Physics glossary

One page per term, each answering the definitional question directly. Where a term has a full walkthrough elsewhere on the site, the entry links it rather than repeating it.

200 terms

A

Absolute zeroAbsolute zero is 0 kelvin, the zero point of the kelvin scale and the lowest temperature there is. In the kinetic model it is where the average translational kinetic energy of the atoms would fall to zero.Absorption spectrumAn absorption spectrum is a continuous spectrum crossed by dark lines, produced when a substance removes the photons whose energies match its own energy level differences. The pattern of gaps identifies what the light passed through.AccelerationAcceleration is the vector rate at which an object's velocity changes, measured in meters per second squared. Velocity changes if its size changes, if its direction changes, or both, so an object can accelerate while moving at constant speed.Adiabatic processAn adiabatic process is one in which no energy is transferred into or out of a system by thermal processes. With no heating or cooling, the change in internal energy equals the work done on the system.Ampere's lawThe line integral of the magnetic field around any closed path equals the vacuum permeability times the current enclosed by that path. In practice it yields a field only when the geometry is symmetric enough to take B outside the integral.AmplitudeAmplitude is the maximum displacement of an oscillating object or a wave from its equilibrium position. It is a positive quantity, measured in meters for a mechanical displacement, and it sets how much energy the motion carries.Angular accelerationAngular acceleration is the rate at which angular velocity changes with time, measured in radians per second squared. It is nonzero only when a rotation is speeding up or slowing down, not merely because something is going round.Angular displacementAngular displacement is the angle, measured in radians, through which a point on a rigid system rotates about a specified axis. It is the rotational counterpart of linear displacement, and one direction of rotation is chosen as positive before any calculation begins.Angular frequencyAngular frequency is the rate at which the phase of an oscillation advances, measured in radians per second rather than cycles per second. It is 2 pi times the ordinary frequency, so ω = 2πf.Angular impulseAngular impulse is the product of the torque exerted on a system and the time interval during which that torque acts. It equals the change in the system's angular momentum, and it is the only way angular momentum ever changes.Angular momentumAngular momentum measures how much rotation a system carries about a chosen axis. For a rigid body it equals the rotational inertia times the angular velocity, its unit is the kilogram meter squared per second, and its value changes if you change the axis.Angular velocityAngular velocity is the rate at which angular position changes with time, measured in radians per second. Every point on a rigid rotating system shares the same angular velocity, however far from the axis of rotation it sits.AntinodeAn antinode is a point on a standing wave where the oscillation reaches maximum amplitude, because the two counter-propagating waves always reinforce there. Antinodes sit halfway between adjacent nodes.Apparent weightApparent weight is the size of the supporting force acting on a system, which is what a scale under it reads. It differs from the gravitational force whenever the system accelerates or a fluid helps hold it up.Archimedes' principleArchimedes' principle states that the buoyant force on an object in a fluid has the same magnitude as the weight of the fluid the object displaces. It is the reasoning behind the buoyant force equation on the AP sheet.Atmospheric pressureAtmospheric pressure is the pressure the air exerts at Earth's surface, about 1.0 times 10 to the fifth pascals at sea level. It is the reference pressure that sits on top of an open column of liquid.Average velocityAverage velocity is the displacement of an object divided by the time interval in which that displacement occurred. It is a vector, and it equals the mean of the starting and ending velocities only when the acceleration is constant.

B

Beat frequencyBeat frequency is the rate at which the loudness of a combined sound rises and falls when two waves of slightly different frequency overlap. It equals the absolute difference of the two frequencies, not their average.Bernoulli's equationBernoulli's equation says that for an ideal fluid in flow, the sum of pressure, the term rho g y and the term one half rho v squared is the same at any two points. It is conservation of mechanical energy per unit volume.Binding energyBinding energy is the energy needed to pull a bound system apart. AP Physics 2 defines it for an atom, as the energy required to remove an electron, and then uses it for a nucleus, as the energy required to separate the nucleons.Biot-Savart lawThe rule giving the magnetic field contributed by a single short segment of current-carrying wire. Adding up those contributions along the whole wire gives the field, whatever the shape of the wire.Blackbody radiationBlackbody radiation is the continuous spectrum an object emits purely because of its temperature. A blackbody is an idealized body that absorbs all radiation falling on it, and at constant temperature it must emit energy in turn.Bohr modelThe Bohr model pictures an atom as electrons moving in circular orbits around a small positive nucleus, held there by the electric force, with only certain orbits allowed. It was the classical picture that first gave hydrogen discrete energy states.Buoyant forceThe buoyant force is the net upward force a fluid exerts on an object placed in it. Its magnitude equals the weight of the fluid the object displaces, so it depends on the fluid density and the displaced volume.

