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.

344 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.Acceleration-time graphAn acceleration-time graph plots acceleration on the vertical axis against time on the horizontal. The signed area between the curve and the time axis gives the change in velocity over that interval, not the velocity itself.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.Alpha decayAlpha decay is the mode of radioactive decay in which a nucleus ejects an alpha particle, a helium nucleus of two protons and two neutrons. The mass number of the parent falls by 4 and the atomic number by 2, so the daughter is a different element.Alpha particleAn alpha particle is a helium nucleus: two protons and two neutrons bound together, with mass number 4 and charge twice the elementary charge. It carries no electrons, which is why it is written with a 2 plus sign, and it is what alpha decay emits.Ampere (A)The ampere is the SI base unit of electric current, equal to one coulomb of charge passing a point each second. Current is measured in amperes through a circuit element, while potential difference is measured in volts across it.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.Amperian LoopAn Amperian loop is a closed imaginary path drawn around a current-carrying conductor, chosen so that Ampere's law can be evaluated on it. It has no physical existence and is selected purely for the symmetry it exposes.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.Angle of incidenceThe angle of incidence is the angle between an incoming ray and the normal, the line drawn perpendicular to the surface at the point where the ray strikes. It is never measured from the surface itself.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.Arc lengthThe distance a point travels along a circular path, equal to the radius multiplied by the angle swept in radians. Symbol s, measured in meters, and printed as s = rθ in the geometry box of all four AP Physics equation booklets.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.Area under a curveThe area under a curve is the region bounded by a plotted line and the horizontal axis over some interval, counted as negative wherever the line dips below the axis. On a physics graph it gives an accumulated quantity such as displacement or impulse.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 AccelerationAverage acceleration is the change in an object's velocity divided by the time interval in which that change occurs, measured in meters per second squared. Only the starting and ending velocities enter it, so it says nothing about what happened in between.Average SpeedAverage speed is the total distance an object travels divided by the total time it takes, in meters per second. It is a scalar, it is never negative, and it is not the magnitude of the average velocity unless the motion never turns around.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.Avogadro's numberAvogadro's number is the count of atoms or molecules in one mole, 6.02 times 10 to the twenty-third per mole. It converts between the two ways the AP gas equations count a sample, moles and individual atoms.Axis of rotationThe line a rigid system turns about, and the reference point every rotational quantity is measured from. Torque, lever arm, rotational inertia and angular momentum all change when a different axis is chosen, so the axis has to be named before any of those numbers means anything.

