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 Glossary of Terms
Physics Glossary

Absorption: What happens when wave passes through a medium and gives up some of its energy.

Acceleration: The rate of change of velocity; the slope of the tangent line on a v-t graph.

Adhesion: The force of attraction between unlike molecules.

Alternating current: An electric current that has one direction during one part of a generating cycle and the opposite direction during the remainder of the cycle.

Ammeter: A device for measuring electrical current.

Ampere: The metric unit of current, one coulomb per second; also "amp."

Amplifier: A device consisting of one or more vacuum tubes (or transistors) and associated circuits, used to increase the strength of a signal.

Amplitude: The amount of vibration, often measured from the center to one side; may have different units depending on the nature of the vibration.

Angle of incidence: The angle between the incident ray and the normal drawn to the point of incidence.

Angle of reflection: The angle between the reflected ray and the normal drawn to the point of incidence.

Angle of refraction: The angle between the refracted ray and the normal drawn to the point of refraction.

Angstrom: A unit of linear measure equal to 10-10 m.

Angular acceleration: The time rate of change of angular velocity.

Angular impulse: The product of a torque and the time interval during which it acts.

Angular momentum: The product of the rotational inertia of a body and its angular velocity.

Angular velocity: The time rate of change of angular displacement.

Angular magnification: The factor by which an image's apparent angular size is increased (or decreased). Cf. magnification.

Armature: A coil of wire formed around an iron or steel core that rotates in the magnetic field of a generator or motor.

Atom: The basic unit of one of the chemical elements.

Atomic mass: The mass of an atom.

Atomic number: The number of protons in the nucleus of an atom.

Atomic weight: The weighted average of the atomic masses of an element's isotopes based on their relative abundance.

Audio signal: The alternating voltage proportional to the sound pressure produced in an electric circuit.

Average velocity: Total displacement divided by elapsed time.

Axis: An arbitrarily chosen point used in the definition of angular momentum. Any object whose direction changes relative to the axis is considered to have angular momentum. No matter what axis is chosen, the angular momentum of a closed system is conserved.

Barometer: A device used to measure the pressure of the atmosphere.

Basic law of electrostatics: Similarly charged objects repel each other. Oppositely charged objects attract each other.

Becquerel: The rate of radioactivity equal to one disintegration per second.

Beta particle: An electron emitted from the nucleus of a radioactive atom.

Betatron: A device that accelerates electrons by means of the transformer principle.

Bevatron: A high-energy synchrotron.

Binding energy: Energy that must be applied to a nucleus to break it up.

Boiling point: The temperature at which the vapor pressure of a liquid equals the pressure of the atmosphere.

Boyle's law: The volume of a dry gas varies inversely with the pressure exerted upon it, provided the temperature is constant.

Calorie: The quantity of heat equal to 4.19 joules.

Capacitance: The ratio of the charge on either plate of a capacitor to the potential difference between the plates. capacitive reactance. Reactance in an a-c circuit containing capacitance which causes a lagging voltage.

Capacitor: A combination of conducting plates separated by layers of a dielectric that is used to store an electric charge.

Capillarity: The elevation or depression of liquids in small-diameter tubes.

Center of curvature: The center of the sphere of which the mirror or lens surface forms a part.

Center of gravity: The point at which all of the weight of a body can be considered to be concentrated.

Centrifugal force: Force that tends to move the particles of a rotating object away from the center of rotation.

Centripetal acceleration: Acceleration directed toward the center of a circular path.

Centripetal force: The force that produces centripetal acceleration.

Charge: A numerical rating of how strongly an object participates in electrical forces.

Charles' law: The volume of a dry gas is directly proportional to its Kelvin temperature, providing the pressure is constant.

Circuit: An electrical device in which charge can come back to its starting point and be recycled rather than getting stuck in a dead end.

Coefficient of area expansion: The change in area per unit area of a solid per degree change in temperature.

Coefficient of cubic expansion: The change in volume per unit volume of a solid or liquid per degree change in temperature.

Coefficient of linear expansion: The change in length per unit length of a solid per degree change in temperature.

Calorimeter: A heat-measuring device consisting of nested metal cups separated by an air space

Coherent: A light wave whose parts are all in phase with each other.

Collision:  An interaction between moving objects that lasts for a certain time.

Component: The part of a velocity, acceleration, or force that is along one particular coordinate axis.

Concave: Describes a surface that is hollowed out like a cave.

Convex: Describes a surface that bulges outward.

Coulomb (C):  The unit of electrical charge. The quantity of electricity equal to the charge on 6.25 x 1018 electrons.

