Physics 2 glossary
Browse existing editorial definitions and examples, then follow deeper reading in the subject’s native course. Shared senses can belong to more than one subject. Historical source definitions and tables remain in the 242-entry Physics source glossary.
Physics 2 printables · Physics 2 interactive and source resources
261 of 261 editorial senses
Absolute pressure
Physics 1 · Physics 2 · Definition
Absolute pressure is pressure measured relative to a perfect vacuum.
It includes the atmospheric contribution that gauge pressure omits and is required in the ideal gas law.
For example, a tire’s absolute pressure is its gauge reading plus the surrounding atmospheric pressure.
Absolute zero
Physics 2 · Definition
Absolute zero is zero kelvin, the lower limit of thermodynamic temperature in the models used here.
It corresponds to minus 273.15 degrees Celsius and does not mean all quantum motion disappears.
For example, an ideal-gas pressure-versus-Celsius graph can be extrapolated toward this limit, although a real gas may condense before reaching it.
Absorption spectrum
Physics 2 · Definition
An absorption spectrum shows radiation removed from a source spectrum as it passes through matter.
Atomic absorption lines correspond to photon energies that can promote atoms between allowed levels.
For example, a cooler gas in front of a continuous light source can produce dark lines at selected wavelengths.
Acceleration
Physics 1 · Physics 2 · Definition
Acceleration is the vector rate of change of velocity with time.
It can change an object’s speed, direction, or both, and it is measured in meters per second squared.
For example, a satellite moving at constant speed in a circular orbit accelerates toward the center.
Adiabatic process
Physics 2 · Definition
An adiabatic process transfers no energy as heat across the chosen system boundary.
Work can still change internal energy, so adiabatic does not generally mean constant temperature.
For example, rapid compression of an insulated gas can warm it through positive work done on the gas.
Alpha decay
Physics 2 · Definition
Alpha decay emits a helium-4 nucleus containing two protons and two neutrons.
The daughter nucleus has an atomic number smaller by two and a mass number smaller by four.
For example, the emitted alpha particle is positively charged because it is a bare nucleus rather than a neutral helium atom.
Ammeter
Physics 2 · Definition
An ammeter measures current through a branch.
It is connected in series with that branch and ideally has negligible resistance so it changes the current as little as possible.
For example, inserting an ammeter into the wire leading to a resistor measures the current through that resistor.
Amplitude
Physics 1 · Physics 2 · Definition
Amplitude is the maximum magnitude of a repeating disturbance measured from its equilibrium value.
It describes the size of the oscillation rather than its frequency or propagation speed.
For example, pulling a spring-mounted mass farther from equilibrium increases its oscillation amplitude when it is released from rest.
Angular acceleration
Physics 1 · Physics 2 · Definition
Angular acceleration is the rate of change of angular velocity.
For a fixed axis its sign follows the chosen positive rotation direction, and its units are radians per second squared.
For example, a fan speeding up has angular acceleration, whereas one turning steadily does not.
Angular displacement
Physics 1 · Physics 2 · Definition
Angular displacement is the change in an object’s rotation angle about a specified axis.
Its sign follows the chosen rotation convention, and it is commonly measured in radians.
For example, a wheel can return to the same orientation after one turn while having an angular displacement of two pi radians.
Angular frequency
Physics 1 · Physics 2 · Definition
Angular frequency measures how rapidly the phase of a sinusoidal oscillation advances.
It equals two pi times the ordinary frequency and is measured in radians per second.
For example, two oscillators at the same ordinary frequency also have the same angular frequency even if their amplitudes differ.
Angular impulse
Physics 1 · Physics 2 · Definition
Angular impulse is the change in angular momentum caused by torque acting over time.
It is the signed area under a torque-versus-time graph about a consistent axis.
For example, applying a braking torque to a wheel for a longer interval changes its angular momentum more.
Angular momentum
Physics 1 · Physics 2 · Definition
Angular momentum describes motion about a specified point or axis.
For rigid rotation about a fixed principal axis it can be written as rotational inertia times angular velocity; a translating particle can also have angular momentum about a point.
For example, a ball moving past a pole has angular momentum about the pole even though the ball’s path is straight.
Angular velocity
Physics 1 · Physics 2 · Definition
Angular velocity describes the rate and direction of rotation about an axis.
For a fixed axis, a signed angular velocity is measured in radians per second, while its magnitude is angular speed.
For example, points on one rigid turntable share the same angular speed even though outer points have greater linear speeds.
Antinode
Physics 2 · Definition
A wave antinode is a location where the specified oscillating quantity has its largest oscillation amplitude in a standing-wave pattern.
Displacement antinodes and pressure antinodes describe different quantities, and their instantaneous values can still pass through equilibrium.
For example, the center of a string in its fundamental two-fixed-end mode is a displacement antinode.
Archimedes’ principle
Physics 1 · Physics 2 · Definition
Archimedes’ principle states that a submerged object’s buoyant-force magnitude equals the weight of the fluid it displaces.
Only the submerged volume contributes to displaced fluid for a partly floating object.
For example, a floating boat displaces enough water for the water’s weight to match the boat’s total weight.
Atomic number
Physics 2 · Definition
Atomic number is the number of protons in an atomic nucleus.
It identifies the element and does not change merely when electrons are gained or lost.
For example, every carbon isotope has the same atomic number even though their neutron counts can differ.
Average velocity
Physics 1 · Physics 2 · Definition
Average velocity is the displacement over an interval divided by the elapsed time.
It is a vector and need not equal the average speed, which uses total distance.
For example, a runner who returns to the starting line has zero average velocity for the full lap.
Avogadro’s constant
Physics 2 · Definition
Avogadro’s constant is the number of specified entities per mole, approximately 6.02 × 10²³ per mole.
It converts between an amount in moles and a count of particles.
For example, doubling the number of moles in a sample doubles its particle count when the entity being counted stays the same.
Beats
Physics 2 · Definition
Beats are periodic changes in resultant amplitude when waves of nearby frequencies interfere.
The beat frequency equals the magnitude of the difference between those frequencies in the simple two-tone model.
For example, two slightly detuned tuning forks produce repeating louder and quieter sound.
Bernoulli’s equation
Physics 1 · Physics 2 · Definition
Bernoulli’s equation relates pressure, fluid speed, and elevation along a streamline in steady ideal flow.
Its usual form assumes an incompressible, nonviscous fluid and no added or removed mechanical energy along the path.
For example, in a horizontal pipe narrowing under those assumptions, higher flow speed accompanies lower pressure.
Beta-minus decay
Physics 2 · Definition
Beta-minus decay transforms a neutron into a proton while emitting an electron and an electron antineutrino.
The nucleus’s atomic number increases by one while its mass number stays fixed.
For example, the emitted electron is created in the nuclear transformation rather than being an orbital electron knocked free.
Beta-plus decay
Physics 2 · Definition
Beta-plus decay transforms a proton into a neutron while emitting a positron and an electron neutrino.
The nucleus’s atomic number decreases by one while its mass number stays fixed, and the reaction must satisfy its energy requirement.
For example, the emitted positron has the same mass as an electron but the opposite electric charge.
Biot–Savart law
Physics 2 · Definition
The Biot–Savart law builds the magnetic field of a steady current by adding contributions from small current elements.
Each contribution depends on current, separation, and the orientation of the current element relative to the observation point.
For example, summing contributions from a very long straight wire gives circular field lines around the wire.
Blackbody
Physics 2 · Definition
A blackbody is an ideal object that absorbs all incident electromagnetic radiation.
Its thermal emission spectrum depends on temperature, and at equilibrium it can emit and absorb equal powers without changing temperature.
For example, heating a blackbody shifts its peak emission toward shorter wavelengths and increases its total emitted power.
Bohr model
Physics 2 · Definition
The Bohr model describes a hydrogen electron using selected allowed orbits and quantized energies.
It connects electric attraction with quantum restrictions but is a limited model rather than a literal general picture of all atoms.
For example, it can account for hydrogen’s discrete line spectrum but does not fully describe multielectron atomic structure.
Bulk modulus
Physics 2 · Definition
Bulk modulus measures resistance to compression through pressure change divided by fractional volume decrease.
A larger modulus means a smaller volume change for the same pressure increase within the model’s range.
For example, a liquid with a high bulk modulus can often be approximated as incompressible in modest-pressure flow problems.
Buoyancy
Physics 1 · Physics 2 · Definition
Buoyancy is the net upward force arising from pressure differences over an object in a fluid under gravity.
Its magnitude equals the weight of the displaced fluid in the usual hydrostatic model.
For example, a fully submerged object can rise, sink, or remain suspended depending on how buoyancy compares with its weight.
Calorimetry
Physics 2 · Definition
Calorimetry measures energy transfers by tracking temperature changes and any relevant phase changes.
The energy balance must include the named objects and account for the container or energy leakage when they matter.
For example, mixing a warm metal sample with cooler water can help estimate the metal’s specific heat.
Capacitance
Physics 2 · Definition
Capacitance is the plate-charge magnitude divided by the potential difference for an ideal capacitor.
It is measured in farads and depends on geometry and the material between the conductors.
For example, increasing the overlap area of ideal parallel plates increases their capacitance when separation and dielectric stay fixed.
Capacitor
Physics 2 · Definition
A capacitor stores energy by separating charge between conducting regions.
In the ideal two-plate model, opposite plate charges create a potential difference related to charge by the capacitance.
For example, disconnecting a charged capacitor from a battery leaves energy stored until a discharge path is provided.
