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.
sink: see load.
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.