B

Banked curveA banked curve is a road or track tilted inward through a turn, so that part of the normal force points toward the center of the circle and helps supply the centripetal acceleration. Tilting lets a vehicle round the bend with no friction needed.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.Best fit lineA best fit line is a single straight line drawn to follow the overall trend of plotted data rather than to pass through any particular points. AP scores it as a line that approximates the trend, and its slope is what the analysis then uses.Beta decayBeta decay is the mode of radioactive decay in which one nucleon changes type inside the nucleus. Beta-minus turns a neutron into a proton and emits an electron and an antineutrino; beta-plus turns a proton into a neutron and emits a positron and a neutrino.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.Boltzmann's constantBoltzmann's constant sets the energy per kelvin in a single atom's average kinetic energy, 1.38 times 10 to the minus 23 joules per kelvin. It is the per-atom counterpart of the universal gas constant.Bright fringeA bright fringe is one of the bands of maximum brightness in an interference or diffraction pattern, formed where the overlapping wavefronts arrive in phase. Its order number m counts outward from the central fringe, which is m equals 0.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.CapacitorA capacitor is a circuit element built from two separated conducting surfaces that hold equal amounts of charge with opposite signs. Storing that charge stores energy in the electric field between the surfaces.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.Center-of-mass velocityThe velocity of the point that moves as though all of a system's mass were concentrated there, equal to the system's total momentum divided by its total mass. It stays constant whenever no net external force acts on the system.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.Charge densityCharge density is the amount of electric charge per unit length, per unit area, or per unit volume of a charge distribution. Integrating it over the length, area or volume of the distribution returns the total charge.Charge distributionA charge distribution is charge spread along a line, over a surface or through a volume rather than concentrated at a point. AP Physics C finds its electric field by integrating the contributions of its pieces; AP Physics 2 stays with small numbers of point charges.Charge polarizationCharge polarization is the separation of positive and negative charge inside an object caused by a nearby charged object or an external electric field. The object's net charge does not change, which is how a neutral object can still be attracted.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.Circuit diagramA circuit diagram, or schematic, represents a circuit by drawing each element as a standard symbol joined by lines standing for wires. It records which elements are electrically connected, not where they physically sit on a bench.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.CoherenceCoherence is a fixed, unchanging phase relationship between wave sources. Coherent sources make an interference pattern that holds still long enough to see, which is why slit experiments use one laser rather than two separate lamps.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.Conservation of electric chargeConservation of electric charge is the rule that a system's net charge stays constant unless charge is transferred to or from that system. Charging by friction, contact or grounding moves electrons between objects; none of them creates or destroys charge.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.Contact forceA contact force is one that exists only while two surfaces are touching, such as the normal force, friction and tension. AP describes contact forces as macroscopic effects of interatomic electric forces, so they are electrical underneath.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.Conventional currentConventional current is current drawn and labelled in the direction positive charge would move. In an ordinary metal wire the actual carriers are electrons moving the other way, and every AP answer still uses the conventional direction.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.Coulomb (C)The coulomb is the SI unit of electric charge, defined as the charge carried past a point by a current of one ampere in one second. As static charge it is enormous, worth about 6.25 times 10 to the eighteenth elementary charges.Critical angleThe critical angle is the angle of incidence at which light passing into a lower index medium refracts to exactly 90 degrees and grazes along the boundary. Past that angle no light is transmitted at all.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.Current DensityCurrent density is the electric current per unit cross-sectional area at a point in a conductor, written J. It is a vector, it points along the direction positive carriers move, and its SI unit is the ampere per square metre.Cyclic processA cyclic process takes a system around a closed path and returns it to its starting state. Because internal energy depends only on the state, the change in internal energy over a full cycle is zero, so the net heating equals minus the net work.

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.Decay constantThe decay constant, written lambda, is the probability per unit time that a given unstable nucleus decays. It has units of inverse seconds, and a larger value means a faster-decaying material.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.Dielectric constantThe dielectric constant, kappa, is the ratio of a material's electric permittivity to the permittivity of free space. It is a pure number with no units, it equals 1 for vacuum, and it is the factor by which the material multiplies a capacitor's capacitance.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.DistanceDistance is a scalar length in meters, with no direction attached. In AP Physics the word does two jobs: the total path length an object covers, and the separation between two points, and the equation sheet uses it in the second sense.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.Double-slit interferenceDouble-slit interference is the pattern of evenly spaced bright and dark bands produced when monochromatic light passes through two narrow slits a distance d apart and falls on a screen. Bright bands sit where the path length difference is a whole number of wavelengths.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.Drift VelocityDrift velocity is the slow average velocity that charge carriers acquire along a conductor when a potential difference is applied, typically well under a millimetre per second even while the current itself appears instantly.