Coulomb's law of electrostatics: The force between two point charges is directly proportional to the product of their magnitudes and inversely proportional ta the square of the distance between them.

Coulomb's law of magnetism: The force between two magnetic poles is directly proportional to the strengths of the poles and inversely proportional to the square of their distance apart.

Couple: Two forces of equal magnitude acting in opposite directions in the same plane, but not along the same line.

Crest: A region of upward displacement in a transverse wave.

Critical angle: That limiting angle of incidence in the optically denser medium that results in an angle of refraction of 90o.

Critical mass: The amount of a particular fissionable material required to make a fission reaction self-sustaining.

Critical point: The upper limit of the temperature-pressure curve of a substance.

Critical pressure: The pressure needed to liquefy a gas at its critical temperature.

Critical temperature: The temperature to which a gas must be cooled before it can be liquefied by pressure.

Current:  The rate at which charge crosses a certain boundary.

Damping: The dissipation of a vibration's energy into heat energy, or the frictional force that causes the loss of energy.

Dielectric. An electric insulator. A non conducting medium.

Dielectric constant. The ratio of the capacitance with a particular material separating the plates of a capacitor

Diffraction: The behavior of a wave when it encounters an obstacle or a non-uniformity in its medium; in general, diffraction causes a wave to bend around obstacles and make patterns of strong and weak waves radiating out beyond the obstacle.

Diffraction angle. The angle that a diffracted wave front forms with the grating plane.

Diffraction grating. An optical surface, either transmitting or reflecting, with several thousand equally spaced and parallel grooves ruled in it.

Diffusion. (1) The penetration of one type of particle into a mass of a second type of particle. (2) The scattering of light by irregular reflection.

Dimensional analysis. The performance of indicated mathematical operations in a problem with the measurement units alone.

Diode. A two-terminal device that will conduct electric current more easily in one direction than in the other.

Direct current. An essentially constant value current in which the movement of charge is in only one direction.

Direct proportion. The relation between two quantities whose graph is a straight line.

Dispersion. The process of separating polychromatic light into its component wavelengths.

Displacement:  A name for the symbol delta-x .

Doppler effect. The change observed in the frequency with which a wave from a given source reaches an observer when the source and the observer are in relative motion.

Ductility. The property of a metal that enables it to be drawn through a die to form a wire.

Driving force:  An external force that pumps energy into a vibrating system.

Electric dipole: An object that has an imbalance between positive charge on one side and negative charge on the other; an object that will experience a torque in an electric field.

Electric field: The force per unit charge exerted on a test charge at a given point in space.

Electrical force: One of the fundamental forces of nature; a non-contact force that can be either repulsive or attractive.

Electron: Thomson's name for the particles of which a cathode ray was made; a subatomic particle.

Energy: A numerical scale used to measure the heat, motion, or other properties that would require fuel or physical effort to put into an object; a scalar quantity with units of joules (J).

Equilibrium: A state in which an object's momentum and angular momentum are constant.

Field: A property of a point in space describing the forces that would be exerted on a particle if it was there.

Fission: The radioactive decay of a nucleus by splitting into two parts.

Fluid:  A gas or a liquid.

Fluid friction:  friction force in which at least one of the object is is a fluid (i.e. either a gas or a liquid).

Focal length: A property of a lens or mirror, equal to the distance from the lens or mirror to the image it forms of an object that is infinitely far away.

Frequency:  The number of cycles per second, the inverse of the period (q.v.).

Fusion: A nuclear reaction in which two nuclei stick together to form one bigger nucleus.

Gamma ray: A form of radioactivity consisting of a very high-frequency form of light.

Gravitational field:  The force per unit mass exerted on a test mass at a given point in space.

Gravity: A general term for the phenomenon of attraction between things having mass. The attraction between our planet and a human-sized object causes the object to fall.

Half-life: The amount of time that a radioactive atom has a probability of 1/2 of surviving without decaying.

Heat: The energy that an object has because of its temperature. Heat is different from temperature (q.v.) because an object with twice as much mass requires twice as much heat to increase its temperature by the same amount. There is a further distinction in the terminology, not emphasized in this book, between heat and thermal energy. See the entry under thermal energy for a discussion of this distinction.

Image:  A place where an object appears to be, because the rays diffusely reflected from any given point on the object have been bent so that they come back together and then spread out again from the image point, or spread apart as if they had originated from the image.

Index of refraction:  An optical property of matter; the speed of light in a vacuum divided by the speed of light in the substance in question.

Independence: The lack of any relationship between two random events.

