Absorption: What happens
when wave passes through a medium and gives up some of its
Acceleration: The rate of change of velocity; the
slope of the tangent line on a v-t graph.
Adhesion: The force of attraction between unlike
Alternating current: An electric current that has
one direction during one part of a generating cycle and
the opposite direction during the remainder of the cycle.
Ammeter: A device for measuring electrical current.
Ampere: The metric unit of
current, one coulomb per second; also "amp."
Amplifier: A device consisting of one or more
vacuum tubes (or transistors) and associated circuits,
used to increase the strength of a signal.
Amplitude: The amount of vibration, often measured
from the center to one side; may have different units
depending on the nature of the vibration.
Angle of incidence: The angle between the incident
ray and the normal drawn to the point of incidence.
Angle of reflection: The angle between the
reflected ray and the normal drawn to the point of
Angle of refraction: The angle between the
refracted ray and the normal drawn to the point of
Angstrom: A unit of linear measure equal to 10-10
Angular acceleration: The time rate of change of
Angular impulse: The product of a torque and the
time interval during which it acts.
Angular momentum: The product of the rotational
inertia of a body and its angular velocity.
Angular velocity: The time rate of change of
Angular magnification: The factor by which an
image's apparent angular size is increased (or decreased).
Armature: A coil of wire formed around an iron or
steel core that rotates in the magnetic field of a
generator or motor.
Atom: The basic unit of one of the chemical
Atomic mass: The mass of an atom.
Atomic number: The number of protons in the nucleus
of an atom.
Atomic weight: The weighted average of the atomic
masses of an element's isotopes based on their relative
Audio signal: The alternating voltage proportional
to the sound pressure produced in an electric circuit.
Average velocity: Total displacement divided by
Axis: An arbitrarily chosen point used in the
definition of angular momentum. Any object whose direction
changes relative to the axis is considered to have angular
momentum. No matter what axis is chosen, the angular
momentum of a closed system is conserved.
Barometer: A device used to measure the pressure of
Basic law of electrostatics: Similarly charged
objects repel each other. Oppositely charged objects
attract each other.
Becquerel: The rate of radioactivity equal to one
disintegration per second.
Beta particle: An electron emitted from the nucleus
of a radioactive atom.
Betatron: A device that accelerates electrons by
means of the transformer principle.
Bevatron: A high-energy synchrotron.
Binding energy: Energy that must be applied to a
nucleus to break it up.
Boiling point: The temperature at which the vapor
pressure of a liquid equals the pressure of the
Boyle's law: The volume of a dry gas varies
inversely with the pressure exerted upon it, provided the
temperature is constant.
Calorie: The quantity of heat equal to 4.19 joules.
Capacitance: The ratio of the charge on either
plate of a capacitor to the potential difference between
the plates. capacitive reactance. Reactance in an a-c
circuit containing capacitance which causes a lagging
Capacitor: A combination of conducting plates
separated by layers of a dielectric that is used to store
an electric charge.
Capillarity: The elevation or depression of liquids
in small-diameter tubes.
Center of curvature: The center of the sphere of
which the mirror or lens surface forms a part.
Center of gravity: The point at which all of the
weight of a body can be considered to be concentrated.
Centrifugal force: Force that tends to move the
particles of a rotating object away from the center of
Centripetal acceleration: Acceleration directed
toward the center of a circular path.
Centripetal force: The force that produces
Charge: A numerical rating of how strongly an
object participates in electrical forces.
Charles' law: The volume of a dry gas is
directly proportional to its Kelvin temperature, providing
the pressure is constant.
Circuit: An electrical device in which charge can
come back to its starting point and be recycled rather
than getting stuck in a dead end.
Coefficient of area expansion: The change in
area per unit area of a solid per degree change in
Coefficient of cubic expansion: The change in
volume per unit volume of a solid or liquid per degree
change in temperature.
Coefficient of linear expansion: The change in
length per unit length of a solid per degree change in
Calorimeter: A heat-measuring device consisting of
nested metal cups separated by an air space
Coherent: A light wave whose
parts are all in phase with each other.
Collision: An interaction between moving objects
that lasts for a certain time.
Component: The part of a velocity, acceleration, or
force that is along one particular coordinate axis.
Concave: Describes a surface that is hollowed out
like a cave.
Convex: Describes a surface that bulges outward.
Coulomb (C): The unit of electrical charge. The
quantity of electricity equal to the charge on 6.25 x 1018
Coulomb's law of electrostatics: The force
between two point charges is directly proportional to the
product of their magnitudes and inversely proportional ta
the square of the distance between them.