Center of mass
Physics 1 · Physics 2 · Definition
The center of mass is the position obtained by weighting each part’s position by its mass.
Its motion represents the translational motion of the whole system under the net external force.
For example, adding a heavy object to one end of a light bar shifts the center of mass toward that end.
Centripetal acceleration
Physics 1 · Physics 2 · Definition
Centripetal acceleration is the inward acceleration component that changes the direction of motion along a curved path.
For circular motion its magnitude is speed squared divided by the circle’s radius.
For example, a car moving at constant speed around a bend still has centripetal acceleration.
Centripetal force
Physics 1 · Physics 2 · Definition
Centripetal force names the inward net-force component required for motion along a circular path.
It is supplied by actual interactions such as tension, friction, gravity, or normal forces rather than being an additional type of force.
For example, gravity supplies the centripetal force on a satellite in a circular orbit.
Charge density
Physics 2 · Definition
Charge density specifies how much electric charge is distributed per unit length, area, or volume.
The appropriate unit and geometry must be chosen before converting between density and total charge.
For example, spreading the same charge uniformly over a larger surface lowers its surface charge density.
Charging by induction
Physics 2 · Definition
Charging by induction uses a nearby charged object to redistribute charge without direct contact with that object.
A grounding connection or separation step can leave the target with net charge while conserving charge for the complete system.
For example, grounding a metal sphere while a negative rod is nearby can let electrons leave before the ground is disconnected.
Circuit node
Physics 2 · Definition
A circuit node is a connected conducting region treated as having one potential in the ideal-wire model.
Branches meet at nodes, so topology rather than the drawing’s shape determines how elements are connected.
For example, moving a wire on a schematic without changing its connections leaves the circuit nodes unchanged.
Coefficient of friction
Physics 1 · Physics 2 · Definition
A coefficient of friction is a dimensionless parameter describing friction between a pair of surfaces in a specified model.
The static coefficient sets a maximum friction magnitude, whereas the kinetic coefficient models friction during sliding.
For example, changing a shoe’s material or the floor condition can change the coefficient without changing the shoe’s mass.
Coherence
Physics 2 · Definition
Coherence describes a sufficiently stable phase relationship between waves for a sustained interference pattern.
It depends on the sources and on the time or distance over which their phase relationship remains predictable.
For example, illuminating two slits with one suitable monochromatic source produces a stable relative phase at the slits.
Compton scattering
Physics 2 · Definition
Compton scattering is the interaction in which a photon changes direction and transfers energy and momentum to an electron.
In the initially stationary free-electron model, a nonzero scattering angle gives the outgoing photon a longer wavelength.
For example, backward scattering gives a larger wavelength increase than forward scattering under the same assumptions.
Conservation of angular momentum
Physics 1 · Physics 2 · Definition
Angular momentum is conserved about a chosen axis when the net external angular impulse about that axis is zero.
Internal interactions can change the distribution of mass and the rotation rates of parts without changing the total.
For example, a rotating skater who pulls their arms inward can spin faster as their rotational inertia decreases.
Conservation of energy
Physics 1 · Physics 2 · Definition
Conservation of energy means total energy is unchanged when the full isolated system is considered.
Energy can transfer between objects or change form, so one selected energy store need not remain constant.
For example, friction can reduce a sliding block’s kinetic energy while increasing the thermal energy of the block and floor.
Conservative force
Physics 1 · Physics 2 · Definition
A conservative force does work that depends only on the starting and ending configurations.
A potential-energy function can describe it, and its work around a closed path is zero.
For example, near-Earth gravity does the same work between two heights whether a ball follows a vertical or a curved path.
Constant-volume process
Physics 2 · Definition
A constant-volume process keeps a system’s volume fixed.
There is no pressure-volume work through the fixed boundary, although heat transfer can change internal energy.
For example, heating gas inside a sealed rigid vessel raises its temperature and pressure without moving its walls.
Constructive interference
Physics 2 · Definition
Constructive interference occurs when overlapping wave contributions reinforce one another.
For equal-frequency sinusoidal waves, in-phase contributions produce increased amplitude at the location considered.
For example, coherent light arriving with an integer-wavelength path difference can form a bright interference fringe.
Continuity equation
Physics 1 · Physics 2 · Definition
The continuity equation expresses conservation of mass in fluid flow.
For steady incompressible flow through one unbranched channel, cross-sectional area times average flow speed is constant.
For example, water moves faster through a narrower part of a pipe when the same volume passes each second.
Convection
Physics 2 · Definition
Convection transfers energy as a fluid moves from place to place.
The circulation can arise from density differences or be driven by a pump or fan.
For example, warmer water can rise while cooler water sinks, carrying thermal energy through a container.
Conventional current
Physics 2 · Definition
Conventional current points in the direction that positive charge would move through a circuit.
In a metal wire, electrons usually drift in the opposite direction.
For example, a current arrow pointing right in a metal wire does not mean the conduction electrons drift right.
Coulomb’s law
Physics 2 · Definition
Coulomb’s law gives the electric force between point charges using their charges and separation.
The magnitude grows with the product of the charge magnitudes and decreases with separation squared, with the medium accounted for in the stated model.
For example, doubling the separation of two fixed point charges reduces the force magnitude to one quarter.
Critical angle
Physics 2 · Definition
The critical angle is the incidence angle at which the refracted ray would travel along the boundary toward a lower-index medium.
At larger incidence angles the ideal model gives total internal reflection.
For example, a ray leaving water for air can refract below the critical angle but undergo total internal reflection above it.
Cross product
Physics 1 · Physics 2 · Definition
A cross product combines two vectors to form a vector perpendicular to both.
Its magnitude depends on both magnitudes and the sine of their included angle, while reversing input order reverses its direction.
For example, a force whose line of action passes through the pivot gives zero torque cross product about that pivot.
Damping
Physics 2 · Definition
Damping is a process that transfers energy out of an oscillation and reduces its amplitude over time.
The motion can remain oscillatory or approach equilibrium without oscillating, depending on the damping strength.
For example, an oscillator moving through a viscous liquid loses mechanical energy to the liquid.
De Broglie wavelength
Physics 2 · Definition
De Broglie wavelength is the wavelength associated with a particle’s momentum in a quantum description.
It is Planck’s constant divided by momentum magnitude, so greater momentum corresponds to shorter wavelength.
For example, electron diffraction is noticeable when the experimental structure has a length scale comparable to the electrons’ de Broglie wavelength.
Decay constant
Physics 2 · Definition
A decay constant describes the decay probability per unit time in an exponential radioactive-decay model.
A larger constant corresponds to a shorter half-life for the same model.
For example, a fixed fractional loss per time interval produces a falling exponential parent-nucleus count rather than a constant absolute loss.
Destructive interference
Physics 2 · Definition
Destructive interference occurs when overlapping wave contributions oppose one another.
Complete cancellation requires equal-magnitude opposite disturbances at the relevant place and time.
For example, equal sinusoidal waves half a cycle out of phase can produce zero net displacement during their overlap.
Diamagnetism
Physics 2 · Definition
Diamagnetism is an induced magnetic response opposing an applied magnetic field.
All materials have a diamagnetic contribution, but stronger magnetic responses can hide it in the net behavior.
For example, a material described as diamagnetic has a net response weakly opposed to the applied field.
Dielectric
Physics 2 · Definition
A dielectric is an insulating material that polarizes in an electric field.
In a capacitor it changes the field response and capacitance; charge or voltage changes depend on whether the battery remains connected.
For example, inserting a dielectric into an isolated charged capacitor can reduce its voltage while its free plate-charge magnitude remains fixed.
Dielectric constant
Physics 2 · Definition
A dielectric constant is a dimensionless measure of a material’s electric permittivity relative to vacuum under specified conditions.
In the ideal fully filled parallel-plate capacitor model, it gives the factor by which the material increases capacitance at fixed geometry.
For example, comparing otherwise identical capacitors with vacuum and a uniform dielectric gives a capacitance ratio equal to that dielectric constant.
Diffraction
Physics 2 · Definition
Diffraction is the spreading and interference of waves near openings or obstacles.
The spreading is especially pronounced when the opening or obstacle size is comparable to wavelength.
For example, sound spreads through a doorway much more noticeably than visible light because sound wavelengths are far larger.
Diffraction grating
Physics 2 · Definition
A diffraction grating is an array of many regularly spaced openings or scattering regions.
Their coherent wave contributions produce narrow reinforced directions determined by spacing and wavelength.
For example, a grating spreads white light into colored orders because different wavelengths reinforce at different angles.
Displacement
Physics 1 · Physics 2 · Definition
Displacement is the vector change in position from the start of an interval to its end.
It depends on those endpoints rather than the length or shape of the path traveled.
For example, walking around a closed track and returning to the start gives zero displacement but a nonzero distance traveled.
Doppler effect
Physics 2 · Definition
The Doppler effect is a difference between emitted and observed frequency caused by relative motion of source and observer.
For sound, both motions must be considered relative to the medium; motion changes the rate at which wavefronts are encountered.
For example, a sound source approaching a stationary listener usually produces a higher observed frequency than the source’s rest frequency.
Drag
Physics 1 · Physics 2 · Definition
Drag is a force from a fluid that opposes an object’s motion relative to that fluid.
Its dependence on speed, shape, and fluid properties varies with the physical regime.
For example, a parachute increases air drag by presenting a large area to the moving air.
Drift velocity
Physics 2 · Definition
Drift velocity is the small average carrier velocity associated with net charge flow in a conductor.
It is distinct from the carriers’ random microscopic motion and from the speed at which a circuit’s electromagnetic changes propagate.
