depends on how much current it can produce for how long. Symbol A.h. A battery with a life of 1 A.h is nominally capable of producing a current of 1 A for 1 h, or 0.5 A for 2 h, etc, before it goes flat. Since the product of current and time has the dimensions of charge, it follows that if you run a battery with 1.00 A.h capacity until it is flat, then 1 A.h or 3.6 ¥ 103 C of mobile charge will have passed through the battery. You can estimate the energy that can be delivered from a battery by taking the product of its life and its emf. For example a 2 V battery with a life of 5 A.h should be able to deliver about (2 V) x (5 A.h) which equals 2 x 5 x 60 x 60 J or 7 kJ.

analogue meter: a meter which is read by noting the position of a pointer against a scale.

angular frequency: frequency multiplied by the number 2π. The "angular" bit is essentially a red herring, but is related to the fact that an angle of 2π radians is one revolution. Usual symbol: w (Greek lower-case omega). The SI unit is the reciprocal second, symbol s-1, but some folks, confused by the "angular" part of the name, use radian per second (rad.s-1).

battery: strictly a collection of electrochemical cells, but commonly refers also to a single electrochemical cell. The key property of a battery or cell is its emf. For a battery consisting of several cells, the total emf is equal to the sum of the cells' individual emfs.

C:         the symbol for coulomb, the SI unit of charge. Not to be confused with the italic symbol C for capacitance.

capacitance: property of a conductor or a pair of conductors which tells how good it is at holding separated charge for a given potential (in the case of one conductor) or potential difference for a pair of conductors. Defined as the quotient: charge divided by potential (difference). The usual symbol is C (printed in italics in books - don't confuse it with the symbol C for coulomb). The SI unit of capacitance is the farad.

capacitor: a two-terminal device designed to have the property of capacitance. It usually consists of two conducting objects separated by insulating material. It can also be thought of as a device for storing energy.

cell:      see electrochemical cell, photovoltaic cell, charging.

charge: the basic electrical property of matter. Usual symbol q, occasionally Q. There are two kinds of charge which we call positive and negative. Of the particles which constitute atoms, every proton has a positive charge of +e, every electron has a charge of -e, while neutrons have no charge. (The value e is often called the electron charge rather than the proton charge because the electron was discovered before the proton.) Normally the total charge of an atom is zero; the number of protons in the nucleus is equal to the number of electrons in the atom; the atom is electrically neutral. Charge does not exist independently of matter.

charging: "charging" a battery is a misnomer for "energising" it. It means putting energy into the battery by forcing a current through the battery, against the battery's emf. The term charging is misleading because the total charge in the battery is always the same; although mobile charge passes through the battery during "charging" and "discharging" what matters is the state of the chemicals in the battery and the energy that you can get from it.

circuit: strictly, a circuit is just one conducting loop containing a string of electrical components joined end to end. In common parlance the meaning is often extended to include any arrangement of components, which may contain many different loops.

circuit diagram: see schematic diagram.

common: a shared connection or part of a circuit to which several different things may be connected. Usually, only one part of a circuit is called common and it has a constant potential. On a meter there may be several different terminals designed for different kinds of measurement (e.g. voltage or large currents) but there is usually one terminal that is common to all functions; the polarity of the common terminal is usually negative.

conductance: the opposite of resistance; a good conductor has a low resistance; defined as the reciprocal of resistance or as the quotient, current through the object divided by the potential difference across it. Usual symbol: G. The SI unit is the siemens, symbol S.

conduction: the process by which charged particles move in an organised way through a material thus forming a current.

conductor: an electrical conductor is any thing or any material which can carry an electric current. (In other contexts a conductor might be something that carries heat from one place to another or a person who minds a travelling tram.) See also insulator, semiconductor.

constant: steady, unchanging, having the same value during some interval of time. A constant current (DC) does not change with time. See also uniform.

conventional current: part of a model which supposes that current consists of moving positive charge. Conventional current goes from the positive terminal of a battery, through a circuit and back into the battery's negative terminal. Even though we know that for a metallic wire a better model describes negatively charged electrons as carrying the current, the concept of conventional current is well established and causes no problems in circuit theory. Unless a context tells you otherwise, assume that all references to current mean conventional current.

coulomb: the SI unit of charge, symbol C, named after Charles Augustin Coulomb (1736 - 1806) who formulated the law of interaction between charged particles. A coulomb of separated charge is a huge quantity.

current: an electric current is something that exists in a closed electrical circuit and is measured using an ammeter. It is not the same as energy or voltage. The name is analogous with water current in a river or an air current which is moving air. What moves in an electric current is electrically charged particles, inside a conductor, whose total charge is zero or neutral. The usual symbol is I; some books use i for changing current. The SI unit of current is the ampere (symbol A). See also conventional current.

