|
|
|
|
|||||
 Physics
|
 |
Glossary of Physics Terms
The following are glossary entries compiled from the Light and Matter series of
free introductory
physics textbooks. Acceleration. The rate of change of velocity; the slope of the tangent line on a v-t graph. See textbook. Alpha decay. The radioactive decay of a nucleus via emission of an alpha particle. See textbook. Alpha particle. A form of radioactivity consisting of helium nuclei. See textbook. Ammeter. A device for measurin electrical current. See textbook. Ampere. The metric unit of current, one coulomb pe second; also "amp." See textbook. Amplitude. The amount of vibration, often measured from the center to one side; may have different units depending on the nature of the vibration. See textbook. Angular magnification. The factor by which an image's apparent angular size is increased (or decreased). Cf. magnification. See textbook. Angular momentum. A measure of rotational motion; a conserved quantity for a closed system. See textbook. Atom. The basic unit of one of the chemical elements. See textbook. Atomic mass. The mass of an atom. See textbook. Atomic number. The number of protons in an atom's nucleus; determines what element it is. See textbook. Attractive. Describes a force that tends to pull the two participating objects together. Cf. repulsive, oblique. See textbook. 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. See textbook. Beta decay. The radioactive decay of a nucleus via the reaction n -> p + e- + or p -> n + e+ + n; so called because an electron or antielectron is also known as a beta particle. See textbook. Beta particle. A form of radioactivity consisting of electrons. See textbook. Cathode ray. The mysterious ray that emanated from the cathode in a vacuum tube; shown by Thomson to be a stream of particles smaller than atoms. See textbook. Center of mass. The balance point of an object. See textbook. Charge. A numerical rating of how strongly an object participates in electrical forces. See textbook. 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. See textbook. Coherent. A light wave whose parts are all in phase with each other. See textbook. Collision. An interaction between moving objects that lasts for a certain time. See textbook. Component. The part of a velocity, acceleration, or force that is along one particular coordinate axis. See textbook. Concave. Describes a surface that is hollowed out like a cave. See textbook. Convex. Describes a surface that bulges outward. See textbook. Coulomb (C). The unit of electrical charge. See textbook. Current. The rate at which charge crosses a certain boundary. See textbook. Damping. the dissipation of a vibration's energy into heat energy, or the frictional force that causes the loss of energy.See textbook. Diffraction. The behavior of a wave when it encounters an obstacle or a nonuniformity 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. See textbook. Diffuse reflection. Reflection from a rough surface, in which a single ray of light is divided up into many weaker reflected rays going in many directions. See textbook. Displacement. (avoided in this book) A name for the symbol delta-x . See textbook. Driving force. An external force that pumps energy into a vibrating system. See textbook. 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. See textbook. Electric field. The force per unit charge exerted on a test charge at a given point in space. See textbook. Electrical force. One of the fundamental forces of nature; a noncontact force that can be either repulsive or attractive. See textbook. Electron. Thomson's name for the particles of which a cathode ray was made; a subatomic particle. See textbook. 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). See textbook. Equilibrium. A state in which an object's momentum and angular momentum are constant. See textbook. Field. A property of a point in space describing the forces that would be exerted on a particle if it was there. See textbook. Fission. The radioactive decay of a nucleus by splitting into two parts. See textbook. Fluid. A gas or a liquid. See textbook. Fluid friction. A friction force in which at least one of the object is is a fluid (i.e. either a gas or a liquid). See textbook. 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. See textbook. Frequency. The number of cycles per second, the inverse of the period (q.v.). See textbook. Fusion. A nuclear reaction in which two nuclei stick together to form one bigger nucleus. See textbook. Gamma ray. Aform of radioactivity consisting of a very high-frequency form of light. See textbook. Gravitational field. The force per unit mass exerted on a test mass at a given point in space. See textbook. 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. See textbook. Half-life. The amount of time that a radioactive atom has a probability of 1/2 of surviving without decaying.See textbook. 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. See textbook. 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. See textbook. 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 question. See textbook. Independence. The lack of any relationship between two random events.See textbook. Induction. The production of an electric field by a changing magnetic field, or vice-versa. See textbook.