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 dipoleAn electric dipole is a pair of equal and opposite point charges held a fixed distance apart. Its net charge is zero, yet it still produces an electric field, and that field weakens with distance faster than a single charge's field does.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 field linesElectric field lines are a simplified diagram of an electric field: continuous curves leaving positive charge and arriving at negative charge, with the field tangent to the line and the spacing showing relative strength. They give directions and comparisons, never numbers.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 inductionElectromagnetic induction is the production of an emf by a changing magnetic flux through a circuit or surface. The change is what matters: a steady magnetic field, however strong, induces nothing, and the emf appears only while the flux is on the move.Electromagnetic spectrumThe electromagnetic spectrum is the full set of electromagnetic wave categories arranged by wavelength, spanning kilometres for radio waves down to picometres for gamma rays. Visible light occupies a narrow band inside 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.ElectronAn electron is the light, negatively charged particle that surrounds the nucleus of an atom and carries the current in a metal wire. Its charge is minus one elementary charge and its mass is about 1800 times smaller than a proton's.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.Electrostatic equilibriumElectrostatic equilibrium is the state an object reaches once its charges have stopped moving. In a conductor it means no electric field inside the conducting material, all excess charge on the surface, and the whole surface at one potential.Elementary chargeThe elementary charge e is the magnitude of the charge carried by one proton or one electron, 1.60 times 10 to the power minus 19 coulombs. AP treats it as the smallest indivisible amount of charge, so every net charge is a whole-number multiple of it.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.EnergyEnergy is a scalar property of a system, measured in joules, that can be converted between types or transferred across the system boundary but is never created or destroyed. There is no single equation for energy, only one per type.Energy bar chartA diagram showing one bar per type of energy stored in a chosen system at one instant, all drawn to a shared scale so that snapshots taken at different instants can be compared. AP Physics 1 asks students to draw them on the free-response section.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.Energy level diagramAn energy level diagram is a picture of an atom's allowed energies, drawn as horizontal lines on a vertical energy axis. Transitions between levels are arrows, and the length of an arrow is the energy of the photon absorbed or emitted.Energy stored in a capacitorThe energy stored in a capacitor equals the work an external agent had to do to separate its charge, and both AP equation sheets give it in one form only: one half the charge times the potential difference. The two familiar alternatives have to be derived.Energy stored in an inductorThe energy stored in an inductor sits in the magnetic field created by its own current, and equals one half the inductance times the square of that current. The AP Physics C E and M sheet prints it; AP Physics 2 has no inductors at all.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.Equilibrium positionThe location at which the net force exerted on an object or system is zero. Every displacement in simple harmonic motion is measured from this point, and an oscillator passes through it at maximum speed with zero acceleration.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.Equivalent capacitanceEquivalent capacitance is the single capacitance that could replace a group of capacitors without changing what the rest of the circuit sees. Capacitors in parallel add directly, while capacitors in series combine as reciprocals, which is the reverse of the rule for resistors.Equivalent resistanceEquivalent resistance is the single resistance that could replace a group of resistors without changing anything the rest of the circuit sees. Resistors in series add directly; resistors in parallel add as reciprocals, so a parallel group resists less than any one of its members.Escape velocityThe speed at which the mechanical energy of a satellite and central-object system equals zero, so the satellite moves away until its speed reaches zero at an infinite distance. It equals the square root of 2GM/r and does not depend on the satellite's mass.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.External forceAn external force is one exerted on a system by something outside the system boundary. Whether a given force counts as external depends entirely on where you drew that boundary, and every conservation of momentum argument turns on the choice.

F

Farad (F)The farad is the SI unit of capacitance, equal to one coulomb of stored charge per volt of potential difference. It is an enormous unit, which is why real capacitors are rated in microfarads and picofarads.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.ForceA force is an interaction between two objects or systems, described by a vector and measured in newtons. It is never something an object owns on its own: name a force and you must be able to name both things involved.Force DiagramA force diagram shows the forces exerted on a rigid system, drawn at the points where they actually act relative to the axis of rotation. AP Physics treats it as a separate representation from the free-body diagram, which ignores where a force is applied.Force-time graphA plot of force on the vertical axis against time on the horizontal. The signed area between the curve and the time axis is the impulse delivered, measured in newton seconds, and the height of the equal-area rectangle is the average force.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

Gamma decayGamma decay is the emission of a high-energy photon by an excited nucleus dropping to a lower energy state. Mass number and atomic number both stay the same, so the nucleus is the same nuclide afterwards, only with less energy.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.Gauss's Law for MagnetismGauss's law for magnetism states that the magnetic flux through any closed surface is exactly zero. It is the formal statement that magnetic field lines always close on themselves, because isolated magnetic poles do not exist.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 forceThe gravitational force is the mutual attraction between any two objects with mass, acting along the line joining their centers of mass. Near an astronomical body AP gives the same force a second name, weight.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.GroundingGrounding is electrically connecting an object to a much larger, approximately neutral system such as Earth, which can absorb or supply charge freely. AP Physics C also treats ground as an idealized point of zero electric potential.

