Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....


absorptionthe gradual conversion of wave energy into heating of the medium; what happens when light hits matter and gives up some of its energy.
accelerationThe rate of change of velocity; the slope of the tangent line on a v - t graph.
alpha decaythe radioactive decay of a nucleus via emission of an alpha particle
alpha particlea form of radioactivity consisting of helium nuclei
ammetera device for measuring electrical current
amperethe metric unit of current, one coulomb pe second; also "amp"
amplitudethe amount of vibration, often measured from the center to one side; may have different units depending on the nature of the vibration
Angular magnificationThe factor by which an image's apparent angular size is increased (or decreased). Cf. magnification.
angular momentuma measure of rotational motion; a conserved quantity for a closed system
atomthe basic unit of one of the chemical elements
atomic massthe mass of an atom
atomic numberthe number of protons in an atom's nucleus; determines what element it is
attractivedescribes a force that tends to pull the two participating objects together
axisAn 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.
beta decaythe radioactive decay of a nucleus via the reaction n → p + e- + ν[bar] or p → n + e+ + ν; so called because an electron or antielectron is also known as a beta particle
beta particlea form of radioactivity consisting of electrons
cathode raythe mysterious ray that emanated from the cathode in a vacuum tube; shown by Thomson to be a stream of particles smaller than atoms
center of massthe balance point of an object
center of massthe balance point or average position of the mass in a system
chargea numerical rating of how strongly an object participates in electrical forces
circuitan electrical device in which charge can come back to its starting point and be recycled rather than getting stuck in a dead end
coherentA light wave whose parts are all in phase with each other.
collisionan interaction between moving objects that lasts for a certain time
componentthe part of a velocity, acceleration, or force that would be perceptible to an observer who could only see the universe projected along a certain one-dimensional axis
concaveDescribes a surface that is hollowed out like a cave.
conic sectiona curve formed by the intersection of a plane and an infinite cone
convexDescribes a surface that bulges outward.
coulomb (C)the unit of electrical charge
currentthe rate at which charge crosses a certain boundary
dampingthe dissipation of a vibration's energy into heat energy, or the frictional force that causes the loss of energy
diffractionThe 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.
diffuse reflectionReflection from a rough surface, in which a single ray of light is divided up into many weaker reflected rays going in many directions.
Doppler effectthe change in a wave's frequency and wavelength due to the motion of the source or the observer or both
driving forcean external force that pumps energy into a vibrating system
elastic collisionone in which no KE is converted into other forms of energy
electric dipolean 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
electric fieldthe force per unit charge exerted on a test charge at a given point in space
electrical forceone of the fundamental forces of nature; a noncontact force that can be either repulsive or attractive
electronThomson's name for the particles of which a cathode ray was made
ellipsea flattened circle; one of the conic sections
energyA 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).
equilibriuma state in which an object's momentum and angular momentum are constant
fielda property of a point in space describing the forces that would be exerted on a particle if it was there e
fissionthe radioactive decay of a nucleus by splitting into two parts
fluid frictiona friction force in which at least one of the object is is a fluid
fluida gas or a liquid
focal lengthA 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.
focusone of two special points inside an ellipse: the ellipse consists of all points such that the sum of the distances to the two foci equals a certain number; a hyperbola also has a focus
frequencythe number of cycles per second, the inverse of the period
fusiona nuclear reaction in which two nuclei stick together to form one bigger nucleus
FWHMthe full width at half-maximum of a probability distribution; a measure of the width of the distribution
gamma raya form of radioactivity consisting of a very high-frequency form of light
gravitational fieldthe force per unit mass exerted on a test mass at a given point in space
gravityA 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-lifethe amount of time that a radioactive atom will survive with probability 1/2 without decaying
heatA form of energy that relates to temperature. Heat is different from temperature because an object with twice as much mass requires twice as much heat to increase its temperature by the same amount. Heat is measured in joules, temperature in degrees. (In standard terminology, there is another, finer distinction between heat and thermal energy, which is discussed in "A Numerical Scale of Energy". In this book, I informally refer to both as heat).
hyperbolaanother conic section; it does not close back on itself
imageA 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.
impulse, I, Jthe amount of momentum transferred, Δp
independencethe lack of any relationship between two random events
Index of refractionAn optical property of matter; the speed of light in a vacuum divided by the speed of light in the substance in question.
inductionthe production of an electric field by a changing magnetic field, or vice-versa
inelastic collisionone in which some KE is converted to other forms of energy
inertial framea frame of reference that is not accelerating, one in which Newton's first law is true
ionan electrically charged atom or molecule
isotopeone of the possible varieties of atoms of a given element, having a certain number of neutrons
kinetic energyThe energy an object possesses because of its motion.
kinetic frictiona friction force between surfaces that are slipping past each other
lightAnything that can travel from one place to another through empty space and can influence matter, but is not affected by gravity.
magnetic dipolean 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
magnetic fielda field of force, defined in terms of the torque exerted on a test dipole
magnificationThe factor by which an image's linear size is increased (or decreased). Cf. angular magnification.
