$Date: 2010-02-16 10:59:30 -0500 (Tue, 16 Feb 2010) $ The NASA QUDT Quantity Ontology James E. Masters QUDT Quantity An ontology about physical quantities and the expression of their magnitude in a chosen unit of measure. Measurable Quantities $Id: n1quantity.n3 85 2010-02-16 15:59:30Z dmekonnen $ The QUDT Ontologies are issued under a Creative Commons Attribution Share Alike 3.0 United States License. Attribution should be made to NASA AMES Research Center and TopQuadrant, Inc. Energy and Work per Mass Amount of Substance Radiant Flux Radiant Flux, or radiant power, is the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light). The power may be the total emitted from a source, or the total landing on a particular surface. Power Power is the rate at which work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time. As a rate of change of work done or the energy of a subsystem, power is: P = W/t where P is power W is work t is time. [Wikipedia] Φ Magnetic Flux per Unit Length Dimensionless U Time Time is a basic component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects. T Electromotive Force Electromotive force is the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals. Energy per Electric Charge ℰ The electromagnetic system of units is used to measure electrical quantities of electric charge, current, and voltage, within the centimeter gram second (or "CGS") metric system of units. In electromagnetic units, electric current is derived the CGS base units length, mass, and time by solving Ampere's Law (expressing the force between two parallel conducting wires) for current and setting the constant of proportionality (k_m) equal to unity. Thus, in the CGS-EMU system, electric current is derived from length, mass, and time. Capacitance Capacitance is the ability of a body to hold an electrical charge; it is quantified as the amount of electric charge stored for a given electric potential. Capacitance is a scalar-valued quantity. Inductance Inductance is an electromagentic quantity that characterizes a circuit's resistance to any change of electric current; a change in the electric current through induces an opposing electromotive force (EMF). Quantitatively, inductance is proportional to the magnetic flux per unit of electric current. L Electric Current per Unit Length Length L Electric Flux The Electric Flux through an area is defined as the electric field multiplied by the area of the surface projected in a plane perpendicular to the field. Electric Flux is a scalar-valued quantity. Auxillary Magnetic Field H Magnetic Fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges. The electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic field. A pure electric field in one reference frame is observed as a combination of both an electric field and a magnetic field in a moving reference frame. The Auxillary Magnetic Field, H characterizes how the true Magnetic Field B influences the organization of magnetic dipoles in a given medium. Magnetic Field B The Magnetic Field, denoted B, is a fundamental field in electrodynamics which characterizes the magnetic force exerted by electric currents. It is closely related to the auxillary magnetic field H (see quantity:AuxillaryMagneticField). B Magnetic Flux Magnetic Flux is the product of the average magnetic field times the perpendicular area that it penetrates. Φ Electric Field E The space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. This electric field exerts a force on other electrically charged objects. In the idealized case, the force exerted between two point charges is inversely proportional to the square of the distance between them. (Coulomb's Law) Resistance The electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. R Length per Unit Electric Current Permittivity Permittivity is a physical quantity that describes how an electric field affects, and is affected by a dielectric medium, and is determined by the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside the material. Permittivity is often a scalar valued quantity, however in the general case it is tensor-valued. ε Electric Charge Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields. The electric charge on a body may be positive or negative. Two positively charged bodies experience a mutual repulsive force, as do two negatively charged bodies. A positively charged body and a negatively charged body experience an attractive force. Q Electric Potential The Electric Potential is a scalar valued quantity associated with an electric field. The electric potential Φ(x) at a point, x, is formally defined as the line integral of the electric field taken along a path from x to the point at infinity. If the electric field is static, i.e. time independent, then the choice of the path is arbitrary; however if the electric field is time dependent, taking the integral along different