**Note** Terms are followed, when appropriate, by the type of quantity-vector or scalar-and the conventional units of the quantity in the SI system. Common abbreviations are also noted.

**absorption spectrum** the specific wavelengths of light absorbed by a gas.

**acceleration** (vector; m/s2) the rate of change of velocity.

**adiabatic** without the exchange of heat with the external universe.

**alternating current** (scalar; A) a type of electrical current that changes direction regularly in time.

**alternating voltage** an electrostatic potential that changes regularly in time.

**ammeter** a device used to measure electrical current.

**ampere** (A) the SI unit of measurement of current; equivalent to C/s.

**amplitude** (scalar; m) the maximal displacement of an object in simple harmonic motion; the peak-to-peak height of a wave.

**angular acceleration** (scalar; radians/s2) the rate of change of angular velocity in time.

**angular displacement** (scalar; radians) the angle between the initial and final radii after a given time of an object moving in a circle.

**angular momentum** (vector; J-s) the cross product of the distance an object is from a point and its momentum with respect to this point.

**angular velocity** (scalar; radians/s) the rate of change of angular displacement in time.

**antinodes** points on a standing wave that have maximum displacement, due to the constructive interference of the component waves.

**atomic mass** the total number of nucleons in a nucleus.

**atomic mass unit** (amu) the unit of mass appropriate to the nucleus of an atom, equivalent to 1/12 of the mass of the carbon nucleus.

**atomic number** the number of protons in a nucleus.

**Avogadro's number** the number of objects or particles in a mole of substance, namely, 6.02 × 10^{23}.

**beats** the effect produced by interference of waves of slightly different frequency, producing a pattern of alternating intensity.

**binding energy** the energy difference between the mass energy of a nucleus and its separate constituent particles. Binding energy is released when nuclei come together during fusion.

**blackbody** an object that absorbs all radiation falling on it and radiates it perfectly.

**Bohr radius** the average radius of an electron orbiting a single proton, equal to 5.29 × 10^{-11} m.

**Boltzmann's constant **a fundamental constant, usually encountered in thermodynamics, with a value of 1.38 × 10^{-23} J/K.

**British thermal unit** (BTU) a unit of energy frequently used in engineering, equivalent to 252 calories or 1.054 kJ.

**bulk modulus** (scalar; N/m^{2}) the ratio of pressure to the resulting compressional strain.

**buoyant force** the upward force on an object placed in fluid.

**calorie** (cal) a unit of energy, defined as the energy required to raise one gram of water one degree Celsius, and equivalent to 4.184 J.

**calorimeter** a device used to measure the specific heat capacity of a substance.

**capacitance** (scalar; F) a measure of how much charge a capacitor can store.

**capacitor** an electrical device for storing separated charge and therefore storing electrostatic potential energy.

**Carnot cycle** an ideal series of states through which a heat engine may move, which maximizes the amount of heat converted into work. The Carnot cycle consists of two isothermal processes and two adiabatic processes.

**centripetal acceleration** (vector; m/s^{2}) the acceleration of an object in uniform circular motion which is pointed toward the center of the circle.

**centripetal force** (vector; N) the force that holds an object in circular motion, pointed toward the center of the circle.

**charge** (scalar; C) An intrinsic property of matter that causes it to produce an electric field and, when it is moving, a magnetic field, and to feel a force due to these fields as well.

**circuit** a closed loop arrangement of elements such as capacitors, resistors, inductors, and batteries, through which electrical current flows.

**compressional strain** (scalar) the fractional volume change of an object due to uniform pressure.

**Compton**** scattering** the deflection of electrons by photons.

**concave mirror **a mirror with an inwardly curved surface, such as the inner surface of a sphere.

**conduction** the transfer of heat through a stationary substance; also the movement of ions or electrons through a material.

**conductor** a material through which electrical charges can easily flow.

**conservation of energy** a fundamental law of physics which states that the energy of a system does not change unless an external force acts upon the system.

**constructive interference** the phenomenon in which waves present simultaneously at the same position add together to form a bigger wave.

**convection **the transport of heat due to the motion of heated substance, such as a gas.

**converging lens** a lens that causes parallel light rays to converge.

**convex mirror** a mirror with an outwardly curved surface, such as the outer surface of a sphere.

**coulomb** (C) the SI unit of measurement of charge.

**critical angle** the angle of incidence for a light ray, below which total internal reflection occurs.

**current** (scalar; A) the amount of electric charge passing a point per unit time.

**daughter nucleus** the nucleus remaining after a radioactive decay.

**degrees** (°) the gradations of a temperature scale; also a unit of measurement of an angle.

**density** (scalar; kg/m^{3}) mass per unit volume.

**destructive interference** the phenomenon in which waves present simultaneously at the same position add together to form a smaller wave, or momentarily cancel entirely.