C

CapacitanceCapacitance is the charge stored on one plate of a capacitor divided by the potential difference between the plates, measured in farads. It is fixed by the geometry of the device and the material in the gap, so storing more charge does not change it.Center of massThe single point at which a system's mass can be treated as concentrated, located at the mass weighted average position of the system's parts. A whole system can be modeled as one object sitting at this point.Centrifugal forceCentrifugal force is the apparent outward force that something moving in a circle seems to feel. It is not a real force and never belongs on a free-body diagram, because no object exerts it; the net force in circular motion points inward, toward the center.Centripetal accelerationThe component of an object's acceleration directed toward the center of its circular path. Its magnitude is the tangential speed squared divided by the radius, and it is present whenever the direction of the velocity changes.Charging by inductionCharging by induction gives an object a net charge without ever touching it with a charged object. A nearby charge polarizes it, a ground connection lets charge escape, and breaking the ground first leaves a permanent charge opposite in sign to the inducing charge.Coefficient of frictionA dimensionless number, written mu, that sets how much friction a given pair of surfaces can produce for a given normal force. It is a property of the two surfaces together rather than of one object, and it has no units because it is a ratio of two forces.Coefficient of restitutionThe coefficient of restitution, written e, is the ratio of the relative speed at which two objects separate after a collision to the relative speed at which they approached. It runs from 1 for an elastic collision down to 0 when the objects stick together.Compton scatteringIn Compton scattering a photon collides with a free electron and comes away with lower energy and a longer wavelength. How much longer depends only on the angle through which the photon's direction turned.Concave mirrorA mirror curved so that its reflecting surface caves inward, bringing parallel rays toward a real focal point in front of it. Its focal length is positive and equal in size to half the radius of curvature.ConductionConduction is the transfer of energy through a material by collisions between neighboring atoms, with no bulk movement of the material itself. It requires matter, so it cannot cross a vacuum.ConductorA conductor is a material whose charge carriers move easily, so charge placed on it redistributes instead of staying put. In electrostatic equilibrium all excess charge sits on the outer surface and the electric field inside the conducting material is zero.Conservative forceA force whose work depends only on the initial and final configurations of a system, never on the path between them, and which does zero net work when the system returns to where it started. Every conservative force has a potential energy attached to it.Constructive interferenceConstructive interference happens where overlapping waves displace the medium in the same direction, so the combined displacement is larger than either wave alone. It occurs where the waves arrive in phase.Continuity equationThe continuity equation states that an incompressible fluid filling a pipe carries the same product of cross sectional area and speed everywhere along it. A narrower section therefore forces the fluid to move faster.ConvectionConvection is the transfer of energy by the bulk movement of a fluid that carries its internal energy along with it. AP Physics 2 names it as one of three thermal processes and treats it qualitatively.Converging lensA lens that is thicker at its centre than at its edges and bends parallel rays inward to a real focal point on the far side. Its focal length is positive, and it forms a real image whenever the object lies beyond the focal point.Convex mirrorA mirror curved so that its reflecting surface bulges outward, spreading parallel rays so they appear to come from a focal point behind it. Its focal length is negative and every image it forms is virtual, upright and reduced.Cross productThe cross product of two vectors is a third vector whose magnitude is the product of their magnitudes times the sine of the angle between them, pointing perpendicular to both in the direction the right hand rule gives.

D

De Broglie wavelengthEvery moving particle has a de Broglie wavelength equal to Planck's constant divided by its momentum. It grows as momentum falls, and wave behavior only matters when the wavelength is comparable to the size of the system.DensityDensity is how much mass a substance packs into each unit of volume, written as the ratio of mass to volume and measured in kilograms per cubic metre. It is the property that decides whether an object floats in a given fluid.Destructive interferenceDestructive interference happens where overlapping waves displace the medium in opposite directions, so the combined displacement shrinks. Complete cancellation also requires the two amplitudes to be equal.DielectricA dielectric is an insulating material placed between the plates of a capacitor. It polarizes in the applied field and sets up a field opposing it, which weakens the net field in the gap and multiplies the capacitance by the dielectric constant kappa.DiffractionThe spreading of a wave around the edges of an obstacle or as it passes through an opening. The effect grows as the size of the opening approaches the wavelength of the wave, and it is negligible when the opening is much wider.Diffraction gratingA surface ruled with many evenly spaced parallel slits. It produces maxima at the same angles a double slit of the same spacing would, but each maximum is far narrower and brighter, which is what makes a grating a measuring instrument.Dimensional analysisDimensional analysis is checking or building an equation by tracking the units on both sides. If the units do not match, the equation is wrong, which makes it the fastest way to eliminate multiple choice options.DisplacementDisplacement is the vector change in an object's position, measured straight from the starting point to the ending point. It has both magnitude and direction, does not depend on the route taken, and is zero for any trip that ends where it began.Diverging lensA lens that is thinner at its centre than at its edges and spreads parallel rays outward, so they appear to come from a focal point on the incoming side. Its focal length is negative and it never forms a real image.Doppler effectThe Doppler effect is the change in the frequency an observer measures when a wave source and the observer move relative to each other. Approaching raises the observed frequency, receding lowers it.Dot productThe dot product of two vectors is a scalar equal to the product of their magnitudes times the cosine of the angle between them. It extracts the part of one vector lying along the other, which is why work carries a cosine.Drag forceA velocity dependent force exerted on an object moving through a fluid, directed opposite to the object's velocity. AP Physics C: Mechanics models it as proportional to speed, which makes the resulting motion exponential in time.