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.HarmonicsHarmonics are the discrete standing wave patterns a confined region allows, numbered from the longest wavelength upward. The first harmonic is the fundamental, and a region with a node at one end and an antinode at the other supports only odd harmonics.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.Henry (H)The henry is the SI unit of inductance, equal to one volt-second per ampere. An inductor of one henry produces one volt of induced emf when the current through it changes at one ampere per second.Hertz (Hz)The hertz is the SI unit of frequency, equal to one cycle per second, which makes it an inverse second. A frequency of 50 Hz means fifty complete repetitions every second.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 batteryAn ideal battery is a model source that holds a fixed potential difference between its terminals however much current it delivers, because it is taken to have no internal resistance. AP problems assume batteries are ideal unless a question says otherwise.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.Inertial reference frameAn inertial reference frame is one in which Newton's first law holds, so an observer in it sees an object with no net force keep a constant velocity. Frames at rest and frames moving at constant velocity are inertial; accelerating frames are not.Instantaneous accelerationInstantaneous acceleration is the acceleration an object has at one particular moment rather than averaged over an interval. On a velocity-time graph it is the slope of the tangent line at that instant, measured in meters per second squared.Instantaneous VelocityInstantaneous velocity is an object's velocity at a single moment rather than over an interval. AP Physics 1 defines it as the rate of change of position, read off a position versus time graph as the slope of the tangent at that point.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.IntensityIntensity is the power a wave transfers per unit area, in watts per square metre. AP Physics 2 defines it as the average power per unit area taken over one period of the wave.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.Internal resistanceInternal resistance is the resistance of the material inside a real battery, modelled as a resistor sitting in series with an ideal battery. Because of it, the potential difference across the terminals drops below the emf as soon as current flows.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.IsotopeIsotopes of an element are nuclei with the same number of protons but different numbers of neutrons, so they share an atomic number and differ in mass number. They behave alike chemically and can behave completely differently in a nucleus.

J

JouleThe SI unit of energy and work, symbol J, equal to one kilogram meter squared per second squared. One joule is the work done by a force of one newton acting through one meter along its own direction.

K

Kelvin (K)The kelvin is the SI base unit of temperature, measured on an absolute scale whose zero is absolute zero. It carries no degree sign, so a temperature is written 300 K, and it is the only scale the gas equations accept.Kepler's third lawThe rule that the square of a circular orbit's period is proportional to the cube of its radius, with the constant of proportionality set only by the mass of the central body. AP writes it as T squared equals 4 pi squared R cubed divided by GM.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.Kinetic theory of gasesKinetic theory explains the bulk properties of a gas as consequences of the motion and collisions of its atoms. Pressure comes from atoms striking a surface, and temperature tracks their average kinetic energy.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

LC circuitAn LC circuit holds only a charged capacitor and an inductor, which pass energy back and forth so the charge oscillates. It is Topic 13.6 of AP Physics C Electricity and Magnetism, the final topic of that course, and it appears in no other AP physics course.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.Lepton numberLepton number is a count of leptons that stays the same through any nuclear decay. In AP Physics 2 the leptons involved are electrons, positrons, neutrinos and antineutrinos, and each antiparticle counts as minus one so the totals balance.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.Linear mass densityLinear mass density is the mass per unit length of a rod, wire or string, written with the Greek letter lambda and measured in kilograms per meter. For a uniform object it is simply the total mass divided by the total length.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.LR circuitAn LR circuit contains a resistor and an inductor, so its current needs time to settle. The instant a switch is thrown the inductor blocks the current entirely, and after a long time it behaves as a plain wire, which is the reverse of what a capacitor does.