paths will produce different results. Φ Electric Current Electric Current is the flow (movement) of electric charge. The amount of electric current through some surface, e.g., a section through a copper conductor, is defined as the amount of electric charge flowing through that surface over time. Current is a scalar-valued quantity. I CGS-EMU System of Quantities Permeability Permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. In general permeability is a tensor-valued quantity. μ Magnetic Dipole Moment The magnetic moment of a system is a measure of the magnitude and the direction of its magnetism. Magnetic moment usually refers to its Magnetic Dipole Moment, and quantifies the contribution of the system's internal magnetism to the external dipolar magnetic field produced by the system (that is, the component of the external magnetic field that is inversely proportional to the cube of the distance to the observer). The Magnetic Dipole Moment is a vector-valued quantity. μ Mass M Electric Displacement Field D In a dielectric material the presence of an electric field E causes the bound charges in the material (atomic nuclei and their electrons) to slightly separate, inducing a local electric dipole moment. The Electric Displacement Field, D, is a vector field that accounts for the effects of free charges within such dielectric materials. Electric Charge per Unit Area D Electric Conductivity Electric conductivity or specific conductance is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current. The conductivity σ is defined as the ratio of the electric current density J to the electric field E: J = σE In isotropic materials, conductivity is scalar-valued, however in general, conductivity is a tensor-valued quantity. σ Magnetization Field M The Magnetization Field is defined as the ratio of magnetic moment per unit volume. It is a vector-valued quantity. M Magnetomotive Force Magnetomotive Force (mmf) is the ability of an electric circuit to produce magnetic flux. Just as the ability of a battery to produce electric current is called its electromotive force or emf, mmf is taken as the work required to move a unit magnet pole from any point through any path which links the electric circuit back the same point in the presence of the magnetic force produced by the electric current in the circuit. Electric Current Density J Electric current density is a measure of the density of flow of electric charge; it is the electric current per unit area of cross section. Electric current density is a vector-valued quantity. J Electric Dipole Moment The Electric Dipole Moment is a measure of the separation of positive and negative electrical charges in a system of (discrete or continuous) charges. It is a vector-valued quantity. If the system of charges is neutral, that is if the sum of all charges is zero, then the dipole moment of the system is independent of the choice of a reference frame; however in a non-neutral system, such as the dipole moment of a single proton, a dependence on the choice of reference point arises. In such cases it is conventional to choose the reference point to be the center of mass of the system or the center of charge, not some arbitrary origin. This convention ensures that the dipole moment is an intrinsic property of the system. Linear Energy Transfer Absolute Humidity Absolute humidity is the mass of water in a particular volume of air. It is a measure of the density of water vapor in an atmosphere. Density Electric Current per Angle Curvature The canonical example of extrinsic curvature is that of a circle, which has curvature equal to the inverse of its radius everywhere. Smaller circles bend more sharply, and hence have higher curvature. The curvature of a smooth curve is defined as the curvature of its osculating circle at each point. The osculating circle of a sufficiently smooth plane curve at a given point on the curve is the circle whose center lies on the inner normal line and whose curvature is the same as that of the given curve at that point. This circle is tangent to the curve at the given point. That is, given a point P on a smooth curve C, the curvature of C at P is defined to be 1/R where R is the radius of the osculating circle of C at P. The magnitude of curvature at points on physical curves can be measured in diopters (also spelled dioptre) — this is the convention in optics. [Wikipedia] Action S S Radiance Radiance is a radiometric measure that describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. Power per Area Angle Power per Area Total Pressure The total pressure is the sum of the dynamic and static pressures, i.e. P_0 = P + q. Pressure Pressure is an effect which occurs when a force is applied on a surface. Pressure is the amount of force acting on a unit area. Pressure is distinct from stress, as the former is the ratio of the component of force