E

Elastic collisionAn elastic collision is one in which the total kinetic energy of the system after the collision equals the total kinetic energy before it. Momentum is conserved too, but momentum is conserved in every collision, so kinetic energy is the test that identifies this case.Elastic potential energyThe energy stored in a stretched or compressed ideal spring, equal to one half the spring constant times the square of the displacement from the spring's natural length.Electric chargeElectric charge is the property of matter that makes it experience and produce electric forces. It comes in two signs, positive and negative, is measured in coulombs, and is quantized: every net charge is a whole-number multiple of the elementary charge.Electric currentElectric current is the rate at which charge passes through a cross section of a conductor, measured in amperes, where one ampere is one coulomb per second. It is not a vector, but it has a direction: conventional current is the way positive charge would move.Electric fieldThe electric field at a point is the electric force per unit charge that a positive test charge placed there would feel. It is a vector measured in newtons per coulomb, and it exists at the point whether or not a charge is there to feel it.Electric fluxElectric flux measures how much electric field passes through a given area. For a field that is constant across the area it is the dot product of the field and the area vector, and in general it is the surface integral of the field over the surface.Electric potentialElectric potential is the electric potential energy per unit charge at a point in space, measured in volts, where one volt is one joule per coulomb. It is a scalar set by the source charges, so it describes the point rather than anything you place there.Electric potential energyElectric potential energy is the energy a configuration of charges holds because of where the charges sit, measured in joules. For two point charges it is the Coulomb constant times the product of the signed charges divided by their separation, with zero taken at infinity.Electric powerElectric power is the rate at which a circuit element transfers, converts or dissipates energy, measured in watts. The AP sheet prints it as P = IΔV, the current in the element multiplied by the potential difference across it.Electromagnetic waveAn electromagnetic wave is a transverse wave made of mutually perpendicular oscillating electric and magnetic fields. It needs no medium, and in vacuum every electromagnetic wave travels at the speed of light.Electromotive forceElectromotive force, or emf, is the potential difference a source such as a battery maintains in order to drive charge around a circuit. Despite the name it is not a force: it is measured in volts, and it equals the terminal potential difference when no current flows.Electron voltAn electron volt is a unit of energy, not of voltage. It is the energy an electron gains crossing a potential difference of one volt, equal to 1.60 times 10 to the minus 19 joules.Emission spectrumAn emission spectrum is the set of bright lines an excited gas gives off, one line for each downward transition between two of its energy levels. The pattern of frequencies is unique to the element, so it identifies the source.Energy levelAn energy level is one of the discrete energies an electron in an atom is allowed to have. The lowest is the ground state, anything above it is an excited state, and levels are drawn as negative numbers because a bound electron needs energy added to escape.EntropyEntropy is a state function that tracks how spread out a system's energy is. AP Physics 2 describes it qualitatively as the tendency of energy to disperse, or the unavailability of some of a system's energy to do work.EquilibriumThe condition in which the net force on a system is zero, so the system's acceleration is zero. Constant nonzero velocity counts: an object cruising in a straight line at steady speed is in equilibrium exactly as much as one sitting still.Equipotential surfaceAn equipotential surface is a surface on which the electric potential has the same value at every point. Electric field lines cross it at right angles, and moving a charge along it does zero work because the potential difference between any two of its points is zero.Experimental uncertaintyExperimental uncertainty is the range within which a measured value plausibly lies, reported as a value plus or minus a range. It is a property of the measurement itself, not evidence that somebody made a mistake.