DC:      literally, direct current, which usually means a steady unchanging current. DC is often used as an adjective to refer to other things associated with direct current; for example DC voltage usually means steady emf or steady potential difference.

decade box: a device which allows you to choose precise and accurate values of some property such as resistance or capacitance by selecting values on a set of knobs, each of which switches in one of ten values. (A decade is a sequence of ten things.)

digital meter: a meter which displays its readings as numbers (digits).

direct current: usually a constant current but the term could refer to a current with a constant direction and a slowly changing value. A battery produces direct current.

direction. Referring to circuits, direction does not mean direction in space but one of two possible ways that you might trace out a circuit or part of a circuit. Such "directions" might be described by terms such as "clockwise" or "from the positive terminal to the negative terminal". To completely specify a current, you need to know its direction as well as its value.

dynamic resistance: a property of a circuit component defined in terms of potential difference (V) across the device and the current (i) through it as dV/di. It is not the same as resistance. For devices which obey Ohm's law dynamic resistance is equal to resistance. The SI unit is the ohm, symbol Ω.

earth: literally just that, or a connection from a circuit to the earth - also called ground. It is useful because the earth can be regarded as a good conductor, which provides a convenient path for the completion of many circuits. Connection is usually made through a wire from the circuit or apparatus to the earth; such a connection is always available through the earth pin of a standard power point. (See also earth potential.)

earth potential: for all practical purposes the earth always stays at the same potential, so it is a convenient reference for specifying potentials, and it is conventionally assigned a potential of zero volts. For example if you see a reference to a potential (rather than a potential difference) of 100 V, that means 100 V above earth potential.

electricity: apart from being the name of the subject, electricity does not have a well-defined technical meaning. How, then, should we translate common usages of the term? To 'generate electricity' usually means to create emf, but when you 'buy electricity' you pay for energy. Some people say that electricity means charge, but if you mean charge, it's probably better to say charge.

electrochemical cell: a device for producing emf from chemical reactions within the cell. Also known colloquially as a battery. When the cell is connected into a suitable circuit it produces a current and delivers energy. In some cells the chemical reactions can be reversed by using another, stronger, source of emf to drive current backwards through the cell and put energy back. (See charging.) An example of such a reversible battery is the 12 V car battery which consists of six 2 V lead-acid cells.

electromagnetic field: an electric field and a magnetic field together. Since electric and magnetic fields are intimately linked to one another it makes sense to have a name which indicates both together. Electromagnetic waves, including light, consist of electromagnetic fields.

electromagnetic induction: a process in which an emf is created either by moving a conductor through a region containing a magnetic field, or by having a magnetic field which changes with time. It is the process used to produce "electricity" (electrical energy) in power generators.

electric field: a physical quantity which has a definite value at each point in space and which determines amongst other things, the electrical force that would be experienced by a charged particle at each point. We think of the field as existing in space even though there may be no particle there to experience the force. Electric field is produced in two ways. (1) An electric field exists in the space surrounding any charged particle. (2) An electric field is created by a magnetic field which varies with time. The SI unit of electric field is the volt per metre, symbol V.m-1. See also field.

electron: type of fundamental particle which carries the smallest possible magnitude of a free charge. The electron's charge is written symbolised as -e. The symbol e represents the value of the fundamental charge: e = 1.60 ¥ 10-19 C. Electrons are constituents of all atoms and are the charged particles which carry the current in a metallic wire.

emf: (pronounced "ee em eff") a physical quantity which describes the ability of an electrical source to deliver energy. You can also think of it as the property of the source which creates current in a circuit. Derived from the nineteenth century term "electromotive force" which is ok (pronounced "okay") as far as the electromotive bit goes, but it is not a force as we define force now. The emf of a battery is responsible for producing a potential difference between the battery's terminals. If the battery is not connected to anything else, that potential difference is equal to the emf. The SI unit is the volt, symbol V. [Not to be confused with electromagnetic field which the popular press sometimes refers to as EMF.]