normal to a surface to the surface area. Stress is a tensor that relates the vector force to the vector area. Force Per Area P_0 Area Area is a quantity expressing the two-dimensional size of a defined part of a surface, typically a region bounded by a closed curve. Velocity Area Time Angular Velocity The change of angle per unit time; specifically, in celestial mechanics, the change in angle of the radius vector per unit time. Compressibility Factor The compressibility factor (Z) is a useful thermodynamic property for modifying the ideal gas law to account for the real gas behaviour. The closer a gas is to a phase change, the larger the deviations from ideal behavior. Values for compressibility are calculated using equations of state (EOS), such as the virial equation and van der Waals equation. The compressibility factor for specific gases can be obtained, with out calculation, from compressibility charts. These charts are created by plotting Z as a function of pressure at constant temperature. Dimensionless Ratio ψ Z Inverse Time Temperature Mass Temperature Absorbed Dose Rate Specific Impulse by Mass Linear Velocity Thermal Resistivity The reciprocal of thermal conductivity is thermal resistivity, measured in kelvin-metres per watt (K*m/W). Linear Thermal Expansion When the temperature of a substance changes, the energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bonds. As a result, solids typically expand in response to heating and contract on cooling; this dimensional response to temperature change is expressed by its coefficient of thermal expansion. Different coefficients of thermal expansion can be defined for a substance depending on whether the expansion is measured by: * linear thermal expansion * area thermal expansion * volumetric thermal expansion These characteristics are closely related. The volumetric thermal expansion coefficient can be defined for both liquids and solids. The linear thermal expansion can only be defined for solids, and is common in engineering applications. Some substances expand when cooled, such as freezing water, so they have negative thermal expansion coefficients. [Wikipedia] Inverse Permittivity Force Per Area Time Polarization Field The Polarization Field is the vector field that expresses the density of permanent or induced electric dipole moments in a dielectric material. The polarization vector P is defined as the ratio of electric dipole moment per unit volume. P Heat Flow Rate per Unit Area Moment of Inertia Coefficient of Heat Transfer Linear Momentum Linear momentum is the product of mass and linear velocity. The SI unit for linear momentum is meter-kilogram per second (m-kg/s). Momentum Quantity of motion. Linear momentum is the quantity obtained by multiplying the mass of a body by its linear speed. Angular momentum is the quantity obtained by multiplying the moment of inertia of a body by its angular speed. The momentum of a system of particles is given by the sum of the momentums of the individual particles which make up the system or by the product of the total mass of the system and the velocity of the center of gravity of the system. The momentum of a continuous medium is given by the integral of the velocity over the mass of the medium or by the product of the total mass of the medium and the velocity of the center of gravity of the medium. Time Temperature Stress Stress is a measure of the average amount of force exerted per unit area of a surface within a deformable body on which internal forces act. In other words, it is a measure of the intensity or internal distribution of the total internal forces acting within a deformable body across imaginary surfaces. These internal forces are produced between the particles in the body as a reaction to external forces applied on the body. Power per Area Quartic Temperature Time Squared Molar Angular Momentum Torque In physics, a torque (τ) is a vector that measures the tendency of a force to rotate an object about some axis [1]. The magnitude of a torque is defined as force times its lever arm [2]. Just as a force is a push or a pull, a torque can be thought of as a twist. The SI unit for torque is newton meters (N m). In U.S. customary units, it is measured in foot pounds (ft lbf) (also known as 'pounds feet'). Mathematically, the torque on a particle (which has the position r in some reference frame) can be defined as the cross product: τ = r x F where r is the particle's position vector relative to the fulcrum F is the force acting on the particles, or, more generally, torque can be defined as the rate of change of angular momentum, τ = dL/dt where L is the angular momentum vector t stands for time. [Wikipedia] Electric Quadrupole Moment The Electric Quadrupole Moment is a quantity which describes the effective shape of the ellipsoid of nuclear charge distribution. A non-zero quadrupole moment Q