F

Faraday's lawThe law of induction: the emf induced around a loop equals the rate at which the magnetic flux through that loop changes. A steady magnetic field, however strong, induces nothing at all.First law of thermodynamicsThe first law of thermodynamics is conservation of energy for a system that can be heated, cooled or worked on: the change in internal energy equals the energy added by heating plus the work done on the system.Focal lengthThe signed distance from a mirror or lens to its focal point. It is positive for a converging optic and negative for a diverging one, and it is the quantity f in the thin lens and mirror equation.Free fallFree fall is motion in which gravity is the only force acting, so the object accelerates downward at the local value of g whichever way it happens to be moving. A ball thrown straight up is in free fall the whole way, including at the top.Free-Body DiagramA free-body diagram shows one chosen object as a point or box with an arrow for every force exerted on it by something else, and nothing else on the page. It is the step that turns a described situation into an equation.FrequencyFrequency is the number of complete cycles a wave or an oscillator goes through each second. Its unit is the hertz, one cycle per second, and it is the reciprocal of the period.Fundamental frequencyThe fundamental frequency is the lowest frequency at which a standing wave can be set up in a given region. It corresponds to the longest wavelength the boundary conditions allow, and it is also called the first harmonic.

G

Gauge pressureGauge pressure is the amount by which the pressure at a point exceeds the surrounding atmospheric pressure. Absolute pressure is gauge pressure plus atmospheric pressure, so the two are never interchangeable.Gaussian surfaceA Gaussian surface is an imaginary three-dimensional closed surface drawn in space so that Gauss's law can be applied to it. Only the charge enclosed by the surface contributes to the net flux through it; charge outside contributes exactly zero.Gravitational fieldThe gravitational force per unit mass at a point in space, measured in newtons per kilogram. A distance r from a mass M its magnitude is G times M divided by r squared, and it points toward the mass that creates it.Gravitational potential energyThe energy stored in the configuration of two masses that attract each other gravitationally. Near a planet's surface it changes by mass times g times the change in height; in general it equals minus G times the product of the masses divided by their separation.

H

Half-lifeHalf-life is the time it takes for half the radioactive nuclei in a sample to decay. It is fixed for a given isotope and does not depend on how much you start with, so a quarter of the original nuclei survive two half-lives whatever the sample size.HeatHeat is energy transferred into or out of a system because of a temperature difference. It names a transfer, not a stored quantity, so an object cannot contain heat any more than it can contain work.Hooke's LawHooke's law is the rule that an ideal spring exerts a force proportional to how far it has been stretched or compressed from its relaxed length, directed opposite to that stretch. In symbols, F = -k Δx.

I

Ideal fluidAn ideal fluid is one that is incompressible and has no viscosity, so its density never changes and no energy is lost to internal friction. AP Physics 1 assumes every fluid is ideal unless a question says otherwise.Ideal gas lawThe ideal gas law states that PV = nRT = N k_B T, linking the pressure, volume, amount and absolute temperature of a gas. Both forms are printed on the AP Physics 2 equation sheet, and T must be in kelvin.ImpulseImpulse is the product of the average force exerted on a system and the time interval during which that force acts. It is a vector pointing along the net force, measured in newton seconds, and it equals the area under a force versus time graph.Index of refractionA dimensionless number for a medium, equal to the speed of light in vacuum divided by the speed of light in that medium. It is 1 in vacuum and greater than 1 in any ordinary transparent material.InductanceThe tendency of a conductor to oppose a change in the electric current through it, measured in henries. A circuit element built to have a large inductance is an inductor, usually a coil.Inelastic collisionAn inelastic collision is one in which the total kinetic energy of the system decreases. Momentum is still conserved; the kinetic energy that disappears is transformed into heat, sound and permanent deformation rather than destroyed.InertiaInertia is a system's resistance to a change in its velocity. Mass is the measure of it: the greater the mass, the smaller the acceleration a given net force produces.InsulatorAn insulator is a material whose charge carriers cannot move easily, so charge stays where it is placed rather than spreading over the surface. Glass, rubber, plastic and dry air are insulators; they polarize in a field but do not conduct.Internal energyInternal energy is the sum of the kinetic energies of the objects making up a system and the potential energy of their configuration. For an ideal gas it depends on temperature and on nothing else.Inverse square lawAn inverse square law is any relationship in which a quantity falls off as one over the distance squared. Doubling the separation cuts the quantity to one quarter of its value, and tripling it cuts it to one ninth.Isobaric processAn isobaric process holds the pressure of a system constant. The work done on the gas is minus the pressure times the change in volume, and the path on a pressure-volume diagram is horizontal.Isochoric processAn isochoric process holds the volume of a system constant. No work is done on or by the gas, so the change in internal energy equals the energy transferred by heating alone.Isolated systemA system whose boundary nothing crosses: no net external force acts on it and no external work is done on it. Every conservation statement in AP Physics is a claim about a system chosen to meet one of those conditions.Isothermal processAn isothermal process holds the temperature of a system constant. For an ideal gas that fixes the internal energy, so the energy transferred by heating and the work done on the gas cancel exactly.