energy: can't be easily defined. It is a physical quantity which, if you do the calculations correctly, always gives the same total energy for the whole universe. Its meaning is best learned through many examples, the same way that we learn normal language. It can be misleading to think of energy as a kind of substance - it is more subtle than that. Energy is what you are asked to pay for when you get your electricity bill. The SI unit of energy is the joule, symbol J.

farad: the SI unit of capacitance, named after Michael Faraday, symbol F. One farad is a very large capacitance; values of capacitors used in typical circuits are in the microfarad range (micro = one millionth).

faraday: an outmoded unit of charge, which we would now define as the charge of a mole of protons, 96 406 coulombs

Faraday, Michael: (1791 - 1867) pioneer researcher in electricity and regarded as one of the all-time greats of physics. He studied electric circuits and electrolysis, and he discovered (contemporaneously with Henry and Lenz) electromagnetic induction.

field: any physical quantity which has a definite value at each point throughout some region of space. The value at each point could be either a scalar (scalar field) or a vector (vector field) whose value consists of both a magnitude and a direction. An example of a scalar field is the temperature of the ocean which varies from place to place. The most familiar vector field is probably Earth's gravitational field which at places near Earth's surface at sea level has a value of about 9.8 N.kg-1 vertically down and is responsible for the familiar free-fall acceleration of 9.8 m.s-2. The main electrical examples are potential (a scalar field) electric field and magnetic field (both vector fields).

frequency: the repetition rate for any process or phenomenon that repeats itself exactly; it is the number of cycles divided by the total time interval taken. It is also equal to the reciprocal of the period, the time taken for one complete cycle. Usual symbol: f. The SI unit of frequency is the hertz, symbol Hz.

ground: see earth.

hertz: the SI unit of frequency, equivalent to one cycle per second; symbol Hz, equivalent to s-1. Named after Heinrich Hertz (1857-1894) who confirmed the existence of radio waves.

hole: an electron that isn't there! Imagine a tiny bubble of nothing, not even air, in a great sea of water. That would be a hole in the water. If some of the surrounding water fills the hole, that creates a new hole, so even though it is really the water that moves, you can think of the hole moving through the water. In some kinds of semiconductor the electrons responsible for electrical conduction are a bit like water which is packed so tightly in the container that it can't move except to fill any holes that might be there. So in electricity a hole is a mobile vacancy in a sea of electrons. In many ways it behaves like a positively charged electron.

impedance: a property of a circuit component, instrument or some other device which encapsulates the relationship between the potential difference (PD) across the device and the current through it. For steady (constant) PD and current, the impedance is equivalent to the resistance, the quotient of PD divided by current, but if the PD and current vary with time, you have to take into account the fact that there may be some delay between cause and effect - potential differences and currents may not change in unison. To specify impedance in such cases you have to know, as well as resistance, a second property called reactance which depends on the relationship among PD, current and the time scale of the changes. The two properties, resistance and reactance, together constitute impedance which cannot be described by a single numerical value. In DC circuits - with steady currents - the reactance of all components can be ignored, so in those cases impedance means the same as resistance. Some folks, such as loudspeaker salespersons, say impedance when they mean resistance.

input: Literally something that is put in to something else or the cause of some effect which you could call the output. For example, the changing potential difference (cause) at the terminals of an oscilloscope is the input which produces the output (effect) on the screen. The input to an amplifier may be the small potential difference generated by a microphone while the output is the current which goes through the loudspeaker. The terminals of a device may be designated as input terminals or output terminals. In the last example, the output wires from the microphone would be connected to the input terminals of the amplifier and the output terminals of the amplifier would be connected to the (input) terminals of the speaker. (A speaker has no output terminals because its output is sound.)

insulator: any thing or type of material which is a very poor conductor of electricity. Electrical wires (conductors) are covered on the outside with insulating material in order to guard against accidental short circuits.

internal resistance: is just resistance. The redundant "internal" is often added when one is referring to something like a battery or an instrument.