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. See
textbook.
Invariant. A quantity that does not change when transformed.See
textbook.
Ion. An electrically charged atom or molecule. See textbook.
Isotope. One of the possible varieties of atoms of a given element, having a certain number of
neutrons. See textbook.
Kinetic energy. The energy an object possesses because of its motion. Cf. potential energy. See textbook.
Light. Anything that can travel from one place to another through empty space and can influence
matter, but is
not affected by gravity. See textbook.
Lorentz transformation. The transformation between frames in relative motion.See textbook.
Magnetic dipole. An object, such 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 of current. See textbook.
Magnetic field. A field of force, defined in terms of the torque exerted on a test dipole. See textbook.
Magnification. The factor by which an image's linear size is increased (or decreased). Cf. angular
magnification. See textbook.
Magnitude. The "amount" associated with a vector; the vector stripped of any information about its
direction. See textbook.
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.) See
textbook.
Mass number. The number of protons plus the number of neutrons in a nucleus; approximately
proportional to its atomic mass. See textbook.
Matter. Anything that is affected by gravity. See textbook.
Millirem. A unit for measuring a person's exposure to radioactivity; cf rem. See textbook.
Mks system. The use of metric units based on the meter, kilogram, and second. Example: meters per
second is the
mks unit of speed, not cm/s or km/hr. See textbook.
Molecule. A group of atoms stuck together. See textbook.
Momentum. A measure of motion, equal to mv for material objects. See
textbook.
Neutron. An uncharged particle, the other types that nuclei are made of. See textbook.
Noninertial frame. An accelerating frame of reference, in which Newton's first law is violated. See textbook.
Nonuniform circular motion. Circular motion in which the magnitude of the velocity vector changes. See textbook.
Normal force. The force that keeps two objects from occupying the same space. See
textbook.
Normalization. The property of probabilities that the sum of the probabilities of all possible
outcomes must equal one.See textbook.
Oblique. Describes a force that acts at some other angle, one that is not a direct repulsion or
attraction. Cf.
attractive, repulsive. See textbook.
Ohm. The metric unit of electrical resistance, one volt per ampere. See
textbook.
Ohmic. Describes a substance in which the flow of current between two points is proportional to the
voltage difference between them. See textbook.
Open circuit. A circuit that does not function because it has a gap in it. See textbook.
Operational definition. A definition that states what operations should be carried out to measure the
thing being
defined. See textbook.
Parabola. The mathematical curve whose graph has y proportional to x2. See
textbook.
Period. The time required for one cycle of a periodic motion (q.v.). See
textbook.
Periodic motion. Motion that repeats itself over and over. See
textbook.
Photon. A particle of light.See textbook.
Photoelectric effect. The ejection, by a photon, of an electron from the surface of an object.See textbook.
Potential energy. The energy having to do with the distance between to objects that interact via a
noncontact
force. Cf. Kinetic energy. See textbook.
Power. The rate of transferring energy; a scalar quantity with units of watts (W). See
textbook.
Probability. The likelihood that something will happen, expressed as a number between zero and one.See textbook.
Probability distribution. A curve that specifies the probabilities of various random values of a
variable; areas under the curve correspond to probabilities.See textbook.
Proton. A positively charged particle, one of the types that nuclei are made of. See textbook.
Quality factor. The number of oscillations required for a system's energy to fall off by a factor of
535 due to
damping. See textbook.
Quantized. Describes quantity such as money or electrical charge, that can only exist in certain
amounts. See textbook.
Quantum number. A numerical label used to classify a quantum state.See
textbook.
Radial. Parallel to the radius of a circle; the in-out direction. Cf. tangential. See
textbook.
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. virtual image. See textbook.
Reflection. What happens when light hits matter and bounces off, retaining at least some of its
energy. See textbook.
Refraction. The change in direction that occurs when a wave encounters the interface between two
media. See textbook.
Rem. A unit for measuring a person's exposure to radioactivity; cf millirem. See textbook.