indicates that the charge distribution is not spherically symmetric. By convention, the value of Q is taken to be positive if the ellipsoid is prolate and negative if it is oblate. In general, the electric quadrupole moment is tensor-valued. Q Thrust to Mass Ratio Amount of Substance Per Unit Volume Concentration Electric Current per Unit Energy Activity Activity is the term used to characterise the number of nuclei which disintegrate in a radioactive substance per unit time. Activity is usually measured in Becquerels (Bq), where 1 Bq is 1 disintegration per second. Stochastic Process Frequency Frequency is the number of occurrences of a repeatiing event per unit time. The repetition of the events may be periodic (i.e. the length of time between event repetitions is fixed) or aperiodic (i.e. the length of time between event repetitions varies). Therefore, we distinguish between periodic and aperiodic frequencies. In the SI system, periodic frequency is measured in hertz (Hz) or multiples of hertz, while aperiodic frequency is measured in becquerel (Bq). Dose Equivalent The equivalent dose to a tissue is found by multiplying the absorbed dose, in gray, by a dimensionless "quality factor" Q, dependent upon radiation type, and by another dimensionless factor N, dependent on all other pertinent factors. N depends upon the part of the body irradiated, the time and volume over which the dose was spread, even the species of the subject. Specific Energy Volumetric Heat Capacity Luminous Energy Luminous Energy is the perceived energy of light. This is sometimes also called the quantity of light. Qv Temperature Amount of Substance International System of Quantities Power Area per Solid Angle Inverse Length Mass per Time Volume Thermal Expansion When the temperature of a substance changes, the energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bonds. As a result, solids typically expand in response to heating and contract on cooling; this dimensional response to temperature change is expressed by its coefficient of thermal expansion. Different coefficients of thermal expansion can be defined for a substance depending on whether the expansion is measured by: * linear thermal expansion * area thermal expansion * volumetric thermal expansion These characteristics are closely related. The volumetric thermal expansion coefficient can be defined for both liquids and solids. The linear thermal expansion can only be defined for solids, and is common in engineering applications. Some substances expand when cooled, such as freezing water, so they have negative thermal expansion coefficients. [Wikipedia] Inverse Length Temperature Gravitational Attraction Catalytic Activity Energy per Square Magnetic Flux Density Energy Density Energy density is defined as energy per unit volume. The SI unit for energy density is the joule per cubic meter. Molar Energy Area Thermal Expansion When the temperature of a substance changes, the energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bonds. As a result, solids typically expand in response to heating and contract on cooling; this dimensional response to temperature change is expressed by its coefficient of thermal expansion. Different coefficients of thermal expansion can be defined for a substance depending on whether the expansion is measured by: * linear thermal expansion * area thermal expansion * volumetric thermal expansion These characteristics are closely related. The volumetric thermal expansion coefficient can be defined for both liquids and solids. The linear thermal expansion can only be defined for solids, and is common in engineering applications. Some substances expand when cooled, such as freezing water, so they have negative thermal expansion coefficients. [Wikipedia] Polarizability Polarizability is the relative tendency of a charge distribution, like the electron cloud of an atom or molecule, to be distorted from its normal shape by an external electric field, which may be caused by the presence of a nearby ion or dipole. The electronic polarizability α is defined as the ratio of the induced dipole moment of an atom to the electric field that produces this dipole moment. Polarizability is often a scalar valued quantity, however in the general case it is tensor-valued. α Power per Electric Charge Amount of Substance N Area Temperature Electric Charge per Amount of Substance Angular Momentum Quantity of rotational motion. Linear momentum is the quantity obtained by multiplying the mass of a body by its linear velocity. Angular momentum is the quantity obtained by multiplying the moment of inertia of a body by its angular velocity. The momentum of a system of particles is given by the sum of the momenta of the individual particles which make up the system or by the product of the total mass of the system and the velocity of the center of gravity of the system. The momentum