**dichroic** having the property of naturally emitting light with only a single polarization.

**dielectric constant** (scalar) an intrinsic property of a substance that indicates the amount of charge induced in the substance when it is placed in an electric field.

**diffraction **the process of a wave spreading as it passes around an object or passes through a narrow opening.

**direct current** (scalar; A) a type of electrical current that flows in only one direction. Direct current flows between the terminals of a simple battery when they are connected to a circuit.

**dispersion** the effect of separating waves, especially light, into constituent wavelengths by passing through a medium with an index of refraction that varies with wavelengths.

**displacement** (vector; m) change in position of an object.

**diverging lens** a lens that causes parallel light rays to diverge.

**Doppler effect** the change in frequency of a wave produced by a moving source. Approaching sources have a higher frequency, and retreating sources have a lower frequency.

**elastic collision** a collision between two objects in which the kinetic energy of both is conserved.

**elastic modulus** (scalar; N/m^{2}) the ratio of stress to strain.

**elasticity **the property of an object to restore its shape after deformation.

**electric field **(vector; V/m) the force felt by a positive unit test charge in a region of space, due to the influence of other charges. Electric fields are produced by stationary and moving charges.

**electric flux **(scalar; V-m) the total sum of electric field vectors passing perpendicularly through a surface. According to Gauss's law, the electric flux through a closed gaussian surface is proportional to the total net charge contained within the surface.

**electromotive force** (emf; scalar; V) the electrostatic potential difference between the terminals of a circuit or battery when no current is flowing.

**electron** the negatively charged fundamental particles present in ordinary matter, surrounding the nucleus.

**electroscope** a simple device for indicating the presence of net electrical charge.

**electrostatic potential** (scalar; V) the amount of energy per unit positive charge required to move a charge between two points within an electric field.

**emf** (scalar; V) the electrostatic potential difference between the terminals of a circuit or battery when no current is flowing. Also called *electromotive force*.

**emissivity **(scalar) an intrinsic property of a material indicating how well it radiates heat.

**energy** (scalar; J) the ability to do work.

**energy level diagram** a diagram illustrating the discrete energies that an electron orbiting a nucleus may possess.

**entropy** (scalar; J/K) a fundamental thermodynamic quantity which measures how much heat energy is unavailable for conversion to work.

**equipotential** **surface** an array of positions within an electric field that all have the same electrostatic potential value. A charge may move along an equipotential surface without requiring or releasing energy.

**equivalence principle** the principle of general relativity which states that experiments conducted in an inertial frame in a gravitational field and experiments conducted in an accelerating reference frame will give the same results.

**ether** The medium on which light waves were once presumed to propagate. The luminous ether does not exist.

**farad** (F) the SI unit of measurement of capacitance; equivalent to A-s/V.

**field lines** a pictorial representation of an electric field or magnetic field or any other vector field.

**focal length** (scalar; m) the distance from the focal point of a lens or mirror to the surface of the lens or mirror.

**focal point** the point at which light rays from a mirror or lens intersect.

**force** (vector; N) a push or pull that causes an object to accelerate.

**force diagram** a diagram displaying all of the forces acting on an object.

**forced vibrations** the vibrations produced in an object connected to another vibrating object, with the effect of amplifying the vibrations of the first object.

**free-body diagram** another name for force diagram.

**frequency** (scalar; Hz) the number of complete cycles of simple harmonic motion per unit time; the reciprocal of period; the number of wave cycles passing a fixed point per unit time.

**friction **a retarding force between two objects that inhibits motion.

**galvanometer** a device used to measure electrical current.

**gaussian surface** an imaginary surface-enclosing charge used to calculate the electric field at points on the surface using Gauss's law.

**general relativity** the theory of mechanics that treats gravitational fields as equivalent to relative acceleration and introduces the notion that mass curves space and time.

**generator** a device that converts mechanical energy into electrical energy.

**half-life** the time required for half of a radioactive sample to decay.

**heat capacity** (scalar; J/K) the amount of heat energy required to change the temperature of an object by one degree.

**heat of fusion **(scalar; J) the heat that must be added per unit mass for a phase change of a substance between solid and liquid states. For a change from liquid to solid, the heat of fusion is released.

**heat of vaporization** (scalar; J) the heat that must be added per unit mass for a phase change of a substance between liquid and gas states. For a change from liquid to solid, the heat of vaporization is released.