K

Kinetic energyThe energy an object has because it is moving, equal to one half its mass times the square of its speed. It is a scalar, it is never negative, and it quadruples when the speed doubles.Kinetic frictionThe friction force between two surfaces that are sliding across each other. Its magnitude is the coefficient of kinetic friction times the normal force, and it acts opposite to the motion of each surface relative to the other.Kirchhoff's junction ruleKirchhoff's junction rule states that the total current entering any junction in a circuit equals the total current leaving it. It is conservation of electric charge: charge does not pile up at a point, so whatever arrives each second must depart each second.Kirchhoff's loop ruleKirchhoff's loop rule states that the potential differences across all the elements in any closed loop of a circuit sum to zero. It is conservation of energy applied to a charge carried once around the loop and back to where it started.

L

Lenz's lawThe direction rule for induction: an induced current flows so that the magnetic field it creates opposes the change in magnetic flux that produced it. It is the source of the minus sign in Faraday's law.Lever armThe lever arm is the perpendicular distance from the axis of rotation to the line of action of a force. Multiplying it by the magnitude of the force gives the torque, which is why a force aimed straight at the axis produces no turning effect at all.LinearizationLinearization is choosing what to plot so that a curved relationship comes out as a straight line, then reading a physical constant off the slope. AP asks for it by name in the Experimental Design and Analysis question.Longitudinal waveA longitudinal wave is one in which the disturbance is parallel to the direction the wave travels, producing regions of compression and rarefaction. Sound is the AP example, and longitudinal waves cannot be polarized.

M

Magnetic fieldA vector field, measured in tesla, that determines the magnetic force on moving charges, on electric currents and on magnetic materials. Its field lines form closed loops, and it is produced by dipoles, never by an isolated magnetic pole.Magnetic fluxThe amount of magnetic field passing through a surface, equal to the field magnitude times the area times the cosine of the angle between the field and the area vector. Measured in webers. Its change, not its size, is what drives induction.Magnetic forceThe force a magnetic field exerts on a moving charge or on a current-carrying wire. It is always perpendicular to both the field and the direction of motion, it is zero for a charge at rest, and it never does work.MagnificationThe ratio of image height to object height for a mirror or lens. A magnitude above 1 means the image is enlarged, below 1 means reduced, and the ratio can also be found from the image and object distances.MassMass is the scalar measure of how much matter a system contains, in kilograms. It plays two separate roles in physics: resistance to acceleration, and the source of gravitational attraction.Mass-energy equivalenceMass-energy equivalence is the statement that mass and energy are two accountings of one conserved thing, linked by E = mc squared. In a nuclear reaction the drop in total mass is exactly the energy released.Mechanical energyThe sum of a system's kinetic and potential energies. It stays constant only when the work done on the system is zero and there are no nonconservative interactions inside the system.Metric prefixesMetric prefixes are the multipliers attached to SI units, such as kilo for a thousand and milli for a thousandth. The AP tables print the set from pico to tera, and each one must be converted away before substitution.MomentumMomentum is the product of an object's mass and its velocity, p = mv. It is a vector pointing the same way as the velocity and is measured in kilogram meters per second, so in one dimension its sign records which way the object is moving.

N

Net ForceThe net force on a system is the vector sum of all the forces exerted on it. Zero net force does not mean no forces: a book resting on a table has two forces on it and a net force of zero.Newton's first lawNewton's first law states that if the net force on a system is zero, the velocity of that system stays constant. Rest is only the special case where that constant is zero; moving in a straight line at steady speed obeys the law just as well.Newton's law of universal gravitationThe rule that any two masses attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers of mass. The force acts along the line joining those centers.Newton's second lawNewton's second law says the acceleration of a system's center of mass is proportional to the net force exerted on it and points in the same direction as that net force. In symbols, net force equals mass times acceleration.Newton's Third LawNewton's third law says that when two objects interact, each exerts a force on the other equal in magnitude and opposite in direction. The two forces of the pair act on different objects, which is why they never cancel each other.NodeA node is a point on a standing wave whose displacement is always zero, because the two counter-propagating waves cancel there at every instant. Adjacent nodes lie half a wavelength apart.Nonconservative forceA force whose work depends on the path an object takes, so no potential energy function can be defined for it. Friction and air resistance are the examples the AP course names.Normal ForceThe normal force is the push a surface exerts on an object touching it, directed perpendicular to that surface. It is whatever size it needs to be to stop the object passing through, so it is not always equal to the weight.Nuclear fissionNuclear fission is the process in which the nucleus of an atom splits into two or more smaller nuclei, along with subatomic particles. Energy is released when the products are more tightly bound per nucleon, so the total mass falls.Nuclear fusionNuclear fusion is the process in which two or more smaller nuclei combine into a larger nucleus, along with subatomic particles. It releases energy when the product is more tightly bound per nucleon than the nuclei that went in.