of a continuous medium is given by the integral of the velocity over the mass of the medium or by the product of the total mass of the medium and the velocity of the center of gravity of the medium. In physics, the angular momentum of an object rotating about some reference point is the measure of the extent to which the object will continue to rotate about that point unless acted upon by an external torque. In particular, if a point mass rotates about an axis, then the angular momentum with respect to a point on the axis is related to the mass of the object, the velocity and the distance of the mass to the axis. While the motion associated with linear momentum has no absolute frame of reference, the rotation associated with angular momentum is sometimes spoken of as being measured relative to the fixed stars. Dynamic Viscosity Viscosity Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In general terms it is the resistance of a liquid to flow, or its "thickness". Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. [Wikipedia] μ Length Temperature Area per Time Standard Gravitational Parameter μ Luminance Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. Area Angle Electric Charge per Mass Electric Charge Line Density Inverse Amount of Substance Kinematic Viscosity The Kinematic Viscosity of a fluid is the dynamic viscosity divided by the fluid density. Absorbed Dose Absorbed dose (also known as Total Ionizing Dose, TID) is a measure of the energy deposited in a medium by ionizing radiation. It is equal to the energy deposited per unit mass of medium, and so has the unit J/kg, which is given the special name Gray (Gy). Note that the absorbed dose is not a good indicator of the likely biological effect. 1 Gy of alpha radiation would be much more biologically damaging than 1 Gy of photon radiation for example. Appropriate weighting factors can be applied reflecting the different relative biological effects to find the equivalent dose. The risk of stoctic effects due to radiation exposure can be quantified using the effective dose, which is a weighted average of the equivalent dose to each organ depending upon its radiosensitivity. When ionising radiation is used to treat cancer, the doctor will usually prescribe the radiotherapy treatment in Gy. When risk from ionising radiation is being discussed, a related unit, the Sievert is used. Inverse Volume Thermal Resistance Length Energy Quartic Electric Dipole Moment per Cubic Energy Square Energy Heat Flow Rate Inverse Square Energy Luminous Flux Luminous Flux or Luminous Power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light. F Volume The volume of a solid object is the three-dimensional concept of how much space it occupies, often quantified numerically. One-dimensional figures (such as lines) and two-dimensional shapes (such as squares) are assigned zero volume in the three-dimensional space. Mass per Electric Charge Cubic Electric Dipole Moment per Square Energy Temperature Θ Temperature per Time Solid Angle The solid angle subtended by a surface S is defined as the surface area of a unit sphere covered by the surface S's projection onto the sphere. A solid angle is related to the surface of a sphere in the same way an ordinary angle is related to the circumference of a circle. Since the total surface area of the unit sphere is 4*pi, the measure of solid angle will always be between 0 and 4*pi. Angle The inclination to each other of two intersecting lines, measured by the arc of a circle intercepted between the two lines forming the angle, the center of the circle being the point of intersection. An acute angle is less than 90°; a right angle 90 °; an obtuse angle, more than 90° but less than 180 °; a straight angle, 180°; a reflex angle, more than 180° but less than 360°; a perigon, 360°. Any angle not a multiple of 90° is an oblique angle. If the sum of two angles is 90°, they are complementary angles; if 180°, supplementary angles; if 360°, explementary angles. Two adjacent angles have a common vertex and lie on opposite sides of a common side. A dihedral angle is the angle between two intersecting planes. A spherical angle is the angle between two intersecting great circles. Mass per Area Time Molar Heat Capacity Volume per Unit Time Luminous Efficacy Luminous Efficacy is the ratio of luminous flux (in lumens) to power (usually measured in watts). Depending on context, the power can be either the radiant flux of the source's output, or it can be the total electric power consumed by the source. Length per Unit Magnetic Flux Force per Electric Charge Plane Angle Force per Unit Length Angular Acceleration Angular acceleration is the rate of change of angular velocity over time. Measurement of the change made in the rate of change of an angle that a spinning object undergoes per unit time. It is a vector quantity. Also called Rotational