For example, electrons can drift slowly along a wire while a circuit responds rapidly when a switch closes.
Elastic collision
Physics 1 · Physics 2 · Definition
An elastic collision conserves the total kinetic energy of the colliding system.
Total momentum is also conserved when the system has negligible net external impulse, although each object’s momentum and energy can change.
For example, ideal gas atoms redistribute their speeds in elastic collisions without changing the pair’s total kinetic energy.
Electric charge
Physics 2 · Definition
Electric charge is a conserved property that determines electric interactions.
It can be positive or negative and is measured in coulombs; ordinary net charges arise from an imbalance of charged particles.
For example, transferring electrons between initially neutral objects leaves one negatively charged and the other positively charged.
Electric current
Physics 2 · Definition
Electric current is the net rate of charge flow through a specified cross section.
It is measured in amperes, or coulombs per second, with direction defined by positive-charge motion.
For example, the same steady current passes through each element of a simple unbranched series loop.
Electric field
Physics 2 · Definition
An electric field assigns the electric force per unit positive test charge to each location.
The field is produced by the sources and is a vector, while the force on a negative charge points opposite the field.
For example, the electric field between ideal parallel capacitor plates can accelerate an electron opposite the field arrows.
Electric flux
Physics 2 · Definition
Electric flux measures the electric field’s normal contribution through a surface.
Its sign depends on the chosen surface normal, and its calculation includes field strength and orientation rather than a literal flow of matter.
For example, a uniform field parallel to a flat surface gives zero electric flux through that surface.
Electric force
Physics 2 · Definition
Electric force is the interaction of an electric field with a charged object.
For point charges at rest, like signs repel and opposite signs attract, with direction along their separation.
For example, an electron and proton exert equal-magnitude opposite forces on one another despite having different masses.
Electric polarization
Physics 2 · Definition
Electric polarization is a separation or rearrangement of positive and negative charge within otherwise neutral matter.
It can create induced dipoles without requiring a net charge to cross the object’s boundary.
For example, a nearby charged rod can slightly shift bound charges in an insulating object and produce attraction.
Electric potential
Physics 2 · Definition
Electric potential is electric potential energy per unit charge relative to a chosen reference.
It is a scalar measured in volts and can be positive, negative, or zero even where the electric field is nonzero.
For example, contributions from equal positive and negative source charges can cancel the potential at a point without canceling their field vectors.
Electric potential energy
Physics 2 · Definition
Electric potential energy is energy associated with the configuration of interacting charges.
Its value depends on the chosen reference, and a charge pair is counted once when calculating a system’s total pair energy.
For example, bringing two positive point charges closer requires positive external work in a slow controlled approach.
Electrical conductor
Physics 2 · Definition
An electrical conductor contains charge carriers that can move through the material.
In electrostatic equilibrium, mobile charges redistribute until the electric field within the conducting material is zero.
For example, excess charge on an isolated metal object resides on its surface after the charges settle.
Electrical insulator
Physics 2 · Definition
An electrical insulator strongly restricts the movement of charge carriers through its bulk.
Its bound charges can still rearrange slightly to polarize in an applied electric field.
For example, a plastic rod can retain localized excess charge even though it does not conduct current like a metal wire.
Electrical resistance
Physics 2 · Definition
Electrical resistance relates the potential difference across an element to the current through it.
It is measured in ohms and depends on the element’s material, geometry, and conditions.
For example, a longer uniform wire has greater resistance when cross-sectional area and material remain fixed.
Electromagnetic induction
Physics 2 · Definition
Electromagnetic induction is the production of an emf associated with changing magnetic flux or with suitable motion through a magnetic field.
An induced current additionally requires a conducting closed path.
For example, moving a magnet toward a wire loop can induce an emf while holding it still produces none from constant flux.
Electromagnetic spectrum
Physics 2 · Definition
The electromagnetic spectrum is the continuous range of electromagnetic-wave frequencies and wavelengths.
Increasing frequency corresponds to decreasing vacuum wavelength, from radio toward gamma-ray wavelengths.
For example, infrared light has a longer wavelength and lower frequency than visible violet light.
Electromagnetic wave
Physics 2 · Definition
An electromagnetic wave contains oscillating electric and magnetic fields that transport energy.
In the plane-wave vacuum model, both fields are perpendicular to each other and to the propagation direction.
For example, visible light and radio waves can both travel through empty space.
Electromotive force
Physics 2 · Definition
Electromotive force is energy supplied per unit charge by a source or induction process.
Despite its name it is measured in volts, rather than newtons, and can exist even when no current flows.
For example, an ideal battery supplies an emf while an induced emf can be produced by changing magnetic flux.
Electron binding energy
Physics 2 · Definition
Electron binding energy is the energy required to remove an electron from its bound atomic state to the specified free-electron reference.
With a free electron at zero energy, a negative bound-state energy has a binding-energy magnitude equal to its distance below zero.
For example, an excited hydrogen atom requires less additional energy to ionize than the same atom in its ground state.
Electronvolt
Physics 2 · Definition
An electronvolt is an energy unit equal to the elementary-charge magnitude multiplied by one volt.
It is approximately 1.60 × 10⁻¹⁹ joule and is not a unit of voltage.
For example, atomic transition energies are often more conveniently expressed in electronvolts than in very small fractions of a joule.
Electrostatic equilibrium
Physics 2 · Definition
Electrostatic equilibrium is a condition in which the charge distribution has settled and there is no sustained motion of free charges.
Within the material of an ideal conductor, the electric field is zero and the connected conductor is at one potential.
For example, charges on a metal surface rearrange until any tangential surface field that would drive them has vanished.
Elementary charge
Physics 2 · Definition
The elementary charge is the magnitude of the charge carried by a proton or electron, approximately 1.60 × 10⁻¹⁹ coulomb.
A proton carries positive elementary charge and an electron carries negative elementary charge.
For example, removing one electron from a neutral atom leaves it with a net charge of one positive elementary charge.
Emission spectrum
Physics 2 · Definition
An emission spectrum shows the frequencies or wavelengths of radiation emitted by a source.
An excited dilute gas produces characteristic bright lines associated with allowed energy-level transitions.
For example, comparing several observed lines with known element spectra can help identify which atoms are present.
Emissivity
Physics 2 · Definition
Emissivity describes a surface’s thermal-radiation emission compared with an ideal blackbody at the same temperature.
It is dimensionless and can depend on wavelength, surface condition, and temperature.
For example, two surfaces of equal area at the same temperature can radiate different powers because their emissivities differ.
Energy level
Physics 2 · Definition
An energy level is an allowed energy of a quantum system.
Bound systems have discrete levels, and a transition between them requires an energy transfer equal to the level difference.
For example, an atom emitting a photon drops to a lower energy level and transfers the difference to that photon.
Entropy
Physics 2 · Definition
Entropy is a thermodynamic state property related to how many microscopic arrangements are compatible with a macroscopic state.
The second law requires the total entropy of an isolated system not to decrease, although a subsystem’s entropy can decrease.
For example, energy spreading from a hot region into a cooler region of an insulated object increases the total entropy.
Equilibrium position
Physics 1 · Physics 2 · Definition
An equilibrium position is a position at which the net force in the modeled coordinate is zero.
In a stable oscillator, displacement to either side produces a restoring force back toward that position.
For example, a mass on a vertical spring oscillates about a position below the spring’s relaxed-length position because gravity shifts the balance.
Equipotential
Physics 2 · Definition
An equipotential is a set of locations with the same electric potential.
Moving a charge along it produces no electric potential-energy change, and electric field vectors are perpendicular to its surface.
For example, concentric spherical surfaces around an isolated point charge are equipotentials.
Escape speed
Physics 1 · Physics 2 · Definition
Escape speed is the minimum launch speed needed to reach indefinitely large separation in a specified gravitational model without further propulsion.
Its value depends on the launch position and attracting mass, with air resistance and other bodies omitted in the simplest model.
For example, launching from a greater distance from the same spherical planet requires a smaller escape speed.
Excited state
Physics 2 · Definition
An excited state is an allowed state with energy above the system’s ground state.
It can release energy through a transition to a lower state when an allowed process occurs.
For example, an excited atom can emit a photon whose energy equals the difference between two levels.
External force
Physics 1 · Physics 2 · Definition
An external force is exerted on part of a chosen system by something outside that system.
The sum of external forces determines the rate of change of the system’s total momentum.
For example, Earth’s gravitational force is external to a system consisting only of a falling ball.
Faraday’s law
Physics 2 · Definition
Faraday’s law relates induced emf to the rate of change of magnetic flux through a circuit.
Changing field strength, loop area, or loop orientation can change flux, while a large constant flux alone produces no emf.
For example, more identical coil turns produce a larger emf when each turn experiences the same flux-change rate.
Ferromagnetism
Physics 2 · Definition
Ferromagnetism is a strong collective magnetic response associated with aligned domains.
Some ferromagnetic materials can retain substantial magnetization after an applied field is removed.
For example, iron can become a permanent magnet when enough domain orientations remain aligned.
Field line
Physics 2 · Definition
A field line is a drawn curve whose tangent shows the local direction of a vector field.
Closer spacing conventionally represents greater field strength, and lines are a representation rather than physical threads.
For example, electric field lines point away from a positive point charge and toward a negative one.
First law of thermodynamics
Physics 2 · Definition
The first law of thermodynamics expresses conservation of energy for a thermodynamic system.
For the closed gas model used here, internal-energy change equals heat transferred into the gas plus work done on it.