J:          symbol for joule, the SI unit of energy.

joule: the SI unit of energy, symbol J. Named after English physicist James Prescott Joule (1818 - 1889) who helped to establish the concept of energy.

kilowatt hour: non-SI unit of energy, used by electricity authorities for billing, equal to 3.6 megajoules. The symbol is kW.h, which is sometimes sloppily written as kWh.

kV:       symbol for kilovolt, 103 V.

load: something which takes electrical energy from a circuit. It is sometimes called an energy sink. A household light globe becomes a load when it is connected to the mains and switched on.

mA:      symbol for milliampere, 10-3 A.

magnetic field: a physical quantity which has a definite value at each point in space. We think of the field as existing in space even though there may be no particle there to experience the force. Magnetic fields can be produced by magnets made of magnetic materials (iron in particular), by electric currents and by electric fields which change with time. The SI unit of magnetic field is the tesla, symbol T. See also field.

milliamp: colloquial for milliampere; one thousandth of an ampere. Symbol: mA.

multimeter: an instrument which can be used for measuring any one of several different electrical quantities, usually potential difference (voltage), current and resistance. The user has to select the quantity to be measured, by selecting some switch settings.

mV:      symbol for millivolt, 10-3 V.

negative: see positive and negative.

neutral: (1) having zero net charge. The wires in a circuit remain neutral even though charged electrons move inside them. (2) A different meaning occurs with the neutral wire or terminal in a household wiring; in that case neutral means having near-zero potential - see under active terminal.

nominal value: literally "named value", usually a rough estimate of the intended value of something. Nobody is too fussed when the real value turns out to be somewhat different.

ohm: SI unit of resistance; symbol Ω (the Greek letter, capital omega).

Ohm, Georg Simon: (1787 - 1854) German scientist who gave his name to the unit of resistance.

Ohm's law: the statement that the resistance of some objects (notably metallic objects), held at constant temperature, is independent of the potential difference across the object or the current through it. There are many interesting objects which don't obey Ohm's law. Some people confuse Ohm's law with the definition of resistance but an object can have a (variable) resistance, even though it does not obey Ohm's law.

open circuit: (1) a break in what was meant to be a circuit or (2) the broken circuit itself. Turning a switch off creates an open circuit. See also short circuit.

output: explained under input.

oscilloscope: an instrument which produces, on a screen, graphs of potential difference (PD) against time or of one potential difference against another potential difference. A dual-trace oscilloscope can simultaneously produce two PD - time graphs with the same time scale.

parallel. Two components are in parallel if, when tracing a path between two points in a circuit, you find that you have the alternative of branching off and tracing through either one component or the other, before those alternative paths rejoin. Whether two things are in parallel or series depends critically on the two points that you are tracing the path between; it makes no sense to say that things are in parallel without reference to those points. See also series.

passive device: a passive circuit component has no emf. The term comes from the idea that it responds to something done to it by an active device, such as a battery. Light globes and capacitors are passive devices.

period: the time interval required for exactly one cycle of a repetitive process or phenomenon of any kind. It is equal to the reciprocal of the process's frequency. Usual symbol, T. The SI unit is the second, symbol s.

photovoltaic cell: a device which produces an emf, and hence a current, in response to light which is absorbed by the cell.

PD:      lazy person's way of writing potential difference.

polarity: the property of a device which means that it has a positive terminal and a negative terminal. For an active device such as a battery, the positive terminal has the higher potential. For a passive device, such as a meter, the positive terminal must be connected to a point in the circuit which has a higher potential than the point where you connect the negative terminal. Some capacitors have a polarity which needs to be observed if they are to work properly.

positive and negative. It is mathematically convenient to call the two kinds of electric charge positive and negative so, for example, there is no external effect from equal amounts of positive and negative charges which are more or less uniformly mixed in some volume of space because they add up to zero charge. A potential difference between two points can also have either a positive or negative value, depending on the order in which you specify the points. See also polarity.

potential: see potential difference.

potential difference: difference in potential between two points in space. Potential is an electrical quantity which has a unique value at every point in space once its value at some reference point has been arbitrarily decided. In circuits we consider potential differences between various pairs of points in the conductors which make up the circuit. The positive terminal of a battery is always at a higher potential than the negative terminal, even though the magnitude of the potential difference may vary. The potential difference between the ends of a circuit component is related to the current through the component and the properties (such as resistance) of the component. Potential difference can be defined as the work per charge done on a small charged particle by electrostatic forces when the particle is moved from one point to another. Usual symbol: ∆V or V.