O

Orbital velocityThe speed required to hold a circular orbit at a given radius, equal to the square root of G times the central mass divided by the orbital radius. It does not depend on the mass of the orbiting satellite.

P

Parallel axis theoremThe parallel axis theorem gives the rotational inertia about any axis parallel to one through the center of mass: add the total mass times the square of the distance between the two axes. The reference axis must pass through the center of mass.Pascal's principlePascal's principle states that a pressure change applied to an enclosed fluid is transmitted undiminished to every point in the fluid and to the walls of its container. A hydraulic lift is the standard payoff.Percent errorPercent error is the difference between a measured value and an accepted value, divided by the accepted value and expressed as a percentage. It requires a known reference value, which is what separates it from percent difference.Perfectly inelastic collisionA perfectly inelastic collision is one in which the objects stick together and move off with a single common velocity. It loses the most kinetic energy that any collision with that momentum can lose, which is not the same as losing all of it.PeriodPeriod is the time one full cycle takes: one complete oscillation of an object, or one complete repetition of a wave at a fixed point. It is measured in seconds and it is the reciprocal of frequency, T = 1/f.PermeabilityThe constant describing how much magnetisation a material takes on in an external magnetic field. The vacuum value, mu naught, is 4 pi times 10 to the minus 7 tesla metres per ampere.PermittivityThe constant describing how an electric field is established in a material or in free space. The vacuum value, epsilon naught, is 8.85 times 10 to the minus 12 in coulombs squared per newton square metre.Photoelectric effectLight shone on a photoactive material ejects electrons from it, but only above a threshold frequency, however bright the light is. The maximum kinetic energy of an ejected electron is the photon energy minus the work function.PhotonA photon is a single quantum of electromagnetic energy: massless, electrically neutral, and carrying an energy proportional to its frequency, E = hf. Turning a light source up sends more photons per second, not more energetic ones.Physical pendulumA physical pendulum is any rigid body that oscillates about a fixed axis under gravity, rather than a point mass on a string. Its period depends on its rotational inertia about the pivot and on the pivot to center of mass distance.Planck's constantPlanck's constant, h, is the proportionality constant between a photon's energy and its frequency. The AP Physics 2 table prints it as 6.63 times 10 to the minus 34 J s, and equivalently as 4.14 times 10 to the minus 15 eV s.PolarizationPolarization is the restriction of a transverse wave's oscillation to a single plane. Only transverse waves can be polarized, and sending a wave through a polarizer generally cuts its intensity.Potential energyEnergy stored in the arrangement of a system whose parts interact through conservative forces. It belongs to the configuration of the whole system rather than to any single object, and only changes in it are physically meaningful.Potential energy curveA graph of a system's potential energy against position. Its slope gives the force, local minima mark stable equilibrium points and local maxima mark unstable ones, so the shape of the curve predicts the motion.PowerThe rate at which energy is transferred or converted, measured in watts, where one watt is one joule per second. Average power is the energy moved divided by the time it took to move it.PressurePressure is the magnitude of the force component perpendicular to a surface divided by the area of that surface. It is a scalar, measured in pascals, where one pascal is one newton per square metre.