For example, compressing an insulated gas can increase its internal energy even though no heat enters.
Fluid
Physics 1 · Physics 2 · Definition
A fluid is a material that flows and continuously deforms under a sustained shear stress.
Liquids and gases are both fluids, although their compressibility and other properties can differ greatly.
For example, both water in a pipe and air flowing through a duct are fluids.
Focal length
Physics 2 · Definition
Focal length is the signed distance from the relevant lens or mirror reference point to its principal focal point in the chosen convention.
It determines how strongly the optical element converges or diverges paraxial rays.
For example, a converging lens with a shorter positive focal length bends initially parallel rays to a nearer focus.
Focal point
Physics 2 · Definition
A focal point is where rays initially parallel to the principal axis converge or appear to diverge after reflection or refraction.
The principal-axis description uses the paraxial model, in which rays remain close to the axis.
For example, a converging thin lens brings parallel incoming rays to its far focal point.
Force
Physics 1 · Physics 2 · Definition
A force describes an interaction that can change an object’s momentum.
It is a vector measured in newtons, so both size and direction matter when forces are added.
For example, a rope can pull a cart while the floor exerts friction in another direction.
Free fall
Physics 1 · Physics 2 · Definition
Free fall is motion in which gravity is the only significant force acting on the object.
An object can be in free fall while moving upward, downward, or sideways.
For example, a thrown ball is approximately in free fall after leaving the hand when air resistance is negligible.
Free-body diagram
Physics 1 · Physics 2 · Definition
A free-body diagram shows the external forces acting on one selected object or system.
Each arrow names a real interaction, and its direction is the direction of that force.
For example, a book resting on a desk has a downward gravitational force and an upward normal force; velocity is not an extra force arrow.
Frequency
Physics 1 · Physics 2 · Definition
Frequency is the number of complete repeating cycles per unit time.
It is measured in hertz, meaning cycles per second, and is the reciprocal of the period.
For example, a vibrating string with a higher frequency produces more cycles each second.
Gamma emission
Physics 2 · Definition
Gamma emission releases a high-energy photon when an excited nucleus moves to a lower nuclear energy state.
It changes neither the proton count nor the neutron count, so the isotope can remain the same.
For example, a daughter nucleus produced in another decay can subsequently emit a gamma photon as it loses excitation energy.
Gamma radiation
Physics 2 · Definition
Gamma radiation consists of high-energy electromagnetic photons, commonly produced in nuclear or particle processes.
The photons carry energy and momentum but have no electric charge or rest mass; the radiation’s name alone does not specify a decay mode.
For example, a gamma photon belongs to the same electromagnetic spectrum as a radio photon but has a much higher frequency and energy.
Gauge pressure
Physics 1 · Physics 2 · Definition
Gauge pressure is the difference between a system’s pressure and a specified surrounding reference pressure.
An ordinary pressure gauge usually uses local atmospheric pressure as its zero.
For example, a container open to the atmosphere has approximately zero gauge pressure while its absolute pressure is not zero.
Gauss’s law
Physics 2 · Definition
Gauss’s law relates net electric flux through a closed surface to the net charge enclosed by that surface.
The law always holds in its applicable electromagnetic model, while using it to calculate a field simply requires sufficient symmetry.
For example, a charge outside a closed surface contributes zero net electric flux through the whole surface even though its field can be nonzero on parts of it.
Gravitational field
Physics 1 · Physics 2 · Definition
A gravitational field gives the gravitational force per unit mass at a location.
Its direction is the direction of the force on a positive test mass, and its units can be written as newtons per kilogram.
For example, near Earth the field points approximately downward, whether or not a ball is placed there.
Gravitational potential energy
Physics 1 · Physics 2 · Definition
Gravitational potential energy is energy associated with the relative positions of attracting masses.
Near Earth, changes can be approximated by mass times gravitational field strength times height change when that field is nearly constant.
For example, lifting a book increases the gravitational potential energy of the book–Earth system.
Gravity
Physics 1 · Physics 2 · Definition
Gravity is the attractive interaction between masses in the Newtonian model.
For spherical bodies or point-mass approximations, its strength decreases with the square of the separation between their centers.
For example, Earth’s gravity provides the inward force that keeps a satellite in orbit.
Ground state
Physics 2 · Definition
The ground state is the lowest-energy state available to the specified quantum system.
Its energy need not be zero because the energy reference is chosen separately.
For example, a hydrogen atom in its ground state is more tightly bound than the same atom in an excited state.
Grounding
Physics 2 · Definition
Grounding connects a conductor to a large charge reservoir that can receive or supply charge.
The connection lets charge redistribute to satisfy the potential conditions; it does not always make an object neutral in the presence of nearby sources.
For example, a grounded sphere near a charged rod can acquire net charge from the reservoir.
Half-life
Physics 2 · Definition
Half-life is the time in which an ensemble of undecayed parent nuclei is expected to decrease to half its initial number.
It is a statistical property of the isotope and does not predict the decay time of one nucleus.
For example, after two half-lives a large ideal sample has about one quarter of its original parent nuclei remaining.
Hall effect
Physics 2 · Definition
The Hall effect is the transverse charge separation produced when moving carriers in a conductor experience a magnetic force.
The separation creates an electric field and voltage that can balance the sideways magnetic force.
For example, reversing the applied magnetic field reverses the side on which a given carrier sign accumulates.
Head loss
Physics 2 · Definition
Head loss describes a decrease in a flowing fluid’s mechanical energy per unit weight.
Friction and fittings can produce losses that the ideal Bernoulli model omits.
For example, a long rough pipe can require a greater pressure difference than a short smooth one to maintain the same flow.
Heat
Physics 2 · Definition
Heat is energy transferred between systems because of a temperature difference.
It describes a transfer rather than a substance or energy store contained in an object.
For example, when a warmer mug transfers energy to a cooler hand, the mug loses internal energy and the hand gains it.
Heat capacity
Physics 2 · Definition
Heat capacity is the energy needed to raise the temperature of an entire object by one temperature unit under specified conditions.
For a uniform sample in the simple constant-specific-heat model, it equals mass times specific heat.
For example, twice as much of the same material has twice the heat capacity when its state and specific heat are unchanged.
Hooke’s law
Physics 1 · Physics 2 · Definition
Hooke’s law models an ideal spring force as proportional to deformation and opposite in direction.
The deformation is measured from the spring’s relaxed length, and the law applies only within the spring’s approximately elastic linear range.
For example, doubling a small extension doubles the restoring-force magnitude of a spring that obeys this law.
Ideal gas
Physics 2 · Definition
An ideal gas model treats particles as having negligible volume and no sustained intermolecular forces between brief elastic collisions.
The equilibrium state obeys the ideal gas law using absolute pressure and kelvin temperature.
For example, a dilute gas far from condensation often approximates this model better than a dense gas near a phase change.
Ideal gas law
Physics 2 · Definition
The ideal gas law relates absolute pressure, occupied volume, amount of gas, and kelvin temperature.
The forms using moles and using particle count express the same state relationship with different constants.
For example, heating a fixed amount of ideal gas in a rigid sealed container raises its pressure.
Impulse
Physics 1 · Physics 2 · Definition
Impulse is the change in linear momentum produced by a force over a time interval.
It equals the signed vector area under a force-versus-time graph and is measured in newton-seconds.
For example, a padded landing spreads the stopping impulse over more time, reducing the average force for the same momentum change.
Incompressible fluid
Physics 1 · Physics 2 · Definition
An incompressible-fluid model treats density as constant despite pressure changes in the modeled situation.
This approximation often works for liquids over modest pressure ranges but is not a universal property of all fluids.
For example, water flowing through a narrowing pipe can often be treated as incompressible.
Inelastic collision
Physics 1 · Physics 2 · Definition
An inelastic collision does not conserve the colliding system’s total kinetic energy.
Momentum can still be conserved when the net external impulse is negligible, and total energy remains conserved when all stores are included.
For example, carts that latch together undergo a completely inelastic collision and convert some initial kinetic energy into other forms.
Inertia
Physics 1 · Physics 2 · Definition
Inertia is an object’s resistance to a change in its velocity.
Mass measures translational inertia, whereas rotational inertia also depends on how mass is distributed relative to an axis.
For example, a loaded cart accelerates less than an empty cart under the same net force.
Inertial frame
Physics 1 · Physics 2 · Definition
An inertial frame is a reference frame in which an object with no net external force moves at constant velocity.
Frames moving at constant velocity relative to an inertial frame are also inertial in the Newtonian model.
For example, a steadily moving train is approximately inertial, whereas a train accelerating out of a station is not.
Inertial mass
Physics 1 · Physics 2 · Definition
Inertial mass measures how strongly an object resists acceleration under a given net force.
It is the mass in Newton’s second law and is measured in kilograms.
For example, comparing accelerations under the same calibrated force lets an experiment compare two objects’ inertial masses.
Instantaneous velocity
Physics 1 · Physics 2 · Definition
Instantaneous velocity is the rate of change of position at a particular moment.
On a position-versus-time graph, its component is the slope of the tangent at that time.
For example, the velocity just before a ball lands differs from the average velocity of its entire flight.
Internal energy
Physics 2 · Definition
Internal energy is energy associated with the microscopic motion and configuration of a system’s constituents.
It excludes the kinetic energy of the whole system’s bulk motion and depends on the thermodynamic state.
For example, a stationary solid can gain internal energy as its atomic vibrations become more energetic.
Internal force
Physics 1 · Physics 2 · Definition
An internal force is an interaction between parts of a chosen system.