**henry** (H) the SI unit of measurement of inductance, equivalent to V-s/A.

**hertz **(Hz) the SI unit of measurement of frequency; equivalent to s^{-1}.

**ideal gas** a collection of identical, infinitesimally small particles that interact only by elastic collisions.

**impulse** (vector; N-s) the product of the average force acting on an object and the time over which it acts.

**index of refraction** (scalar) an intrinsic property of a transparent substance, which measures the speed of light in the material compared to the speed of light in a vacuum.

**inductance **(scalar; H) a property of a circuit that is the proportionality constant between the rate of change of the current in that circuit and the emf that this changing current produces.

**induction** the process by which electromotive forces are generated in a circuit due to changes in a magnetic field.

**inelastic collision** a collision between objects in which the kinetic energy changes, for example, due to deformation or frictional loss.

**inertial frame** a set of coordinates that is not accelerating.

**insulator** a material through which electrical charges cannot flow.

**intensity** (scalar; W/m^{2}) the amount of energy carried by a wave across a unit area in a unit time.

**isobaric** at constant pressure.

**isochoric** at constant volume.

**isothermal** at constant temperature.

**isotopes** atoms with nuclei possessing the same number of protons but different numbers of neutrons.

**joule** (J) the SI unit of measurement of energy; equivalent to kg-m2/s2.

**Kelvin** (K) the SI unit of measurement of temperature.

**kilogram** (kg) the SI unit of measurement of mass.

**kinetic energy** (scalar; J) the energy of an object in motion.

**kinetic friction** friction that acts to resist motion of an object that is already moving.

**kinetic theory of gases** a model of an ideal gas that treats it as a collection of molecules undergoing motion according to Newton's laws of classical mechanics and predicts macroscopic quantities such as pressure and temperature in terms of molecular properties, such as the velocity of the molecules.

**latent heat** (scalar; J/kg) the heat required for a phase change of a substance per unit mass.

**law of conservation of momentum** a fundamental law of physics which states that the linear momentum of a system does not change unless an external force acts upon the system.

**line spectrum **the specific wavelengths of light absorbed and emitted by a gas. The line spectrum is different for each type of gas.

**linear momentum** (vector; kg-m/s^{2}) the product of mass and velocity.

**lines of force** a pictorial representation of an electric field or magnetic field or any other vector field. Also called *field lines*.

**longitudinal wave **a wave in which points on the wave move parallel to the direction of wave propagation.

**Lorentz contraction** the effect that an observer moving with respect to a given object will find the object to be shortened compared with the measurement by an observer at rest relative to the object.

**loudness** (scalar; decibels) the intensity of a sound wave compared to a set standard, namely, the lowest threshold of human hearing.

**luminous ether** Ether is the medium on which light waves were once presumed to propagate. The luminous ether does not exist.

**magnetic field **(vector; T) the force felt by a unit positive test charge moving through space, due to the influence of magnets or moving charges. Magnetic fields are produced by moving charges.

**magnetic flux** (scalar; T-m^{2}) the total sum of magnetic field vectors passing perpendicularly through a surface. According to Faraday's law, the rate of change of the magnetic flux through a surface is proportional to electromotive force generated on the closed loop containing the surface.

**mass** (scalar; kg) the intrinsic property of matter that causes it to resist acceleration.

**mass spectrometer** an instrument that separates ionized atoms or molecules based upon their ratio of charge to mass.

**mole** the measure of the number of objects or particles. One mole is 6.02 × 10^{23} objects (Avogadro's number).

**moment of inertia** (scalar; kg-m^{2}) the intrinsic property of an object that causes it to resist changes in rotational motion.

**momentum** (vector; kg-m/s) see linear momentum or angular momentum.

**mutual inductance** when a change in one circuit causes a change in a different circuit.

**neutron** the fundamental particle, possessing no charge, that is present in the nuclei of ordinary matter.

**Newton** the unit of measurement for force; equivalent to kg-m/s^{2}.

**nodal points** points on a standing wave that do not move at all due to the destructive interference of the component waves.

**normal force** (vector; N) the force exerted by a surface on an object sitting upon it pointing perpendicular to the surface.

**normal** perpendicular.

**north pole** one end of a magnet; the end that attracts the south pole.

**nuclear fission** the process of a nucleus splitting into two nearly equal-sized nuclei.

**nuclear fusion** the process of two light nuclei coming together to form a heavier nucleus.

**nucleon** any particle present in the nucleus of an atom, that is, a proton or neutron.

**nucleus** the positively charged central collection of protons and neutrons in an atom.

**ohm** (W) the SI unit of measurement of resistance; equivalent to V/A.

**opaque** having the property of not allowing light to pass through.

**parallel** two lines or surfaces that never intersect. Also, for circuit elements, elements that are connected so that they have the same electrostatic potential difference across them.