R

RadianThe radian is the unit of angle defined as arc length divided by radius. Because it is a ratio of two lengths it carries no dimensions, and one full revolution contains 2 pi radians, roughly 6.283.Radioactive decayRadioactive decay is the spontaneous transformation of a nucleus into one or more different nuclei, or into a lower energy level of the same nucleus. When a given nucleus will decay cannot be predicted, only the probability per unit time.RC circuitAn RC circuit contains at least one resistor and one capacitor, so its currents and voltages change over time as the capacitor charges or discharges. An uncharged capacitor acts like a wire the instant the switch closes and like a break in the circuit once fully charged.Real imageAn image formed where light rays actually cross after reflecting or refracting. Because real light arrives there, a real image can be caught on a screen, and for a single mirror or lens it is always inverted.Reference frameA reference frame is the coordinate system an observer uses to measure positions, velocities and times. The frame chosen determines the direction and magnitude of what gets measured, so a velocity means nothing until the frame is named.ReflectionThe bouncing of light back from a surface. The angle of incidence equals the angle of reflection, with both angles measured from the normal, the line drawn perpendicular to the surface at the point where the ray strikes.RefractionThe change in direction of a light ray as it crosses from one medium into another. It happens because the speed of light differs between the two media, and the frequency of the light stays the same while its wavelength changes.Relative velocityRelative velocity is the velocity of one object as measured by an observer moving with another. You get it by subtracting the observer's velocity from the object's velocity, with both taken in the same reference frame.ResistanceResistance measures how strongly an object opposes the movement of electric charge through it, in ohms, where one ohm is one volt per ampere. It belongs to a particular object, depending on the material and on that object's length and cross-sectional area.ResistivityResistivity is the property of a material that quantifies how strongly it opposes the motion of electric charge, measured in ohm meters. Every sample of a substance shares the same resistivity whatever its shape, and in a real conductor it typically rises with temperature.ResonanceResonance is the large-amplitude response an oscillating system gives when an external force drives it at its natural frequency, the frequency at which the system oscillates when displaced from equilibrium and left alone.Restoring ForceA restoring force is a force exerted in the direction opposite to an object's displacement from an equilibrium position, so it always acts to bring the object back toward that position. Springs are one example, not the definition.Right-hand ruleA set of hand conventions for finding the direction of a magnetic quantity that is perpendicular to two others. Always the right hand, always conventional current, and the result reverses for a negative charge.Rolling without slippingRolling without slipping is motion in which a round object turns and advances together, so its contact point never slides across the surface. The speed of the center of mass and the angular speed are locked together by the radius.Root-mean-square speedThe root-mean-square speed is the speed an atom would need in order to have exactly the average kinetic energy of the sample. It is the square root of the mean of the squared speeds, which is not the average speed.Rotational equilibriumRotational equilibrium is the condition in which the net torque on a system is zero, so its angular velocity stays constant. A system spinning at a steady rate is in rotational equilibrium just as much as one that is not turning at all.Rotational inertiaRotational inertia measures how strongly a rigid system resists a change in its rotation. It depends on the total mass and on how that mass is distributed relative to the chosen axis, so a single object has many values, one for each axis.Rotational kinetic energyRotational kinetic energy is the energy a system has because it is spinning, equal to one half the rotational inertia times the square of the angular velocity. It is a scalar measured in joules, and it adds to translational kinetic energy rather than replacing it.

S

ScalarA scalar is a quantity fully described by a magnitude and a unit, with no direction attached. Mass, time, distance, speed, energy, temperature and electric charge are scalars, and a minus sign on one of them is not a direction.Scientific notationScientific notation writes a number as one digit before the decimal point multiplied by a power of ten. It keeps very large and very small physical quantities readable and makes their order of magnitude obvious at a glance.Second law of thermodynamicsThe second law of thermodynamics states that the total entropy of an isolated system can never decrease, and is constant only when every process the system undergoes is reversible.SI unitsSI units are the international system of measurement built on seven base units, including the metre, kilogram and second. Every equation on the AP tables of information is written in SI, so numbers must be converted before substitution.Significant figuresSignificant figures are the digits of a measured value that carry real information about its precision. A product or quotient keeps the fewest significant figures of its inputs, and a sum keeps the fewest decimal places.Simple pendulumA simple pendulum is a point mass hanging from a light string of fixed length, swinging under gravity. Displaced by a small angle it moves in simple harmonic motion with a period set only by the string length and g.Snell's lawThe law of refraction: the index of refraction times the sine of the angle equals the same product on the other side of a boundary. Both angles are measured from the normal to the surface, not from the surface itself.SolenoidA long coil of wire that produces a nearly uniform magnetic field along its interior axis and almost none outside. The interior field is the vacuum permeability times the turns per unit length times the current.Specific heatSpecific heat is the energy needed to raise one kilogram of a material by one kelvin, measured in joules per kilogram per kelvin. A large specific heat means a small temperature change for the same energy.SpeedSpeed is a scalar measure of how fast an object is moving, with no direction attached. Average speed is total distance divided by total elapsed time, and instantaneous speed is the magnitude of the velocity at a single moment.Spring ConstantThe spring constant is a measure of how stiff a spring is: the force it takes to stretch or compress it by one unit of length. It is written k and measured in newtons per meter, and a larger k means a stiffer spring.Standing waveA standing wave is the pattern produced when two identical waves travel in opposite directions through the same confined region. Instead of a moving waveform it has fixed points of zero displacement and fixed points of maximum oscillation.Static frictionThe friction force between two surfaces that are not sliding on each other. It takes whatever value and direction is needed to prevent slipping, up to a maximum equal to the coefficient of static friction times the normal force.Stopping potentialStopping potential is the reverse potential difference that just halts the fastest photoelectrons, so no current flows. Multiplied by the elementary charge it gives the maximum kinetic energy of the ejected electrons.Superposition principleThe superposition principle says that where two or more waves overlap, the displacement at each point is the sum of the displacements each wave would produce on its own, and the waves then continue on unchanged.