Newton’s third-law partners cancel in the total force sum for the whole system, although individual parts can accelerate.
For example, two skaters pushing each other exert internal forces when the two skaters are analyzed together.
Internal resistance
Physics 2 · Definition
Internal resistance models energy dissipation inside a real electrical source.
When the source delivers current, its terminal voltage can be smaller than its emf because of this internal voltage drop.
For example, connecting a heavy load to a battery can lower the voltage measured at its terminals.
Ion
Physics 2 · Definition
An ion is an atom or group of atoms with a nonzero net electric charge.
For an atomic ion, the electron count differs from the proton count while the element remains fixed by its protons.
For example, removing an electron from a neutral atom makes a positive ion.
Isobaric process
Physics 2 · Definition
An isobaric process occurs at constant pressure.
The volume and temperature can change, and a moving boundary can allow pressure-volume work.
For example, heating a gas beneath a freely moving loaded piston can increase its volume while its pressure stays approximately constant.
Isolated system
Physics 1 · Physics 2 · Definition
An isolated system exchanges neither energy nor matter with its surroundings.
In a momentum problem, conservation of total momentum specifically requires zero net external impulse over the interval analyzed.
For example, two colliding carts can be treated as momentum-isolated horizontally when the external horizontal impulse is negligible.
Isothermal process
Physics 2 · Definition
An isothermal process occurs at constant temperature.
For a fixed amount of ideal gas, internal energy remains constant even though heat and work can be transferred.
For example, a gas expanding slowly while exchanging energy with a temperature reservoir can remain isothermal.
Isotope
Physics 2 · Definition
Isotopes are atoms of the same element with different numbers of neutrons.
They share an atomic number but have different mass numbers and can differ in nuclear stability.
For example, carbon-12 and carbon-14 are isotopes because both have six protons but different neutron counts.
Kelvin temperature scale
Physics 2 · Definition
The kelvin scale measures thermodynamic temperature from absolute zero.
A temperature interval of one kelvin has the same size as one degree Celsius, but their zero points differ.
For example, temperature ratios in the ideal gas law must use kelvin values rather than Celsius values.
Kinematics
Physics 1 · Physics 2 · Definition
Kinematics describes motion using position, velocity, acceleration, and time without first analyzing its forces.
The familiar constant-acceleration equations apply only over intervals in which that acceleration is constant.
For example, a ball falling near Earth with negligible air resistance can be modeled with constant vertical acceleration.
Kinetic energy
Physics 1 · Physics 2 · Definition
Kinetic energy is energy associated with motion relative to a chosen reference frame.
For a nonrelativistic translating object it is one half of its mass times speed squared.
For example, doubling a cart’s speed quadruples its translational kinetic energy when its mass stays fixed.
Kinetic friction
Physics 1 · Physics 2 · Definition
Kinetic friction acts between surfaces that slide relative to one another.
It opposes their relative slipping and is often modeled with magnitude equal to the kinetic-friction coefficient times the normal force.
For example, a rapidly moving belt can exert forward kinetic friction on a slower box sliding on it.
Kirchhoff’s junction rule
Physics 2 · Definition
Kirchhoff’s junction rule says total current entering a node equals total current leaving when charge does not accumulate there.
It follows from conservation of charge and applies regardless of how the branches are drawn.
For example, current entering a branching connection divides among the outgoing paths without charge being consumed at the node.
Kirchhoff’s loop rule
Physics 2 · Definition
Kirchhoff’s loop rule sets the signed sum of potential changes around a closed circuit loop to zero in the circuit model used here.
It expresses energy accounting per unit charge and requires consistent signs for sources and components.
For example, a battery’s potential rise balances the total resistor drops around a simple steady loop.
Latent heat
Physics 2 · Definition
Specific latent heat is the energy transferred per unit mass during a specified phase change at the stated conditions.
During an ideal equilibrium phase change at constant pressure, energy can change microscopic arrangement while temperature stays constant.
For example, melting ice requires energy even while an ice–water mixture remains at its melting temperature.
Lenz’s law
Physics 2 · Definition
Lenz’s law gives an induced current direction whose magnetic effect opposes the change in flux that produced it.
It opposes the change rather than necessarily opposing the existing field itself.
For example, when an outward magnetic flux decreases, the induced current can create an outward field that resists that decrease.
Lever arm
Physics 1 · Physics 2 · Definition
A lever arm is the perpendicular distance from a chosen rotation axis to a force’s line of action.
It determines the torque magnitude for a given force and need not equal the distance to the point of application.
For example, a force aimed directly toward a hinge has zero lever arm and produces no torque about that hinge.
Linear momentum
Physics 1 · Physics 2 · Definition
Linear momentum is a vector describing an object’s motion and equals mass times velocity in the nonrelativistic model.
The vector sum of momenta is conserved when the net external impulse on the system is zero.
For example, two identical carts moving equally fast in opposite directions have zero total linear momentum but nonzero total kinetic energy.
Longitudinal wave
Physics 2 · Definition
In a longitudinal wave, the disturbance is parallel to the direction of propagation.
Compression and rarefaction can travel while particles oscillate locally along that direction.
For example, sound in air is a longitudinal pressure disturbance rather than a stream of air traveling all the way from source to listener.
Magnetic dipole
Physics 2 · Definition
A magnetic dipole has a paired north–south magnetic character, modeled by a small magnet or a current loop.
Its field lines form closed loops, and an external field can exert a turning torque on it.
For example, cutting a bar magnet creates smaller dipoles rather than isolated north and south poles.
Magnetic domain
Physics 2 · Definition
A magnetic domain is a region of a ferromagnetic material in which many microscopic magnetic dipoles are aligned.
Different domains can have different orientations, so their contributions may largely cancel before magnetization.
For example, an applied magnetic field can favor domains aligned with it and leave a piece of iron magnetized.
Magnetic field
Physics 2 · Definition
A magnetic field assigns a vector direction and strength to each location and predicts interactions with moving charges, currents, and magnetic materials.
Its strength is measured in teslas, and the force on a moving charge need not point along the field.
For example, a compass aligns with the local field while a moving charge can be deflected perpendicular to it.
Magnetic flux
Physics 2 · Definition
Magnetic flux measures the magnetic field’s perpendicular contribution through a chosen surface.
For a uniform field and flat surface it depends on field strength, area, and the angle to the area normal, and it is measured in webers.
For example, rotating a loop from face-on to edge-on to a uniform field reduces its flux magnitude toward zero.
Magnetic permeability
Physics 2 · Definition
Magnetic permeability relates a material’s magnetic response to an applied field in a specified model.
The response can depend on field strength, direction, and temperature, so one constant need not describe every condition.
For example, a ferromagnetic core approaching saturation does not keep increasing its magnetization in direct proportion to the applied field.
Magnitude
Physics 1 · Physics 2 · Definition
Magnitude is the nonnegative size of a vector quantity.
It does not specify direction, and it differs from a signed component.
For example, a westward velocity can have a negative x component while its speed, the velocity magnitude, stays positive.
Mass defect
Physics 2 · Definition
Mass defect is the difference between the summed rest masses of separated constituents and the rest mass of their bound system.
For a nucleus, the constituents include all its free protons and neutrons.
For example, a positive nuclear mass defect corresponds to energy released when those nucleons assemble into the bound nucleus.
Mass density
Physics 1 · Physics 2 · Definition
Mass density is mass per unit volume of a material or sample.
It is commonly measured in kilograms per cubic meter and can vary with temperature or pressure.
For example, equal-volume samples of water and oil generally have different masses because their densities differ.
Mass number
Physics 2 · Definition
Mass number is the total number of protons and neutrons in a nucleus.
It is a count rather than the atom’s measured mass in kilograms or atomic mass units.
For example, subtracting atomic number from mass number gives the neutron count of an isotope.
Maxwell–Boltzmann distribution
Physics 2 · Definition
A Maxwell–Boltzmann speed distribution describes the range of particle speeds in a classical gas at thermal equilibrium.
For the same species, a higher temperature shifts the distribution toward larger speeds and broadens it while preserving the normalized total population.
For example, the tallest point identifies the most likely speed rather than the speed of every particle or the rms speed.
Mechanical energy
Physics 1 · Physics 2 · Definition
Mechanical energy is the sum of the kinetic and potential energies included in a model.
It stays constant when transfers to or from other energy stores and across the chosen boundary are absent.
For example, a pendulum’s mechanical energy can remain approximately constant while energy shifts between motion and gravitational configuration.
Mechanical equilibrium
Physics 1 · Physics 2 · Definition
Mechanical equilibrium means there is no translational or angular acceleration for the system being considered.
For a rigid body, both the net external force and the net external torque must vanish.
For example, a stationary ladder supported against a wall requires both force balance and torque balance.
Mechanical work
Physics 1 · Physics 2 · Definition
Mechanical work is energy transferred by a force through displacement of its point of application.
For a constant force on a particle, it depends on the force component along the particle’s displacement, and it can be positive, negative, or zero.
For example, a magnetic force perpendicular to a particle’s displacement does zero work on that particle.
Mole
Physics 2 · Definition
A mole is an amount of substance containing Avogadro’s number of specified entities.
Those entities must be named, such as atoms, molecules, or ions, and a mole is not itself a mass unit.
For example, one mole of helium atoms and one mole of oxygen molecules contain the same number of specified entities but have different masses.
Net force
Physics 1 · Physics 2 · Definition
Net force is the vector sum of all forces acting on the object or system under consideration.
For constant mass in an inertial frame, it equals mass times acceleration.