M

Magnetic domainA magnetic domain is a region inside a ferromagnetic material in which the atomic magnetic dipoles already point the same way. Magnetising the material aligns the domains with each other; in unmagnetised iron they point in scattered directions and cancel.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.MagnitudeThe magnitude of a quantity is its size with the direction stripped off. A magnitude is never negative, it always carries the unit, and for a vector it is written with the bare symbol or with vertical bars around the arrow form.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.Maxwell's equationsMaxwell's equations are the four laws that together describe electromagnetism: Gauss's law, Gauss's law for magnetism, Faraday's law of induction, and Ampere's law with Maxwell's addition. AP Physics C names and numbers them in exactly that order.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.Mechanical waveA mechanical wave is one that requires a medium in which to propagate. Sound in air and a wave on a string are the AP examples, and the properties of the medium, not the source, set the speed.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.ModelA model in AP Physics is a physical, mathematical, or conceptual representation of a system of ideas, events, or processes. Choosing a model means choosing which features of a real situation to keep and which to ignore.Mole (mol)A mole is a counting unit. It denotes a fixed number of things, Avogadro's number of them, in the same way a dozen denotes twelve, so it measures neither a mass nor a volume.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.Momentum ChartA momentum chart is a bar-style representation of the momentum carried by each part of a system before and after an interaction. AP Physics lists it as one of the representations students are expected to create and interpret.Monatomic ideal gasA monatomic ideal gas is an ideal gas whose particles are single atoms, so all of their energy is translational kinetic energy. The AP Physics 2 exam conventions state that ideal gases are monatomic unless a question says otherwise.Motion MapA motion map is a representation that shows where a moving object is at a sequence of equally spaced instants, usually with arrows for velocity and sometimes acceleration. AP Physics names it as a representation students should be able to create.

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.Net torqueThe signed sum of every torque exerted on a rigid system about one chosen axis. It equals the system's rotational inertia times its angular acceleration, and it is the slope of a graph of the system's angular momentum against time.Net workThe signed sum of the work done on a system by every force exerted on it. The work-energy theorem equates the change in kinetic energy to net work, not to the work done by any one force on its own.NeutrinoA neutrino is a subatomic particle with no electric charge and negligible mass, emitted alongside a positron in beta-plus decay. Its antiparticle, the antineutrino, is emitted alongside an electron in beta-minus decay.NeutronA neutron is a nucleon with no electric charge and about the same mass as a proton. Neutrons do not change which element an atom is, but the number of them fixes which isotope it is.NewtonThe SI unit of force, symbol N, equal to one kilogram meter per second squared. One newton is the net force that gives a 1 kg mass an acceleration of 1 m/s squared, and a 1.0 kg object weighs 9.8 N near Earth's surface.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.NucleonA nucleon is a particle in the nucleus, meaning a proton or a neutron. The nucleon number of a nuclide is the total count of the two, written as the mass number A, and it is conserved in every nuclear reaction.

O

ObjectIn AP Physics an object is a physical thing whose internal structure and properties are ignored. Calling something an object is a modeling decision: it says the thing has mass and position but no size, no shape and no moving parts.Object distanceObject distance is how far an object sits from a mirror or lens, written s sub o in the mirror and thin-lens equation. For a mirror it is measured to the mirror surface; for a thin lens it is measured to the midline of the lens.Ohm (Ω)The ohm is the SI unit of electrical resistance, equal to one volt per ampere. It is defined by that ratio and not by any material property, so an object has a resistance in ohms even when the ratio changes with current.Ohmic materialAn ohmic material is one whose resistance stays constant no matter how much current it carries. Ohm's law is a property that some materials have rather than a law all matter obeys, and a straight line through the origin on a current against voltage graph is the test.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.OscillationRepeated back-and-forth motion of a system about an equilibrium position, driven by a restoring force. Simple harmonic motion is the special case in which the magnitude of that restoring force is proportional to the displacement, and not every oscillation qualifies.