power: rate of transfer of energy. For a steady rate (constant power) it can be expressed as E/∆t where E is the energy transferred in the time interval ∆t. Usual symbol: P. The SI unit is the watt (symbol W).

reactance: see under impedance.

resistance: a property of an object associated with energy dissipation which occurs when a current exists in the object. For a passive device (one which has no emf) resistance is defined as the quotient: steady potential difference (∆V) between two points on the object divided by the associated current (I) through the object; R = ∆V/I. The value of the resistance depends on the contact points chosen but for many circuit components the two connection points are usually obvious. A thing whose resistance is independent of the potential difference is said to obey Ohm's law.

resistor: a two-terminal device designed to have the property of resistance. It is usually desirable that a resistor should obey Ohm's law and have a resistance that is fairly stable against temperature changes. Most resistors are painted with a code consisting of coloured bands which tell you the resistance.

scalar: a physical quantity which can be specified by a single numerical value, including the unit. (Strictly the value must be the same in all reference frames or coordinate systems that are not moving relative to one another.) Most electrical quantities including charge, potential and emf, are scalars. Electric field and magnetic filed are not scalars - they are vector quantities.

schematic diagram: a circuit diagram which shows the logic of the connections rather than the actual layout of the components. Schematic diagrams have most of the connections drawn as lines parallel to an edge of the page. Those lines represent conducting paths which ideally should have zero resistance. (In practice, they have small resistance.)

semiconductor: a kind of material intermediate between a conductor and an insulator. Semiconductors are used to make transistors, diodes and photovoltaic cells.

series. Circuit components are said to be connected in series with each other if they form a chain without branches. In the more complicated case of a circuit you have to specify two points in the circuit before you can make sense of the terms series and parallel. Two components are in series if, when tracing a path between two points in a circuit, you find that the path goes through one component and then the other. See also parallel.

short circuit: a conducting path, or part of a circuit, with negligible or relatively low resistance. The term is most commonly used to indicate an accidental path, which causes a much bigger current than the one you wanted. See also open circuit.

siemens: the SI unit of conductance, symbol S, equivalent to the reciprocal ohm (Ω-1); named after William (Wilhelm) Siemens (1823 - 1883), engineer.

static resistance: exactly the same as resistance. The "static" qualifier is added in order to emphasise that one does not mean dynamic resistance.

source: usually means a source or giver of electrical energy in a circuit, so it is a general term for something that has an emf. It could also mean the source of a signal, but signal sources also have emf. When current rather, than emf, is important one may refer to a current source, but a source is still a source.

terminal: part of a component or a circuit to which something else gets connected. For example a battery has two terminals both of which have to be joined into a circuit before you get anything from the battery.

uniform: having the same value at all points within a region of space. For example, a uniform electric field has the same magnitude and direction at all points within some identifiable region. Not the same as constant.

V:         symbol for volt, the SI unit of potential difference and emf.

vector: a physical quantity whose value consists of a numerical value, including the unit, and a direction in space. (An alternative way of specifying each value is to give three components). Electric and magnetic fields are both vector quantities.

volt: the SI unit of potential, potential difference and emf, symbol V, named after Alessandro Volta.

Volta, Alessandro: (1745 - 1827) Italian physicist who gave his name to lots of things electrical.

voltage: a colloquial term which could mean either emf or potential difference. It usually means potential difference. If you know which one you mean it is better to use the more exact term.

voltmeter: an instrument for measuring potential difference. (It does not measure emf directly; values of emf have to be inferred from other measurements.)

W:        symbol for watt, the SI unit of power.

watt: the SI unit of power, symbol W, equivalent to 1 joule per second. Named after Scottish engineer, James Watt (1736-1819).

work: energy which is transferred by any mechanism other than heat flow. Work is done on a charged particle when it moves between two places with different potentials. A source of emf can increase the energy of a charged particle that passes through it. The SI unit is the joule, symbol J.

Acknowledgements: Thanks to Ian Cooper and Michael Large for helpful criticisms and suggestions.