Coulomb's law of magnetism: The force between
two magnetic poles is directly proportional to the
strengths of the poles and inversely proportional to the
square of their distance apart.
Couple: Two forces of equal magnitude acting in
opposite directions in the same plane, but not along the
Crest: A region of upward displacement in a
Critical angle: That limiting angle of incidence
in the optically denser medium that results in an angle of
refraction of 90o.
Critical mass: The amount of a particular
fissionable material required to make a fission reaction
Critical point: The upper limit of the
temperature-pressure curve of a substance.
Critical pressure: The pressure needed to
liquefy a gas at its critical temperature.
Critical temperature: The temperature to which a
gas must be cooled before it can be liquefied by pressure.
Current: The rate at which charge crosses a
Damping: The dissipation of a vibration's energy
into heat energy, or the frictional force that causes the
loss of energy.
Dielectric. An electric insulator. A non
Dielectric constant. The ratio of the
capacitance with a particular material separating the
plates of a capacitor
Diffraction: The behavior of a wave when it
encounters an obstacle or a non-uniformity in its medium;
in general, diffraction causes a wave to bend around
obstacles and make patterns of strong and weak waves
radiating out beyond the obstacle.
Diffraction angle. The angle that a diffracted
wave front forms with the grating plane.
Diffraction grating. An optical surface, either
transmitting or reflecting, with several thousand equally
spaced and parallel grooves ruled in it.
Diffusion. (1) The penetration of one type of
particle into a mass of a second type of particle. (2) The
scattering of light by irregular reflection.
Dimensional analysis. The performance of
indicated mathematical operations in a problem with the
measurement units alone.
Diode. A two-terminal device that will conduct
electric current more easily in one direction than in the
Direct current. An essentially constant value
current in which the movement of charge is in only one
Direct proportion. The relation between two
quantities whose graph is a straight line.
Dispersion. The process of separating
polychromatic light into its component wavelengths.
Displacement: A name for the symbol
Doppler effect. The change observed in the
frequency with which a wave from a given source reaches an
observer when the source and the observer are in relative
Ductility. The property of a metal that enables
it to be drawn through a die to form a wire.
Driving force: An external force that
pumps energy into a vibrating system.
Electric dipole: An object that has an imbalance
between positive charge on one side and negative charge on
the other; an object that will experience a torque in an
Electric field: The force per unit charge exerted
on a test charge at a given point in space.
Electrical force: One of the fundamental forces of
nature; a non-contact force that can be either repulsive
Electron: Thomson's name for the particles of which
a cathode ray was made; a subatomic particle.
Energy: A numerical scale used to measure the heat,
motion, or other properties that would require fuel or
physical effort to put into an object; a scalar quantity
with units of joules (J).
Equilibrium: A state in which an object's momentum
and angular momentum are constant.
Field: A property of a point in space describing
the forces that would be exerted on a particle if it was
Fission: The radioactive decay of a nucleus by
splitting into two parts.
Fluid: A gas or a liquid.
Fluid friction: friction force in which at
least one of the object is is a fluid (i.e. either a gas
or a liquid).
Focal length: A property of a lens or mirror, equal
to the distance from the lens or mirror to the image it
forms of an object that is infinitely far away.
Frequency: The number of cycles per second,
the inverse of the period (q.v.).
Fusion: A nuclear reaction in which two nuclei
stick together to form one bigger nucleus.
Gamma ray: A form of radioactivity consisting of a
very high-frequency form of light.
Gravitational field: The
force per unit mass exerted on a test mass at a given
point in space.
Gravity: A general term for the phenomenon of
attraction between things having mass. The attraction
between our planet and a human-sized object causes the
object to fall.
Half-life: The amount of time that a radioactive
atom has a probability of 1/2 of surviving without
Heat: The energy that an object has because of its
temperature. Heat is different from temperature (q.v.)
because an object with twice as much mass requires twice
as much heat to increase its temperature by the same
amount. There is a further distinction in the terminology,
not emphasized in this book, between heat and thermal
energy. See the entry under thermal energy for a
discussion of this distinction.
Image: A place where
an object appears to be, because the rays diffusely
reflected from any given point on the object have been
bent so that they come back together and then spread out
again from the image point, or spread apart as if they had
originated from the image.
Index of refraction: An
optical property of matter; the speed of light in a vacuum
divided by the speed of light in the substance in
Independence: The lack of any relationship between
two random events.