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.Parallel-plate capacitorA parallel-plate capacitor is two separated parallel conducting surfaces holding equal amounts of charge with opposite signs. The field between the plates is uniform in magnitude and direction except near the edges.ParticleA particle is a body treated as having mass and a position but no size, shape or internal structure. AP Physics usually calls this an object; particle appears mainly where a system is built from several separate masses.Pascal (Pa)The pascal is the SI unit of pressure, equal to one newton spread over one square metre. It is a very small amount of pressure: atmospheric pressure at sea level is about 1.0 times 10 to the fifth pascals.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.Phase angleThe constant added inside the cosine of the simple harmonic motion equation x = A cos(ωt + φ). Measured in radians, it records where in its cycle an oscillator sits at t = 0, so it encodes the starting position and the starting direction together.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.Plane mirrorA plane mirror is a flat mirror. Its focal point is an infinite distance away, so every image it forms is virtual, upright, the same size as the object, and as far behind the mirror as the object stands in front of it.Point chargeA point charge is a model in which the physical size of a charged object is treated as negligible for the situation being analysed. It is an idealisation about size, and it is what lets a single distance r stand for the separation in Coulomb's law.Point MassA point mass is a body idealized as all of its mass concentrated at a single location, with no size and no shape. It is what AP Physics means when it says an object may be treated as a single point.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.PositionPosition is where an object is, measured as a vector from a chosen origin. It is the quantity every other kinematic quantity is built from: displacement is a change in position, and velocity is the rate at which position changes.Position-time graphA position-time graph plots an object's position on the vertical axis against time on the horizontal. The slope at any point gives the velocity there, the curvature shows whether the object is accelerating, and the area underneath carries no physical meaning.PositronA positron is the antiparticle of the electron: same mass, opposite charge, so it carries plus one elementary charge. It is emitted in beta-plus decay, where a proton inside a nucleus changes into a neutron.Potential differencePotential difference is the change in electric potential between two points, equal to the change in electric potential energy per unit charge moved between them. Measured in volts, it is what a voltmeter reads and what every circuit equation contains.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.Potential GradientA potential gradient is the rate at which electric potential changes with position. The electric field component in any direction equals the negative of that rate, so the field points from high potential toward low.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.ProjectileA projectile is an object that has been launched and is then moving under gravity alone, with no engine and no air resistance. AP models its flight as motion with zero acceleration in one dimension and constant nonzero acceleration in the other.ProtonA proton is a positively charged nucleon, carrying exactly one elementary charge. The number of protons in a nucleus is the atomic number Z, and it is what decides which element the atom is.PV diagramA PV diagram is a graph of a gas's pressure against its volume, with pressure on the vertical axis. Each point is one state of the gas, each curve is one process, and the area under a curve gives the magnitude of the work.

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.Ray diagramA ray diagram is a sketch of the path light takes before and after it meets a mirror or lens, drawn with straight rays, and used to find where an image forms and what it looks like.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.RepresentationCollege Board defines a representation as a method of understanding and communicating understandings about physics. A representation is a depiction of a model, or of some aspect of that model, such as a graph, a diagram, an equation or a written description.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.Resistive forceA resistive force is a velocity dependent force pointing opposite an object's velocity, the standard AP Physics C example being force equal to minus k times v. Faster motion means a bigger opposing force, which is what makes the resulting motion exponential.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.ResistorA resistor is a circuit element made to have a known resistance, drawn as a zigzag on a schematic. It converts electrical energy into thermal energy, and on both AP exams it is assumed to be ohmic unless a question says otherwise.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.Rigid SystemA rigid system is a system that does not change shape, but whose different points may move in different directions and with different speeds. It is the model AP Physics uses whenever rotation matters and the object model is no longer enough.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.Rotational workThe energy a torque transfers into or out of a rigid system as that system turns, equal to the torque times the angular displacement in radians. It is measured in joules and it is the rotational counterpart of force times distance.