**parent nucleus** the original nucleus during a radioactive decay.

**pascal** (Pa) the SI unit of measurement of pressure; equivalent to N/m^{2}.

**period **(scalar; s) the time of one complete cycle of simple harmonic motion; the time for one wave cycle to pass a fixed point.

**permeability constant** (m_{o}) a fundamental constant of the universe, which is the proportionality constant in Ampere's law and has the value 1.26 × 10^{-6} T-m/A.

**permittivity of free space** (e_{o}) a fundamental constant of the universe that is the proportionality constant in Coulomb's law.

**phase** the physical state of an object, for example, solid, liquid, or gas. Also, a property of waves indicating the displacement at the initial time.

**photoelectric effect** the emission of electrons by certain metals when the appropriate wavelength of light is shined on them.

**photoelectrons** the electrons emitted in the photoelectric effect.

**photons** discrete units of energy; the particles associated with electromagnetic radiation (light).

**pitch **(scalar; Hz) the frequency of a sound wave.

**Planck's constant** (h) a fundamental constant of the universe, appearing in quantum mechanics, with the value 6.626 × 10^{-34} J-s.

**polarization** a property of transverse waves, especially light waves, that indicates the orientation of the wave displacement with respect to a coordinate system.

**polarizer** a device that permits light of only one polarization to pass through.

**potential energy** (scalar; J) the energy of an object due to its position or internal structure.

**power** (scalar; W) the rate of change in energy, or the rate of doing work.

**pressure **(scalar; Pa) the ratio of force to the area to which the force is applied.

**proton **the fundamental particle, possessing positive charge, that is present in the nuclei of ordinary matter.

**quanta** discrete units of energy.

**quantum mechanics** the laws of physics that apply to objects on the atomic scale.

**quantum number** a number describing the quantum state of an object, such as the state of an electron orbiting a nucleus.

**radians** the SI unit of measurement of the size of an angle. A complete circle is 2p radians.

**radiation** the transport of heat in the form of electromagnetic waves.

**radioactivity** the process by which some nuclei spontaneously split and emit particles.

**range** the horizontal distance a projectile travels.

**ray **a straight line representation of the path of a light wave.

**ray diagram** a drawing of light rays used to analyze a set of optical devices, such as a lens or mirror.

**RC circuit** a circuit that contains a resistor and a capacitor in series with one another.

**reactance** (scalar; W) the apparent resistance of capacitors and inductors to alternating current.

**real image** an image produced in an optical device, such as a lens or mirror, that is formed by converging rays.

**refraction** the bending of light when it passes across the boundary between two media with different indices of refraction.

**resistance** (scalar; W) the proportionality constant between the applied electrostatic potential difference and the resulting current in a circuit.

**resistivity** (scalar; W-m) a measure of how well current flows through a material, per unit length and cross-sectional area.

**resistor** a circuit element that impedes the flow of current.

**resonance** a vibration produced in an object at its natural vibration frequency, or a multiple of it, due to a nearby object vibrating at this frequency.

**rest mass** (scalar; kg) the mass of an object, measured by an observer at rest with respect to the object.

**restoring force** the force exerted by a spring when it is stretched or compressed, which tends to restore the spring to its resting length.

**resultant** the sum of two or more vectors.

**rigid body** an object in which the relative distance between internal points does not change.

**RL circuit **a circuit that contains a resistor and an inductor in series with one another.

**RLC circuit **a circuit that contains a resistor, a capacitor, and an inductor in series with one another.

**root mean square** (rms) a way of averaging, equal to the square root of the average of the squares of a quantity.

**rotational inertia** (scalar; kg-m^{2}) the intrinsic property of an object that causes it to resist changes in rotational motion. Also called *moment of inertia*.

**Rydberg constant** (R) a constant found in the description of the line spectra of gases.

**scalar** a quantity with magnitude but without direction; expressed as a simple number.

**self-inductance** the generation of an electromotive force in a closed circuit due to a changing current in that circuit.

**series** for circuit elements, elements that are connected so that they have the same electrical current conducted through them.

**shear modulus** (scalar; N/m^{2}) the ratio of shear stress to shear strain.

**shear strain** (scalar) the ratio of the horizontal distance a sheared face moves to the height of the object.

**shear stress** (scalar; N/m^{2}) the ratio of the tangential force to the area of the face being stressed.

**simple harmonic motion** (SHM) the motion of an object with acceleration proportional to the displacement, resulting in repetitive motion.