Induction:  The production of an electric field by a changing magnetic field, or vice-versa.

Inertial frame: A frame of reference that is not accelerating, one in which Newton's first law is true.

Kinetic friction:  A friction force between surfaces that are slipping past each other.

Invariant: A quantity that does not change when transformed.

Ion:  An electrically charged atom or molecule.

Isotope: of the possible varieties of atoms of a given element, having a certain number of neutrons.

Kinetic energy: The energy an object possesses because of its motion. Cf. potential energy.

Lorentz transformation: The transformation between frames in relative motion.

Magnetic dipole: Known as a current loop, an atom, or a bar magnet, that experiences torques due to magnetic forces; the strength of magnetic dipoles is measured by comparison with a standard dipole consisting of a square loop of wire of a given size and carrying a given amount of current.

Magnetic field: Force, defined in terms of the torque exerted on a test dipole.

Magnification:  The factor by which an image's linear size is increased (or decreased). Cf. angular magnification.

Magnitude:  The "amount" associated with a vector; the vector stripped of any information about its direction.

Mass:  A numerical measure of how difficult it is to change an object's motion. (In the context of relativity, some books use the word "mass" to mean what we refer to as mass multiplied by gamma.)

Neutron:  An uncharged particle, the other types that nuclei are made of.

Open circuit:  A circuit that does not function because it has a gap in it.

Parabola: The mathematical curve whose graph has y proportional to x2.

Period: The time required for one cycle of a periodic motion (q.v.).

Photon: A particle of light.

Photoelectric effect:  The ejection, by a photon, of an electron from the surface of an object.

Power:  The rate of transferring energy; a scalar quantity with units of watts (W).

Proton: A positively charged particle, one of the types that nuclei are made of.

Quality factor (Q) is the number of oscillations required for a system's energy to fall off by a factor of 535 due to damping.

Quantized: Describes quantity such as money or electrical charge, that can only exist in certain amounts.

Radial: Parallel to the radius of a circle; the in-out direction. Cf. tangential.

Real image: A place where an object appears to be, because the rays diffusely reflected from any given point on the object have been bent so that they come back together and then spread out again from the new point. Cf. virtual image.

Reflection: What happens when light hits matter and bounces off, retaining at least some of its energy.

Refraction:  The change in direction that occurs when a wave encounters the interface between two media.

Repulsive:  Describes a force that tends to push the two participating objects apart. Cf. attractive, oblique.

Resistance:  The ratio of the voltage difference to the current in an object made of an ohmic substance.

Resonance: The tendency of a vibrating system to respond most strongly to a driving force whose frequency is close to its own natural frequency of vibration.

Scalar:  A quantity that has no direction in space, only an amount. Cf. vector.

Short circuit:  A circuit that does not function because charge is given a low-resistance "shortcut" path that it can follow, instead of the path that makes it do something useful.

Significant figures;  Digits that contribute to the accuracy of a measurement.

Simple harmonic motion:  Motion whose x-t graph is a sine wave.

Sink: A point at which field vectors converge.

Source:  A point from which field vectors diverge; often used more inclusively to refer to points of either convergence or divergence.

Specular reflection: Reflection from a smooth surface, in which the light ray leaves at the same angle at which it came in.

Spin:  The built-in angular momentum possessed by a particle even when at rest.

Tangential: Tangent to a curve. In circular motion, used to mean tangent to the circle, perpendicular to the radial direction Cf. radial.

Thermal energy.. Properly, thermal energy is used to mean the total amount of energy possessed by an object, while heat indicates the amount of thermal energy transferred in or out.

Torque: The rate of change of angular momentum; a numerical measure of a force's ability to twist on an object.

Transformation: The mathematical relationship between the variables such as x and t, as observed in different frames of reference.

Uniform circular motion:  Circular motion in which the magnitude of the velocity vector remains constant.

Vector: A quantity that has both an amount (magnitude) and a direction in space. Cf. scalar.

Velocity: The rate of change of position; the slope of the tangent line on an x-t graph.

Virtual image: Like a real image, but the rays don't actually cross again; they only appear to have come from the point on the image. Cf. real image.

Volt: The metric unit of voltage, one joule per coulomb.

Voltage:  Electrical potential energy per unit charge that will be possessed by a charged particle at a certain point in space.

Voltmeter: A device for measuring voltage differences.

Weak nuclear force: The force responsible for beta decay.

Weight:  force of gravity on an object, equal to mg.

Work: The amount of energy transferred into or out of a system, excluding energy transferred by heat conduction.  