Repulsive. Describes a force that tends to push the two participating objects apart. Cf. attractive,
oblique. See textbook.
Resistance. The ratio of the voltage difference to the current in an object made of an ohmic
substance. See textbook.
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. See textbook.
Rest mass. Referred to as mass in this book; written as m0 in some books. Cf. mass.See textbook.
Scalar. A quantity that has no direction in space, only an amount. Cf. vector. See
textbook.
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 something useful. See textbook.
Significant figures. Digits that contribute to the accuracy of a measurement. See
textbook.
Simple harmonic motion. Motion whose x-t graph is a sine wave. See
textbook.
Sink. A point at which field vectors converge. See textbook.
Source. A point from which field vectors diverge; often used more inclusively to refer to points of
either convergence or divergence. See textbook.
Specular reflection. Reflection from a smooth surface, in which the light ray leaves at the same
angle at which it came in. See textbook.
Speed. (avoided in this book) The absolute value of or, in more then one dimension, the magnitude of
the
velocity, i.e. the velocity stripped of any information about its direction
Spring constant. The constant of proportionality between force and elongation of a spring or other
object under
strain. See textbook.
Spin. The built-in angular momentum possessed by a particle even when at rest.See textbook.
Stable equilibrium. One in which a force always acts to bring the object back to a certain point. See textbook.
Static friction. A friction force between surfaces that are not slipping past each other. See textbook.
Steady state. The behavior of a vibrating system after it has had plenty of time to settle into a
steady response
to a driving force. In the steady state, the same amount of energy is pumped into the system during each
cycle as is
lost to damping during the same period. See textbook.
Strong nuclear force. The force that holds nuclei together against electrical repulsion. See textbook.
Systeme International. Fancy name for the metric system. See
textbook.
Tangential. Tangent to a curve. In circular motion, used to mean tangent to the circle, perpendicular
to the
radial direction Cf. radial. See textbook.
Temperature. What a thermometer measures. Objects left in contact with each other tend to reach the
same
temperature. Roughly speaking, temperature measures the average kinetic energy per molecule. For the
distinction between
temperature and heat, see the glossary entry for heat. See textbook.
Thermal energy. Careful writers make a distinction between heat and thermal energy, but the
distinction is often
ignored in casual speech, even among physicists. 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. The term
heat is used in
this book to include both meanings. See textbook.
Torque. The rate of change of angular momentum; a numerical measure of a force's ability to twist on
an object.
See textbook.
Transformation. The mathematical relationship between the variables such as x and t, as observed in
different frames of reference.See textbook.
Uniform circular motion. Circular motion in which the magnitude of the velocity vector remains
constant.See textbook.
Vector. A quantity that has both an amount (magnitude) and a direction in space. Cf. scalar. See textbook.
Unstable equilibrium. One in which any deviation of the object from its equilibrium position results
in a force
pushing it even farther away. See textbook.
Velocity. The rate of change of position; the slope of the tangent line on an x-t graph. See textbook.
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. See textbook.
Volt. The metric unit of voltage, one joule per coulomb. See
textbook.
Voltage. Electrical potential energy per unit charge that will be possessed by a charged particle at
a certain point in space. See textbook.
Voltmeter. A device for measuring voltage differences. See textbook.
Wave-particle duality. The idea that light is both a wave and a particle.See textbook.
Wavefunction. The numerical measure of an electron wave, or in general of the wave corresponding to
any quantum mechanical particle.See textbook.
Weak nuclear force. The force responsible for beta decay. See
textbook.
Weight. The force of gravity on an object, equal to mg. See textbook.
Work. The amount of energy transferred into or out of a system, excluding energy transferred by heat
conduction.
See textbook.
|
|
What is lightandmatter.com? This site offers physics textbooks that you can download for free or buy in print. The books have been adopted at 15 colleges and universities, and 15 high schools. If you're having a hard time finding something, the best place to start is probably the site map. |
|
|
(c) Copyright 1998 Benjamin Crowell. All rights reserved. |
|||||