For example, equal upward and downward forces give zero net force even though neither force is zero.
Neutrino
Physics 2 · Definition
Neutrinos are electrically neutral elementary particles with very small masses and weak interactions with ordinary matter.
Neutrinos and antineutrinos carry energy, momentum, and lepton number in beta-decay accounting.
For example, an electron antineutrino accompanies the electron emitted in beta-minus decay.
Nonconservative force
Physics 1 · Physics 2 · Definition
A nonconservative force can do work that depends on the path followed.
Its effects cannot be described by a single configuration-dependent potential energy for that interaction alone.
For example, sliding friction transfers more mechanical energy into thermal energy along a longer sliding path under the same conditions.
Normal force
Physics 1 · Physics 2 · Definition
A normal force is a contact force perpendicular to the local surface of contact.
Its magnitude is set by the motion and the other forces, so it need not equal the object’s weight.
For example, a wall’s normal force on a leaning ladder points horizontally rather than upward.
Nuclear binding energy
Physics 2 · Definition
Nuclear binding energy is the energy required to separate a nucleus into free protons and neutrons.
It equals the mass defect multiplied by the square of the speed of light, with both proton and neutron masses included.
For example, forming a bound nucleus releases energy because its rest mass is smaller than that of its separated nucleons.
Nuclear fission
Physics 2 · Definition
Nuclear fission splits a nucleus into smaller nuclei and may also emit other particles.
The energy outcome depends on the reaction’s mass balance, so not every arbitrary splitting releases energy.
For example, fission of an appropriate heavy nucleus can release kinetic energy, photons, and neutrons that can initiate further reactions.
Nuclear fusion
Physics 2 · Definition
Nuclear fusion combines smaller nuclei into a larger nucleus and can also emit other particles.
Energy release depends on the reaction’s mass balance, and initially the charged nuclei must come close despite electric repulsion.
For example, suitable light-hydrogen-isotope reactions can release energy when their products are more tightly bound.
Ohm’s law
Physics 2 · Definition
Ohm’s law describes an element whose current is proportional to its potential difference under the stated conditions.
Such an element has constant resistance over the modeled range, giving a straight-line current–voltage relationship through the origin.
For example, a fixed-temperature ideal resistor follows this model, whereas a warming filament bulb need not.
Optical magnification
Physics 2 · Definition
Optical magnification is the ratio of image height to object height in the chosen signed convention.
Its magnitude gives the size ratio, while its sign indicates upright or inverted orientation.
For example, an inverted image half the object’s height has negative magnification with magnitude one half.
Optical path length
Physics 2 · Definition
Optical path length accounts for wave phase accumulated along a path through a medium.
For a uniform medium it equals refractive index times geometric path length at the relevant frequency.
For example, equal geometric distances through air and glass generally correspond to different optical path lengths.
Orbital period
Physics 1 · Physics 2 · Definition
An orbital period is the time needed for one complete orbit.
In a two-body model it depends on the attracting mass and the orbit’s size rather than just the orbiting object’s mass.
For example, a larger circular orbit around the same planet has a longer orbital period.
Oscillation period
Physics 1 · Physics 2 · Definition
An oscillation period is the time required for one complete repeating cycle.
It is the reciprocal of frequency and is measured in seconds.
For example, measuring the time for several pendulum swings and dividing by the number of cycles estimates the period.
Oscillation phase
Physics 1 · Physics 2 · Definition
Phase specifies the position within a repeating oscillation cycle.
A phase difference tells how much one oscillation leads or lags another at a chosen place and time.
For example, two sinusoidal waves half a cycle out of phase can cancel when their amplitudes are equal.
Parallel connection
Physics 2 · Definition
A parallel connection joins elements across the same two nodes.
They therefore have the same potential difference, while their branch currents can differ.
For example, two different resistors connected in parallel across a battery generally carry different currents.
Parallel-axis theorem
Physics 1 · Physics 2 · Definition
The parallel-axis theorem relates rotational inertia about a center-of-mass axis to inertia about a parallel displaced axis.
The displaced-axis value is the center-of-mass value plus total mass times the squared separation of the axes.
For example, a uniform rod has greater rotational inertia about one end than about a parallel axis through its middle.
Paramagnetism
Physics 2 · Definition
Paramagnetism is a usually weak tendency for a material’s magnetic dipoles to align with an applied field.
The additional alignment generally disappears when the field is removed.
For example, aluminum has a weak positive magnetic response under the conditions described in the course model.
Pendulum
Physics 1 · Physics 2 · Definition
A pendulum is a system that swings about a support under a restoring torque.
A simple pendulum approximates a small mass on a light string and follows simple harmonic motion only for sufficiently small angles.
For example, a short string pendulum completes small swings more quickly than a longer one under the same gravitational field.
Permittivity
Physics 2 · Definition
Permittivity describes a medium’s electric response and relates electric fields to charge in the chosen model.
A material’s polarization can make its response differ from that of vacuum.
For example, placing a dielectric between capacitor plates can change the field and capacitance because the material polarizes.
Photoelectric effect
Physics 2 · Definition
The photoelectric effect is electron emission from a material after it absorbs electromagnetic radiation with sufficient energy.
In the ordinary single-photon model, the photon must meet the material’s work-function threshold before an electron can escape.
For example, making below-threshold light brighter does not cause emission in that model because each photon still has insufficient energy.
Photon
Physics 2 · Definition
A photon is a quantum of electromagnetic radiation with energy proportional to its frequency.
It has zero rest mass and no electric charge but carries energy and momentum.
For example, increasing light’s frequency increases the energy of each photon, while increasing the number of same-frequency photons increases the total energy.
Physical system
Physics 1 · Physics 2 · Definition
A physical system is the object or collection of objects chosen for an analysis.
Its boundary determines which forces and energy transfers count as internal or external.
For example, friction between a block and a floor is an external interaction for the block alone but internal when both are included.
Planck’s constant
Physics 2 · Definition
Planck’s constant relates the energy of a photon to its frequency.
It also sets the scale connecting particle momentum to de Broglie wavelength.
For example, two photons of different frequencies have energies in the same ratio as their frequencies because the constant is the same for both.
Positron
Physics 2 · Definition
A positron is the electron’s antiparticle.
It has the same rest mass as an electron but carries positive elementary charge.
For example, beta-plus decay emits a positron along with an electron neutrino.
Potential difference
Physics 2 · Definition
Potential difference is the change in electric potential between two locations.
It represents electric potential-energy change per unit charge and is measured in volts.
For example, the potential difference across a resistor describes the energy transferred per coulomb passing through it.
Potential energy
Physics 1 · Physics 2 · Definition
Potential energy describes energy associated with the configuration of interacting parts of a system.
Its numerical zero is a chosen reference, while differences in potential energy determine physical energy changes.
For example, separating Earth and a ball changes their gravitational potential energy even though the energy belongs to their interaction.
Power
Physics 1 · Physics 2 · Definition
Power is the rate at which energy is transferred or converted.
It is measured in watts, with one watt equal to one joule per second.
For example, two motors can lift the same load through the same height but have different average powers if they take different times.
Pressure
Physics 1 · Physics 2 · Definition
Pressure is the normal force per unit area exerted on a surface.
It is a scalar measured in pascals, with one pascal equal to one newton per square meter.
For example, a gas exerts pressure throughout its volume through many microscopic collisions, even when its bulk motion is zero.
Projectile motion
Physics 1 · Physics 2 · Definition
Projectile motion describes an object after launch when gravity is the only significant force in the chosen model.
Near Earth, horizontal and vertical motion can be analyzed separately using the same time interval and approximately constant downward acceleration.
For example, a ball rolling off a table continues horizontally while its downward velocity increases.
Quantization
Physics 2 · Definition
Quantization means a quantity can take only certain allowed values in the stated quantum system.
Bound-state boundary conditions can restrict energy to discrete levels rather than a continuous range.
For example, an electron bound in a hydrogen atom cannot have an arbitrary energy between its allowed levels.
Radian
Physics 1 · Physics 2 · Definition
A radian is an angle measure defined by arc length divided by radius.
One complete turn is two pi radians, and angle formulas connecting linear and rotational motion use this measure.
For example, an arc whose length equals the circle’s radius subtends one radian at the center.
Radioactive decay
Physics 2 · Definition
Radioactive decay is a spontaneous nuclear transformation to different nuclei or to a lower energy state of the same nucleus.
Individual decay times are unpredictable, while probabilities describe the behavior of a large sample.
For example, gamma emission can lower a nucleus’s energy without changing its isotope.
RC circuit
Physics 2 · Definition
An RC circuit contains resistance and capacitance that produce time-dependent charging or discharging.
Its initial and long-time behavior depends on the capacitor’s initial voltage and its actual connection to the source and resistors.
For example, a discharged capacitor in a simple series charging loop initially carries current that decreases toward zero as its voltage rises.
RC time constant
Physics 2 · Definition
An RC time constant sets the characteristic time scale of capacitor charging or discharging in a specified resistance–capacitance connection.
For a simple series RC loop it equals resistance times capacitance and is measured in seconds.
For example, increasing the relevant resistance stretches the charging curve when the capacitance remains fixed.
Real image
Physics 2 · Definition
A real image forms where light rays physically converge after leaving an optical element.
It can be projected onto a screen placed at the image location.
For example, a projector’s lens forms a real image of its source on a screen.
Reference frame
Physics 1 · Physics 2 · Definition
A reference frame specifies the observer and coordinates used to describe positions and motion.
Different observers can assign different velocities to the same object.