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.Single-slit diffractionSingle-slit diffraction is the banded pattern monochromatic light produces after passing through one narrow opening of width a. The condition printed on the AP Physics 2 sheet locates the dark fringes of that pattern, not the bright ones.SlopeSlope is how much the vertical quantity on a graph changes per unit change in the horizontal quantity, found as rise divided by run. On a physics graph it carries units, and those units identify which physical quantity the slope represents.Small-angle approximationThe rule that sin θ is approximately equal to θ when θ is small and measured in radians. AP uses it to make a pendulum's restoring torque proportional to angle, which is the condition the pendulum period formula quietly depends on.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.Sound waveA sound wave is modelled as a mechanical longitudinal wave: it needs a medium, and the disturbance is a pressure variation along the direction of travel. Its frequency relates to pitch and its amplitude to loudness.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.Stefan-Boltzmann lawThe Stefan-Boltzmann law gives the power a blackbody radiates as P = A sigma T^4: emitting surface area times the Stefan-Boltzmann constant times absolute temperature raised to the fourth power, with T in kelvin.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.Strong forceThe strong force is the attraction that acts between nucleons at nuclear distances and holds a nucleus together against the electrical repulsion of its protons. AP Physics 2 treats it qualitatively and gives it no equation.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.SystemA system is a collection of objects that are analyzed together. It is chosen by the person solving the problem, not fixed by the situation, and the choice decides which forces count as external and which quantities come out constant.

T

Tangential accelerationTangential acceleration is the part of an object's acceleration directed along its circular path, and it is the part that changes the object's speed. Centripetal acceleration points at the center instead and changes only the direction of motion.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.Tesla (T)The tesla is the SI unit of magnetic field strength, equal to one newton per ampere-metre. It is a large unit: Earth's field at the surface is a few tens of microtesla, while a hospital MRI magnet runs at 1.5 or 3 tesla.Test chargeA test charge is a point charge small enough in magnitude that its own presence does not significantly affect the electric field it is being used to measure. That smallness is what makes E equals F over q a definition of the field rather than a rearrangement.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 energyEnergy stored in the disordered motion of a system's particles. A system has thermal energy, unlike heat, which names a transfer rather than a store, and nonconservative forces such as friction dissipate mechanical energy into it.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 lens equationThe thin lens equation relates image distance, object distance and focal length: one over the image distance plus one over the object distance equals one over the focal length. The same printed equation serves mirrors.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.Threshold frequencyThreshold frequency is the minimum frequency of light that can eject electrons from a given material by the photoelectric effect. Below it no electrons come off however bright the beam, and the value depends on the material through its work function.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.Unit vectorA unit vector is a vector whose magnitude is exactly one, so it carries a direction and no size. AP Physics C writes vectors in unit vector notation, using i-hat, j-hat and k-hat for the x, y and z directions.Universal gas constantThe universal gas constant relates the pressure, volume, amount and temperature of an ideal gas per mole of substance. The AP Physics 2 table prints it as 8.31 joules per mole per kelvin, and it equals Avogadro's number times Boltzmann's constant.

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.Vector fieldA vector field attaches a vector to every point in a region of space, so each location carries both a magnitude and a direction. Gravitational, electric and magnetic fields are vector fields, and AP draws them as vector field maps.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.Volt (V)The volt is the SI unit of electric potential and potential difference, equal to one joule per coulomb. It measures energy per unit charge, so a volt is not itself a quantity of energy.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

WattThe SI unit of power, symbol W, equal to one joule per second or one kilogram meter squared per second cubed. A watt measures how fast energy is being transferred, so it describes a rate and never an amount.WaveA wave is a disturbance that transfers energy between two locations without transferring matter between them. AP Physics 2 models it as continuous and periodic, with a well defined wavelength and frequency.Wave pulseA wave pulse is a single disturbance that transfers energy without transferring matter between two locations. Because it does not repeat, it has no wavelength and no frequency, only a shape, a speed and an amplitude.Wave speedWave speed is how fast a disturbance travels through a medium, in metres per second. It is fixed by the medium and the type of wave rather than by the source, so changing the frequency changes the wavelength instead.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.WavefrontA wavefront is a surface joining points of a wave that are all at the same phase, such as the line running along one crest. It is the bridge between wave language and ray language in geometric optics.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.Wien's displacement lawWien's law says the peak wavelength a blackbody emits is inversely proportional to its absolute temperature, λmax = b/T, where Wien's constant b = 2.90 x 10^-3 m K. A hotter body peaks at a shorter wavelength.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.

Y

Y-interceptThe y-intercept of a graph is the value the vertical variable takes where the horizontal variable is zero. On a linearized physics plot it is usually a physical constant the question is asking for, not a leftover of the algebra.