Foot-Candile, Equivalent - The term formerly used for foot-lambert.
Friction, Angle Of - The angle whose tangent is equal to the coefficient , of static friction. Also known as the limiting angle of friction.
Foot-Lambert - A unit of luminance. It is the luminance of uniform diffuser, emitting 1 lumen/ft2 (or 3.426 cd/m2).
Forbush Decrease - The decrease in cosmic ray intensity associated with a large magnetic storm.
Force - The name given to the physical agent which, acting upon a body, tends to cause a change in its momentum or to produce an interanal strain. By extension the term is used to denote such physical agencies as electromotive force, magnetomotive force, coercive force, etc. Force is a vector quantity, unit force being that which produces unit acceleration of unit mass. In CGS units this is the dyne. In MKS and SI units it is the Newton. In the Imperial. (pound-foot-second) system it is the poundal.
Force, Centrifugal - An apparent force experienced by a body in reference frame which is rotating with respect to a stationary frame. For a body of mass m, moving in a circle of radius r with angular velocity ω, this force is equal to mω2r and its direction is outwards along the radius. It is, mathematically, an imaginary inertial force introduced to make valid the use of Newton's third Jaw of motion in a moving frame of reference, and may be regarded as the reaction to the centripetal force.
Force, Centripetal - The force which acts on a body which is moving about a fixed point so as to deflect that body from a straight path. For a body of mass m, moving in a circle of radius, r with angular velocity ω, this force is equal to mω2r and its direction is inwards along the radius: the corresponding centripetal acceleration is ω2r
Forced Vibrations - The vibrations induced in a system (electrical. mechanical, nuclear, etc.) by an external periodic force. The system oscillates at a frequency equal to that of the applied force and the amplitude is at a maximum when this frequency is the same as the natural frequency of the system, when resonance is said to occurFormant - A specific set of resonances by which a vowel sound is characterized. For the production of a given vowel the vocal chords resonate at the formant frequencies in the cavities of the mount, both vocal chords and mouth cavities being adjusted accordingly.
Fortrat Diagram - For the spectral lines of a diatomic molecule: a series of parabolae obtained by plotting wave number against the whole numbers Which serve to identify successive lines, from which, by projection on the wave number axis, may be obtained the positions of the spectral lines in each branch.
Foucault Current - Another name for eddy current.
Fourier Law - States that the amount of heat flowing per unit time, dQ/ds across an infinitesimally small area, § A, is given by, - KδA dθ/ds` where K is the thermal conductivity of the material involved and dθ/ds` is the temperature gradient normal to the area.
Fringing Field - The field in an electron microscope produced by scattered electrons. Confusion of the final image by this is prevented by a suitable stop.
Front (Meteorology) - A surface of discontinuity between. air masses at different temperatures, especially at the intersection of the surface with the surface of the Earth.
Frost Point - The temperature at which the water vapour present in the atmosphere will pass direttly into the solid state without passing through an intermediate liquid state. It is always below O°C.
Fraunhofer lines - Dark lines in the solar spectrum, produced mainly by selective absorption in the relatively cool gaseous envelope surrounding the incandescent photosphere but partly by similar absorption in the Earth's atmosphere. There are thousands of such lines, tothe most prominent of which Fraunhofer assigned reference letters A-H.The elements and wavelengths to which they refer are as follows: Å, terrestrial oxygen 7594Å (extreme red); B, terrestrial oxygen, 6867Å(red); C, hydrogen, 6563Å (red); D1 and D2, sodium, 5896Å and 5890Å doublet (yellow); E, iron, 5270Å (green); F,hydrogen, 4861A (blue); G, iron and calcium (group), 430sÅ (violet); H calcium, 3968Å (extreme violet). (Note: lA =10-10 m).
Free Electron Model - For an electronic system containing more than one. electron: a model for the solution of the quantum mechanical equations for the system, according to which the electrons concerned are assumed to have no interactions with each other. The model has been very successful in the study of the electric at conductivity of metals.
Free Energy - That portion of the energy of a system which is available for conversion to work; (1) Helmholtz/ree energy: given by U-TS where U is the internal energy, S the entropy, and T the temperature. It is especially useful in describing systems where the independent variables are temperatre, volume, and composition. It is also known as the thermodynmic work function: (2) Gibbs free energy: given by U-TS + pV, where p is the pressure and V the volume. It is especially. useful in describing systems where the independent variables are temperature, pressure and composition.
Freezing - The process of solidification from the liquid state. It is believed that the long-range order characteristic of a crystalline solid is preceded in the liquid by some form of short-range order.
Freezing Point - Of a liquid: the temperature at which it solidifies at atmospheric pressure.
Frequency - (1) Of a periodic phenomenon: the number of oscillations or cycles occurring in unit time. for a progressive wave' motion the frequency is given by the speed divided by the wavelength. (2) Of a class in statistical distribution: the number of members of that class.
Frequency Converter - A heterodyne device for changing an incoming radio signal from its original carrier frequency to a fixed intermediate carrier frequency.
Frequency Modulation - The process of imparting information to a carrier wave by causing the frequency of the carrier to very in accordance with an input signal.
Frequency Multiplier - An electrical network for generating harmonics from a sinusoidal input and selecting a higher harmonic as its output.
Frequency, Natural - The frequency of free oscillation. For a resonant electrical circuit the frequency, known as the resontintfrequency, is given by 1/(2π√LC)where L is the inductance and C the capacitance of the circuit.
Fresnel Biprism - A single prism of very obtuse angle that can be considered as two prisms of very acute angle placed base to base. It is used for producing two coherent images from the same source.
Fresnel Diffraction - 'Diffraction at an aperture, when either the source of radiation or the point of observation, or both, arc at a finite distance from the aperture, i.e., when the wave fronts are spherical.
Fresnel Ellipsoid - Ray Ellipsoid. A geometrical construction for the determination of propagation velocities and vibration directions in a doubly refracting crystal. It takes the form of an ellipsoid, the semi axes of which are proportional to the reciprocal of the principal indices of refraction, and the directions of which are parallel to the corresponding vibration directions.
Fresnel-Huyghens Principle - See: Huyghens principle.
Fugacity - A fictitious pressure which, when substituted for the partial pressure, enables some of the thermodynamic equations for a real gas to be written in the same simple forms as those for a perfect gas. The fugacity p* of a gas in a real gas mixture may be defined by Where μ is the chemical potential of the gas, A is a constant, T is the temperature, and R is gas constant.
Full Radiation - Synonym for black-body radiation.
Full Radiator - A black- body radiator.
Full-Wave Circuit - A high-voltage circuit in which alternate half-cycles are reversed.
Fundamental Frequency - Of a periodic quantity (e.g. a musical note or electrical wave form): the frequency of the lowest harmonic component of that quantity.
Fundamental Interval - Of a thermometer: the difference between the reading at the ice point and the steam point. A platinum-resistance thermometer, for example, may have a fundamental resistance of about 10 ohms.
Fundamental Particles: Elementary Particles. Thos particles which are held to be simple, such as neutrons, protons, electrons, mesons, hyperons, and photons. They are characterized by their mass, electric charge, spin, magnetic moment, or interaction properties as appropriate. Some are stable, e.g. neutrinos, electrons. and protons. Others are unstable, e.g. neutrons, mesons, and hyperons.
Fundamental Particles, Interations Between. Four types of interaction which occur between fundamental particles. Their relative strengths are: strong (about 1), electromagnetic (1/137,), weak (about 10-14 ),and gravitational (about 10-39).
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