For example, a seated passenger is at rest relative to a train but moving relative to the station.
Reflection
Physics 2 · Definition
Reflection occurs when a wave returns from a boundary into the region from which it arrived.
For a light ray at a surface, the reflection angle equals the incidence angle when both are measured from the local normal.
For example, a smooth mirror produces an orderly image while a rough surface scatters reflected rays in many directions.
Refraction
Physics 2 · Definition
Refraction is a change in wave propagation at a boundary where wave speed changes.
For light entering a new medium, frequency stays fixed while speed and wavelength can change, and the direction generally bends unless incidence is normal.
For example, a light ray entering water obliquely from air bends toward the normal.
Refractive index
Physics 2 · Definition
Refractive index is the ratio of light’s speed in vacuum to its speed in the material at the relevant frequency.
It is dimensionless and can depend on wavelength.
For example, glass usually has a larger refractive index than air, so visible light travels more slowly through the glass.
Resistivity
Physics 2 · Definition
Resistivity is a material property describing resistance to electrical current under specified conditions.
It is distinct from the resistance of a particular object and is measured in ohm-meters.
For example, changing a wire’s length changes its resistance without changing the material’s resistivity when temperature and composition stay fixed.
Resonance
Physics 2 · Definition
Resonance is a large oscillatory response when a periodic driving influence has a frequency near a system’s natural frequency.
The response also depends on damping and on how the driving influence couples to the system.
For example, repeated pushes timed to a swing’s motion can build its amplitude.
Restoring force
Physics 1 · Physics 2 · Definition
A restoring force points toward an equilibrium position when an object is displaced from it.
Simple harmonic motion requires the net restoring force to be proportional to displacement and opposite in direction.
For example, a compressed ideal spring pushes its attached object back toward the spring’s relaxed length.
Resultant vector
Physics 1 · Physics 2 · Definition
A resultant vector is the vector sum of two or more vectors.
Its components are the sums of the corresponding components of the vectors being combined.
For example, two equal forces in opposite directions have a zero resultant even though both forces are present.
Right-hand rule
Physics 1 · Physics 2 · Definition
A right-hand rule is a convention for determining the direction of a vector cross product or a current-produced magnetic field.
The chosen gesture must use the correct input vectors in their correct order, with reversal when a negative charge requires it.
For example, the thumb-and-curl rule around a straight wire uses conventional current as the thumb direction.
Rolling without slipping
Physics 1 · Physics 2 · Definition
Rolling without slipping means the point of contact has no instantaneous relative motion along the contacting surface.
For a wheel rolling on a stationary surface, the center-of-mass speed equals angular speed times radius.
For example, a wheel rolling smoothly along a road has a momentarily stationary contact point while its top moves faster than its center.
Root-mean-square speed
Physics 2 · Definition
Root-mean-square speed is the square root of the mean of the particles’ squared speeds.
It represents a kinetic-energy average and is different from both average speed and vector-average velocity.
For example, an equilibrium gas can have zero vector-average velocity while its rms speed is substantial.
Rotational equilibrium
Physics 1 · Physics 2 · Definition
Rotational equilibrium means the net external torque about the chosen axis is zero.
A rigid body’s angular velocity then stays constant when its rotational inertia is fixed; it need not be at rest.
For example, opposing torques can balance on a wheel that is already rotating steadily.
Rotational inertia
Physics 1 · Physics 2 · Definition
Rotational inertia measures how strongly a mass distribution resists a change in angular velocity about a specified axis.
Mass farther from that axis contributes more strongly because each contribution depends on squared distance.
For example, moving small masses outward along a rotating rod increases its rotational inertia without changing its total mass.
Rotational kinetic energy
Physics 1 · Physics 2 · Definition
Rotational kinetic energy is the kinetic energy associated with rotation about an axis.
For a rigid body about a fixed axis it is one half of rotational inertia times angular speed squared.
For example, a rolling wheel can possess both translational kinetic energy of its center of mass and rotational kinetic energy about that center.
Scalar
Physics 1 · Physics 2 · Definition
A scalar is a quantity described by a value and unit without a spatial direction.
A scalar can have a sign when its definition or reference permits one; scalar does not mean always positive.
For example, mass is a scalar, whereas the velocity of that mass also needs a direction.
Second law of thermodynamics
Physics 2 · Definition
The second law of thermodynamics states that the total entropy of an isolated system cannot decrease.
It explains the preferred direction of spontaneous processes without replacing conservation of energy.
For example, a cooler object does not spontaneously transfer net thermal energy to a hotter object without another compensating process.
Series connection
Physics 2 · Definition
A series connection places elements along one unbranched path.
The same current passes through the series elements, while their potential differences can differ.
For example, adding another resistor to a simple series loop raises the total resistance and reduces current from a fixed ideal source.
Short circuit
Physics 2 · Definition
A short circuit provides a very low-resistance path between nodes that would otherwise have a potential difference.
It can bypass components and draw a large current limited by the real source and wiring resistance.
For example, a wire directly across an ideal resistor can bypass that resistor; a real battery short can heat the wiring dangerously.
Simple harmonic motion
Physics 1 · Physics 2 · Definition
Simple harmonic motion occurs when the net restoring force is proportional to displacement from equilibrium and points in the opposite direction.
The resulting position varies sinusoidally with time in the ideal model.
For example, a mass attached to an ideal spring can execute simple harmonic motion when damping is negligible.
Snell’s law
Physics 2 · Definition
Snell’s law relates the incidence and refraction angles to the refractive indices of two media.
Both angles are measured from the normal to the boundary, rather than from the surface.
For example, entering a higher-index material at oblique incidence bends a light ray toward the normal.
Specific gravity
Physics 2 · Definition
Specific gravity is a material’s density divided by a chosen reference density.
It is dimensionless; for liquids and solids the reference is usually water at a specified temperature.
For example, a liquid with specific gravity below one is less dense than the reference water.
Specific heat
Physics 2 · Definition
Specific heat is the energy needed per unit mass for a unit temperature increase under the stated conditions.
Its units are joules per kilogram per kelvin, and the simple calorimetry model treats it as constant without a phase change.
For example, equal masses of two materials can require different energy transfers for the same temperature rise.
Spring constant
Physics 1 · Physics 2 · Definition
A spring constant measures the stiffness of a spring within its approximately linear range.
Its units are newtons per meter, and a larger value means more force is needed for the same deformation.
For example, two springs stretched by the same distance exert different forces if one is stiffer.
Spring potential energy
Physics 1 · Physics 2 · Definition
Spring potential energy is energy stored in an ideal spring’s deformation from its relaxed length.
It is proportional to the square of deformation, so equal extension and compression magnitudes store equal energy.
For example, compressing a spring before releasing a toy transfers energy that can later become kinetic energy.
Standing wave
Physics 2 · Definition
A standing wave is a spatially fixed interference pattern formed by suitable waves traveling in opposite directions.
Its nodes remain at zero displacement while antinodes mark positions of maximum oscillation amplitude.
For example, a string fixed at both ends can support standing waves whose wavelengths fit the boundary conditions.
State variable
Physics 2 · Definition
A state variable is determined by a system’s present equilibrium state rather than by the route used to reach it.
Internal energy and entropy are state properties, whereas heat and work describe transfers along a process.
For example, two gas processes with identical endpoints have the same internal-energy change but can involve different amounts of work.
Static friction
Physics 1 · Physics 2 · Definition
Static friction opposes the tendency for relative sliding between surfaces that are not sliding at their contact.
In the simple model it adjusts up to a maximum set by the coefficient of static friction and normal force.
For example, static friction can push a walking person forward when the foot pushes backward on the ground.
Stefan–Boltzmann constant
Physics 2 · Definition
The Stefan–Boltzmann constant is the proportionality constant in the ideal blackbody law linking emitted power to surface area and the fourth power of kelvin temperature.
Its value is approximately 5.67 × 10⁻⁸ W/(m²·K⁴), so multiplying it by area and temperature to the fourth power gives watts.
For example, the same constant applies to two ideal blackbodies of different sizes, while their areas and temperatures determine their different emitted powers.
Stefan–Boltzmann law
Physics 2 · Definition
The Stefan–Boltzmann law gives an ideal blackbody’s total emitted power as proportional to surface area and kelvin temperature to the fourth power.
Total emitted power differs from net radiative transfer because surrounding objects can also supply radiation.
For example, doubling a blackbody’s surface area doubles its emitted power when its temperature stays fixed.
Stopping potential
Physics 2 · Definition
Stopping potential is the smallest retarding-voltage magnitude that stops the fastest photoelectrons from reaching the collector.
Its electric energy barrier measures the maximum kinetic energy of the emitted electrons.
For example, increasing above-threshold light frequency raises the maximum kinetic energy and therefore raises the required stopping-potential magnitude.
Streamline
Physics 1 · Physics 2 · Definition
A streamline is a curve tangent to the local fluid velocity at each point at one instant.
In steady flow its shape stays fixed and fluid particles follow it, while in changing flow a streamline need not be a particle’s full path.
For example, streamline arrows around a smooth obstacle show the local direction of the passing fluid.
Strong interaction
Physics 2 · Definition
The strong interaction binds protons and neutrons at nuclear distances.
It acts over a very short effective range between nucleons, while electric repulsion between protons also affects nuclear stability.
For example, bringing nuclei close enough for fusion requires overcoming their electric repulsion before nuclear binding can occur.
Superposition
Physics 1 · Physics 2 · Definition
Superposition means adding the separate contributions to obtain a total response in a linear model.