Induction: The production of an electric
field by a changing magnetic field, or vice-versa.
Inertial frame: A frame of reference that is not
accelerating, one in which Newton's first law is true.
Kinetic friction: A friction force between
surfaces that are slipping past each other.
Invariant: A quantity that does not change when
Ion: An electrically charged atom or molecule.
Isotope: of the possible varieties of atoms of a
given element, having a certain number of neutrons.
Kinetic energy: The energy an object possesses
because of its motion. Cf. potential energy.
Lorentz transformation: The transformation between
frames in relative motion.
Magnetic dipole: Known as a current loop, an atom,
or a bar magnet, that experiences torques due to magnetic
forces; the strength of magnetic dipoles is measured by
comparison with a standard dipole consisting of a square
loop of wire of a given size and carrying a given amount
Magnetic field: Force, defined in terms of the
torque exerted on a test dipole.
Magnification: The factor by which an image's
linear size is increased (or decreased). Cf. angular
Magnitude: The "amount" associated with a vector;
the vector stripped of any information about its
Mass: A numerical measure of how difficult it
is to change an object's motion. (In the context of
relativity, some books use the word "mass" to mean what we
refer to as mass multiplied by gamma.)
Neutron: An uncharged particle, the other
types that nuclei are made of.
Open circuit: A circuit that does not function
because it has a gap in it.
Parabola: The mathematical curve whose graph has y
proportional to x2.
Period: The time required for one cycle of a
periodic motion (q.v.).
Photon: A particle of light.
Photoelectric effect: The ejection, by a
photon, of an electron from the surface of an object.
Power: The rate of transferring energy; a
scalar quantity with units of watts (W).
Proton: A positively charged particle, one of the
types that nuclei are made of.
Quality factor (Q) is the number of oscillations
required for a system's energy to fall off by a factor of
535 due to damping.
Quantized: Describes quantity such as money or
electrical charge, that can only exist in certain amounts.
Radial: Parallel to the radius of a circle; the
in-out direction. Cf. tangential.
Real image: A place where an object appears to be,
because the rays diffusely reflected from any given point
on the object have been bent so that they come back
together and then spread out again from the new point. Cf.
Reflection: What happens
when light hits matter and bounces off, retaining at least
some of its energy.
Refraction: The change in direction that
occurs when a wave encounters the interface between two
Repulsive: Describes a force that tends to push
the two participating objects apart. Cf. attractive,
Resistance: The ratio of the voltage difference to
the current in an object made of an ohmic substance.
Resonance: The tendency of a vibrating system to
respond most strongly to a driving force whose frequency
is close to its own natural frequency of vibration.
Scalar: A quantity that has no direction in
space, only an amount. Cf. vector.
Short circuit: A circuit that does not function
because charge is given a low-resistance "shortcut" path
that it can follow, instead of the path that makes it do
Significant figures; Digits that contribute to the
accuracy of a measurement.
Simple harmonic motion: Motion whose x-t graph is
a sine wave.
Sink: A point at which field vectors converge.
Source: A point from which field vectors
diverge; often used more inclusively to refer to points of
either convergence or divergence.
Specular reflection: Reflection from a smooth
surface, in which the light ray leaves at the same angle
at which it came in.
Spin: The built-in angular momentum possessed
by a particle even when at rest.
Tangential: Tangent to a curve. In circular motion,
used to mean tangent to the circle, perpendicular to the
radial direction Cf. radial.
Thermal energy.. Properly, thermal energy is used
to mean the total amount of energy possessed by an object,
while heat indicates the amount of thermal energy
transferred in or out.
Torque: The rate of change of angular momentum; a
numerical measure of a force's ability to twist on an
Transformation: The mathematical relationship
between the variables such as x and t, as observed in
different frames of reference.
Uniform circular motion: Circular motion in which
the magnitude of the velocity vector remains constant.
Vector: A quantity that has both an amount
(magnitude) and a direction in space. Cf. scalar.
Velocity: The rate of change of position; the slope
of the tangent line on an x-t graph.
Virtual image: Like a real image, but the rays
don't actually cross again; they only appear to have come
from the point on the image. Cf. real image.
Volt: The metric unit of voltage, one joule per
Voltage: Electrical potential energy per unit
charge that will be possessed by a charged particle at a
certain point in space.
Voltmeter: A device for measuring voltage
Weak nuclear force: The force responsible for beta
Weight: force of gravity on an object, equal
Work: The amount of energy transferred into or out
of a system, excluding energy transferred by heat