T

TemperatureTemperature is the property of a system that tracks the average kinetic energy of its atoms. It is intensive, so it does not depend on how much of the substance is present, and every gas equation needs it in kelvin.TensionTension is the force a string, rope, cable or chain exerts along its own length when something pulls on it. It can only pull, never push, and in an ideal string it has the same value at every point along the string.Terminal velocityTerminal velocity is the maximum speed an object reaches when a constant driving force and an opposing resistive force act on it. At that speed the two forces balance, the net force is zero, and the acceleration stops.Thermal conductivityThermal conductivity is an intrinsic property of a material that sets how readily it conducts energy, measured in watts per meter per kelvin. On its own it does not give a transfer rate: area, thickness and temperature difference do the rest.Thermal equilibriumThermal equilibrium is the state two systems in thermal contact reach when no net energy is transferred between them by thermal processes. The condition for it is equal temperature, not equal internal energy.Thermal radiationThermal radiation is energy an object emits as electromagnetic waves because of its temperature. It needs no medium, which makes it the only one of the three thermal processes that crosses a vacuum.Thin-film interferenceThe interference between light reflected from the top surface of a thin transparent layer and light reflected from its bottom surface. The colours on a soap bubble and on an oil slick come from it, and so do antireflection coatings.Time constantThe time constant is the characteristic time that sets how quickly a circuit settles toward its final state. For a resistor and capacitor it is resistance times capacitance, and one time constant takes a charging capacitor to about 63 percent of its final charge.TorqueTorque measures how effectively a force twists a rigid system about an axis. It equals the force multiplied by the perpendicular distance from the axis to the force's line of action, its unit is the newton meter, and it is not a form of energy.Total internal reflectionThe complete reflection of light back into a medium when it strikes the boundary with a lower-index medium at an angle of incidence beyond the critical angle. No light at all is transmitted across the boundary.Transverse waveA transverse wave is one in which the disturbance is perpendicular to the direction the wave travels. Waves on a string, surface ripples and light are transverse, and only transverse waves can be polarized.

U

Uniform circular motionMotion around a circular path at constant speed. The speed never changes but the velocity does, because its direction turns continuously, so the object accelerates toward the center of the circle at every instant.

V

VectorA vector is a quantity that needs both a magnitude and a direction to be fully specified. Displacement, velocity, acceleration, force and momentum are vectors, and they combine by vector addition rather than by ordinary arithmetic.Vector componentsThe components of a vector are the perpendicular pieces it splits into along the chosen axes, usually an x part and a y part. Added back together they reproduce the original vector exactly, in both size and direction.VelocityVelocity is a vector that gives both how fast an object is moving and in which direction. Used with no qualifier it means instantaneous velocity, the rate of change of position at a single moment, measured in meters per second.Velocity-time graphA velocity-time graph plots velocity on the vertical axis against time on the horizontal. Its slope at any point gives the acceleration, and the signed area between the curve and the time axis gives the displacement.Virtual imageAn image formed where reflected or refracted rays only appear to originate, because they are diverging when they reach the eye. No light reaches that location, so a virtual image cannot be projected on a screen.Volume flow rateVolume flow rate is the volume of fluid that passes a cross section each second, equal to the cross sectional area multiplied by the flow speed, in cubic metres per second.

W

Wave-particle dualityWave-particle duality is the finding that light and matter each behave as waves in some experiments and as particles in others, with no single classical picture covering both. Which behavior appears depends on what the experiment measures.WavelengthWavelength is the distance between two successive corresponding positions on a wave, such as peak to peak or trough to trough. It is a length, measured in meters, and written with the Greek letter lambda.WeightWeight is the gravitational force exerted on an object, equal to mass times the gravitational field strength. It is a vector measured in newtons, and unlike mass it changes if the object moves to another planet.WorkThe energy transferred into or out of a system by a force acting over a distance. Only the component of the force parallel to the displacement counts, so work is a scalar measured in joules and can be positive, negative or zero.Work functionThe work function of a material, written phi, is the minimum energy needed to free an electron from atoms in that material. It sets the threshold frequency for the photoelectric effect and is subtracted from the photon energy to leave kinetic energy.