**simple pendulum** a mass swinging on the end of a massless string under the influence of gravity.

**solenoid** a long straight coil of wire.

**south pole** one end of a magnet; the end that attracts the north pole.

**special relativity** the theory of mechanics for objects moving with velocities close to the speed of light.

**specific heat capacity** (scalar; J/kg-K) the heat capacity of a substance per unit mass.

**spectrum** the distribution of wavelengths or frequencies of electromagnetic radiation.

**spring constant** (scalar; N/m) the constant of proportionality between the applied force and the resulting change in length of a given spring.

**standard pressure** a convenient measure of pressure equivalent to 1 atmosphere or 1.01 × 10^{5} pascal.

**standard temperature** zero degrees Celsius.

**standard volume** the volume of an ideal gas at standard temperature and standard pressure, namely, 22.4 liters.

**standing wave** a wave produced by the superposition of waves traveling in opposite directions, such that the sum is a wave pulse that does not move along the medium (see *nodal points, antinodes*).

**static equilibrium** the state of an object when all forces acting on it sum to zero.

**static friction** friction that resists an object's beginning motion.

**stationary state** a quantum state of an electron in which it does not emit radiation.

**superposition principle** the rule for adding waves at the same point together, which states that the resulting wave is the vector sum of all the independent waves.

**sympathetic vibration** a vibration produced in an object at its natural vibration frequency, or a multiple of it, due to a nearby object vibrating at this frequency. Also called *resonance*.

**temperature** (scalar; K, °C) a measure of how hot or cold an object is with respect to a reference object.

**tensile strain** (scalar) the fractional change in length of an object due to an applied tensile force.

**tensile stress** (scalar; N/m^{2}) the ratio of tensile force to the cross-sectional area perpendicular to the force.

**tesla **(T) the SI unit of measurement of the magnitude of a magnetic field; equivalent to N/A-m.

**thermal conductivity **(scalar; J/m-K) an intrinsic property of a material indicating how well heat transfers through the material.

**thermal contact** contact between objects that allows them to influence each other's temperatures.

**thermal equilibrium** the state of objects in thermal contact when they no longer change each other's temperature.

**thermal expansion **the phenomenon that most substances increase in volume as their temperature increases.

**thermodynamics** the branch of physics concerned with very general properties of matter and energy. It also describes the macroscopic properties of matter in terms of the microscopic properties of its components.

**thermometer** a tool for measuring temperature.

**threshold frequency** the minimum frequency that light must have in order to cause photoelectrons to be emitted by a given metal.

**time dilation** the effect that time moves more slowly in an inertial frame moving with respect to a stationary one.

**toroid** a doughnut shaped coil of wire; a solenoid bent into a closed circle.

**torque** (vector; N-m) a push or pull that tends to cause an object to rotate about a fixed point; the rotational analog of force.

**total internal reflection** the process in which light traveling from a material with a higher index of refraction to a material with a lower index of refraction is reflected at the boundary and no light actually crosses the boundary.

**transformer** a device that is used to pass an alternating voltage from one circuit to another. In the process, the voltage may be increased or decreased.

**transmutation** the process of one nucleus turning into another through radioactivity.

**transparent** having the property of allowing light to pass through.

**transverse wave** a wave pulse in which points on the wave move perpendicular to the direction of wave propagation.

**triple point of water** the temperature of water at which ice, liquid water, and water vapor coexist at thermal equilibrium, defined as 0.01°C or 273.15 K.

**uncertainty principle** the statement that, due to the laws of quantum mechanics, it is impossible to simultaneously exactly measure a particle's position and momentum or to exactly measure a particle's energy for a finite amount of time.

**universal gravitational constant** (G) proportionality constant in Newton's law of universal gravitation, a fundamental constant of the universe, with the value 6.67 × 10^{-11} N-m^{2}/kg.

**vector** a quantity with magnitude and direction.

**vector component** the projection of a vector upon a given coordinate axis.

**velocity** (vector; m/s^{2}) the rate of change of position in time.

**virtual image** an image produced in an optical device, such as a lens or mirror, that is formed by the extension of diverging rays.

**volt** (V) the SI unit of measurement of electrostatic potential; equivalent to J/C.

**voltmeter **a device used to measure electrostatic potential differences between two points.

**watt **(W) the SI unit of measurement of power; equivalent to J/s.

**wavelength** the distance between identical points in a wave cycle.

**weight **(vector; N) the product of mass and gravitational force.

**work** (scalar; J) the product of the force exerted on an object and the distance through which the object moves as a result. Work results in a change in energy.

**work function** the energy required to release an electron from a metal due to the photoelectric effect.

**Young's modulus** (scalar; N/m^{2}) the ratio of tensile stress to tensile strain.