Vector fields add vectorially, scalar potentials add algebraically, and wave disturbances add with their signs at each place and time.
For example, equal but opposite electric-field contributions can give zero net field at one point while both sources remain present.
Temperature
Physics 2 · Definition
Temperature is a state quantity that determines the direction of net energy transfer through thermal contact.
For a classical ideal gas it is proportional to the average translational kinetic energy per particle, rather than to the total energy of the sample.
For example, a small hot metal sample can have a higher temperature but less total internal energy than a large cooler sample.
Tension
Physics 1 · Physics 2 · Definition
Tension is a pulling force transmitted through a stretched rope, string, or similar connector.
It acts along the connector away from the object it pulls, and an ideal massless rope has the same tension throughout an uninterrupted segment.
For example, a rope supporting an accelerating load can have a tension different from the load’s weight.
Terminal speed
Physics 2 · Definition
Terminal speed is a constant speed reached when the forces along the direction of motion balance.
For a falling object in fluid, increasing drag can eventually balance weight after buoyancy is also accounted for.
For example, a falling raindrop can reach approximately constant downward velocity instead of accelerating indefinitely.
Thermal conduction
Physics 2 · Definition
Thermal conduction transfers energy through microscopic interactions without bulk flow of the material.
Particles and, in metals, mobile electrons can carry energy from hotter regions toward cooler regions.
For example, the handle of a metal spoon warms when its other end is placed in hot water.
Thermal conductivity
Physics 2 · Definition
Thermal conductivity measures how readily a material conducts energy through a temperature gradient.
It is a material property with units watts per meter per kelvin, distinct from the sample’s heat capacity or thickness.
For example, a metal and an insulating foam slab of the same dimensions can have very different conduction rates.
Thermal equilibrium
Physics 2 · Definition
Thermal equilibrium between systems means their temperatures are equal and there is no net energy transfer between them through thermal processes.
Microscopic energy exchanges can continue while their opposing contributions balance.
For example, water and a metal sample left together in an insulated cup approach a common equilibrium temperature.
Thermal expansion
Physics 2 · Definition
Thermal expansion is a change in a material’s dimensions as its temperature changes.
A simple coefficient model approximates the change over a limited temperature range and depends on the material and geometry.
For example, leaving gaps between sections of a metal rail allows space for expansion in warmer conditions.
Thermal radiation
Physics 2 · Definition
Thermal radiation is electromagnetic energy emitted by matter because of its temperature.
It can transfer energy through a vacuum, and net radiative transfer between ordinary bodies is from hotter to cooler.
For example, sunlight warming a surface does not need air to carry the energy across space.
Thin lens
Physics 2 · Definition
A thin-lens model treats a lens’s thickness as negligible compared with the relevant object and image distances.
The model relates focal length to signed object and image distances and is most accurate for paraxial rays.
For example, a converging lens can form a real or a virtual image depending on the object’s position relative to its focal point.
Thin-film interference
Physics 2 · Definition
Thin-film interference occurs when waves reflected from different film interfaces overlap.
The resulting phase difference includes both travel through the film and any phase reversals at reflection.
For example, the colors of a soap film change with thickness because different wavelengths are reinforced.
Threshold frequency
Physics 2 · Definition
A threshold frequency is the lowest light frequency that meets a surface’s work-function requirement in the single-photon photoelectric model.
Below it, increasing intensity alone does not produce emitted electrons in that model.
For example, a material with a larger work function requires higher-frequency photons to reach the emission threshold.
Torque
Physics 1 · Physics 2 · Definition
Torque describes a force’s turning effect about a chosen axis or point.
Its magnitude is force times the perpendicular lever arm, and its direction or sign describes the rotation tendency.
For example, pushing near the handle of a door produces more torque than the same perpendicular push near the hinge.
Total internal reflection
Physics 2 · Definition
Total internal reflection occurs when light travels toward a lower-index medium at an incidence angle above the critical angle.
Both the higher-to-lower index direction and the angle condition are required.
For example, light can remain guided inside an optical fiber by repeated total internal reflection at its boundary.
Transverse wave
Physics 2 · Definition
In a transverse wave, the disturbance is perpendicular to the direction of propagation.
The disturbance direction and the energy-transfer direction must therefore be distinguished.
For example, a rope pulse can travel horizontally while rope segments move up and down.
Unit vector
Physics 1 · Physics 2 · Definition
A unit vector has magnitude one and specifies a direction.
Multiplying it by a signed component gives the vector contribution along that direction.
For example, the Cartesian unit vectors identify the positive x, y, and z directions.
Vector
Physics 1 · Physics 2 · Chemistry · Definition
A vector is a quantity with a magnitude and a direction, represented by an arrow or by components along chosen axes.
Vectors add by adding corresponding components rather than simply adding their magnitudes.
For example, an eastward displacement followed by a northward displacement produces a resultant pointing northeast.
The zero vector has zero magnitude and no unique direction.
Vector component
Physics 1 · Physics 2 · Definition
A vector component describes the part of a vector along a chosen axis.
Its signed value changes when the axes change, even though the physical vector remains the same.
For example, a diagonal launch velocity can be resolved into horizontal and vertical components that share the same elapsed time.
Velocity
Physics 1 · Physics 2 · Definition
Velocity describes how position changes with time and includes direction.
Instantaneous velocity describes one moment, while average velocity is displacement divided by elapsed time.
For example, a car turning a corner can change velocity while maintaining the same speed.
Virtual image
Physics 2 · Definition
A virtual image is located where backward extensions of outgoing rays appear to meet.
The rays do not physically converge at that location, so placing a screen there does not capture that image.
For example, a plane mirror produces an image that appears behind the mirror.
Viscosity
Physics 1 · Physics 2 · Definition
Viscosity measures a fluid’s resistance to relative motion between neighboring layers.
Viscous effects can dissipate mechanical energy and alter the velocity profile of flowing fluid.
For example, honey flows more reluctantly than water under comparable conditions because it is more viscous.
Voltmeter
Physics 2 · Definition
A voltmeter measures potential difference between two nodes.
It is connected in parallel and ideally has very high resistance so it draws negligible current.
For example, placing its probes on opposite ends of a resistor measures that resistor’s voltage drop.
Wave
Physics 2 · Definition
A wave is a propagating disturbance that can transfer energy without transporting the medium as a whole.
Mechanical waves require a material medium, while electromagnetic waves can propagate through vacuum.
For example, a pulse travels along a rope while individual rope segments move locally around their equilibrium positions.
Wave intensity
Physics 2 · Definition
Wave intensity is the time-averaged energy-transfer rate per area perpendicular to propagation.
It is measured in watts per square meter and differs from total power because area matters.
For example, spreading the same sound power over a larger area reduces the intensity there.
Wave interference
Physics 2 · Definition
Wave interference is the combination of overlapping wave disturbances.
In a linear medium, the signed disturbances add at each location and time without permanently destroying the passing waves.
For example, two overlapping upward rope pulses temporarily make a larger upward disturbance.
Wave node
Physics 2 · Definition
A wave node is a location where the specified oscillating quantity has zero oscillation amplitude in a standing-wave pattern.
Displacement nodes and pressure nodes in a sound standing wave need not occur at the same positions, so the quantity must be named.
For example, the fixed ends of an ideal vibrating string are displacement nodes.
Wave polarization
Physics 2 · Definition
Wave polarization specifies the orientation of a transverse wave’s oscillation.
For light, an ideal linear polarizer selects an electric-field component along its transmission direction.
For example, crossed ideal linear polarizers block light transmitted through the first polarizer.
Wave–particle duality
Physics 2 · Definition
Wave–particle duality describes quantum observations that require both wave-like and particle-like features in appropriate models.
Localized detection events can build an interference pattern, so neither an ordinary classical particle picture nor a classical wave picture is sufficient alone.
For example, electrons detected one at a time can collectively form a double-slit interference pattern.
Wavefront
Physics 2 · Definition
A wavefront is a surface joining points of equal wave phase at a given time.
Rays are perpendicular to wavefronts in the uniform isotropic ray model used here.
For example, a distant source can be approximated by nearly plane wavefronts over a small region.
Wavelength
Physics 2 · Definition
Wavelength is the spatial distance between neighboring points at the same phase of a periodic wave.
It connects propagation speed and frequency through speed equal to frequency times wavelength.
For example, neighboring crests of a sinusoidal water wave are separated by one wavelength.
Wavenumber
Physics 2 · Definition
Angular wavenumber measures the change of wave phase per unit distance.
In the sinusoidal convention used in the source reading, it equals two pi divided by wavelength and is measured in radians per meter.
For example, shortening a wave’s wavelength increases its angular wavenumber.
Weight
Physics 1 · Physics 2 · Definition
Weight is the gravitational force exerted on an object by a specified attracting body.
Near Earth’s surface its magnitude is approximately mass times the local gravitational field strength, and it is measured in newtons.
For example, an astronaut has less weight on the Moon while retaining the same mass.
Wien’s displacement law
Physics 2 · Definition
Wien’s displacement law relates a blackbody spectrum’s peak wavelength to its kelvin temperature.
The peak wavelength is inversely proportional to temperature when the spectrum is expressed per unit wavelength.
For example, a hotter blackbody peaks at a shorter wavelength, though it still emits across a broad range.
Work function
Physics 2 · Definition
A work function is the minimum energy needed to remove an electron from the specified material surface.
For single-photon photoemission, the photon energy first pays this energy requirement before contributing to the emitted electron’s kinetic energy.
For example, two different surfaces can have different threshold frequencies because their work functions differ.