magnitudethe "amount" associated with a vector
mass numberthe number of protons plus the number of neutrons in a nucleus; approximately proportional to its atomic mass
massA numerical measure of how difficult it is to change an object's motion.
matterAnything that is affected by gravity.
mediuma physical substance whose vibrations constitute a wave
millirema unit for measuring a person's exposure to radioactivity
mks systemThe 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.
moleculea group of atoms stuck together
moment of inertia, Ithe proportionality constant in the equation L = 2πI/T
momentuma measure of motion, equal to mv for material objects
neutronan uncharged particle, the other types that nuclei are made of
noninertial framean accelerating frame of reference, in which Newton's first law is violated
nonuniform circular motioncircular motion in which the magnitude of the velocity vector changes
normal forcethe force that keeps two objects from occupying the same space
normalizationthe property of probabilities that the sum of the probabilities of all possible outcomes must equal one
obliquedescribes a force that acts at some other angle, one that is not a direct repulsion or attraction
ohmthe metric unit of electrical resistance, one volt per ampere
ohmicdescribes a substance in which the flow of current between two points is proportional to the voltage difference between them
open circuita circuit that does not function because it has a gap in it
operational definitionA definition that states what operations should be carried out to measure the thing being defined.
parabolathe mathematical curve whose graph has y proportional to x2
periodthe time required for a planet to complete one orbit; more generally, the time for one repetition of some repeating motion
periodthe time required for one cycle of a periodic motion
periodic motionmotion that repeats itself over and over
photoelectric effectthe ejection, by a photon, of an electron from the surface of an object
photona particle of light
potential energythe energy having to do with the distance between to objects that interact via a noncontact force
powerThe rate of transferring energy; a scalar quantity with units of watts (W).
probability distributiona curve that specifies the probabilities of various random values of a variable; areas under the curve correspond to probabilities
probabilitythe likelihood that something will happen, expressed as a number between zero and one
protona positively charged particle, one of the types that nuclei are made of
quality factorthe number of oscillations required for a system's energy to fall off by a factor of 535 due to damping
quantizeddescribes quantity such as money or electrical charge, that can only exist in certain amounts
quantum numbera numerical label used to classify a quantum state
radialparallel to the radius of a circle; the in-out direction
real imageA 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.
reflectionthe bouncing back of part of a wave from a boundary; what happens when light hits matter and bounces off, retaining at least some of its energy.
refractionThe change in direction that occurs when a wave encounters the interface between two media.
remA unit for measuring a person's exposure to radioactivity; cf millirem.
repulsivedescribes a force that tends to push the two participating objects apart
resistancethe ratio of the voltage difference to the current in an object made of an ohmic substance
resonancethe tendency of a vibrating system to respond most strongly to a driving force whose frequency is close to its own natural frequency of vibration
scalara quantity that has no direction in space, only an amount
short circuita 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
significant figuresDigits that contribute to the accuracy of a measurement.
simple harmonic motionmotion whose x-t graph is a sine wave
sinka point at which field vectors converge
sourcea point from which field vectors diverge; often used more inclusively to refer to points of either convergence or divergence
specular reflectionReflection from a smooth surface, in which the light ray leaves at the same angle at which it came in.
spinthe built-in angular momentum possessed by a particle even when at rest
spring constantthe constant of proportionality between force and elongation of a spring or other object under strain
stable equilibriumone in which a force always acts to bring the object back to a certain point
standing wavea wave pattern that stays in one place
static frictiona friction force between surfaces that are not slipping past each other
steady statethe behavior of a vibrating system after it has had plenty of time to settle into a steady response to a driving force
strong nuclear forcethe force that holds nuclei together against electrical repulsion
superpositionthe adding together of waves that overlap with each other
Système InternationalA fancy name for the metric system.
tangentialtangent to the circle, perpendicular to the radial direction
temperatureWhat a thermometer measures. Objects left in contact with each other tend to reach the same temperature. Cf. heat. As discussed in more detail in section "Heat is Kinetic Energy", temperature is essentially a measure of the average kinetic energy per molecule.
thermal energyCareful 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.
torquethe rate of change of angular momentum; a numerical measure of a force's ability to twist on an object
transmissionthe continuation of part of a wave through a boundary
uniform circular motioncircular motion in which the magnitude of the velocity vector remains constant
unstable equilibriumone in which any deviation of the object from its equilibrium position results in a force pushing it even farther away
vectora quantity that has both an amount (magnitude) and a direction in space
velocitythe rate of change of position; the slope of the tangent line on an x - t graph.
virtual imageLike 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.
voltthe metric unit of voltage, one joule per coulomb
voltageelectrical potential energy per unit charge that will be possessed by a charged particle at a certain point in space
voltmetera device for measuring voltage differences
wave-particle dualitythe idea that light is both a wave and a particle
wavefunctionthe numerical measure of an electron wave, or in general of the wave corresponding to any quantum mechanical particle
wavelengththe distance in space between repetitions of a periodic wave
weak nuclear forcethe force responsible for beta decay
weightthe force of gravity on an object, equal to mg
workthe amount of energy transferred into or out of a system, excluding energy transferred by heat conduction

Last Update: 2010-11-11