acceleration. In SI units, it is measured in radians per second squared (rad/s^2), and is usually denoted by the Greek letter alpha. Acceleration Acceleration is the (instantaneous) rate of change of velocity. Acceleration may be either linear acceleration, or angular acceleration. It is a vector quantity with dimension length/time^2 for linear acceleration, or in the case of angular acceleration, with dimension angle/time^2. In SI units, linear acceleration is measured in meters/second^2 (m·s^-2) and angular acceleration is measured in radians/second^2. In common speech, the term acceleration is only used for an increase in speed. In physics, any increase or decrease in speed is referred to as acceleration and similarly, motion in a circle at constant speed is also an acceleration, since the direction component of the velocity is changing. Linear Acceleration Mass per Area Specific Heat Pressure Specific heat at a constant pressure. Energy and Work Inverse Magnetic Flux Thermal Insulance Electric Charge Volume Density ρ Thermal Conductivity Luminous Intensity Luminous Intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle. The weighting is determined by the luminosity function, a standardized model of the sensitivity of the human eye to different wavelengths. J Radiant Intensity Radiant Intensity is a measure of the intensity of electromagnetic radiation. It is defined as power per unit solid angle. Exposure Length Mass Power Area Molar Volume V_m Specific Heat Capacity Thermal Diffusivity Amount of Substance per Unit Mass Molar Mass Luminous Flux per Area Mass per Length Energy per Area Electric Charge Specific Heat Volume Specific heat per constant volume. Energy per Area Inverse Energy Force Force is an influence that causes mass to accelerate. It may be experienced as a lift, a push, or a pull. Force is defined by Newton's Second Law as F = m · a, where F is force, m is mass and a is acceleration. Net force is mathematically equal to the time rate of change of the momentum of the body on which it acts. Since momentum is a vector quantity (has both a magnitude and direction), force also is a vector quantity. Length Molar Energy Temperature per Magnetic Flux Density Specific Volume ν Specific volume (ν) is the volume occupied by a unit of mass of a material. It is equal to the inverse of density. ν Heat Capacity Energy per Temperature Cp Illuminance Illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Data Rate The frequency derived from the period of time required to transmit one bit. This represents the amount of data transferred per second by a communications channel or a computing or storage device. Data rate is measured in units of bits per second (written "b/s" or "bps"), bytes per second (Bps), or baud. When applied to data rate, the multiplier prefixes "kilo-", "mega-", "giga-", etc. (and their abbreviations, "k", "M", "G", etc.) always denote powers of 1000. For example, 64 kbps is 64,000 bits per second. This contrasts with units of storage which use different prefixes to denote multiplication by powers of 1024, e.g. 1 kibibit = 1024 bits. Respiratory Rate Static Pressure Static Pressure is the pressure at a nominated point in a fluid. Every point in a steadily flowing fluid, regardless of the fluid speed at that point, has its own static pressure P, dynamic pressure q, and total pressure P_0. The total pressure is the sum of the dynamic and static pressures, i.e. P_0 = P + q. Heart Rate Luminous Emmitance Luminous Emittance is the luminous flux per unit area emitted from a surface. Circulation In fluid dynamics, circulation is the line integral around a closed curve of the fluid velocity. It has dimensions of length squared over time. Γ First Moment of Area The first moment of area is the summation of area times distance to an axis. It is a measure of the distribution of the area of a shape in relationship to an axis. Mass Amount of Substance Temperature Thermal Energy Thermodynamic Entropy Thermodynamic Entropy is a measure of the unavailability of a system’s energy to do work. It is a measure of the randomness of molecules in a system and is central to the second law of thermodynamics and the fundamental thermodynamic relation, which deal with physical processes and whether they occur spontaneously. Spontaneous changes, in isolated systems, occur with an increase in entropy. Spontaneous changes tend to smooth out differences in temperature, pressure, density, and chemical potential that may exist in a system, and entropy is thus a measure of how far this smoothing-out process has progressed. It can be seen that the dimensions of entropy are energy divided by temperature, which is the same as the dimensions of Boltzmann's constant (kB) and heat capacity. The SI unit of entropy is joule per kelvin. [Wikipedia] Thrust Thrust is a reaction force described quantitatively by Newton's Second and Third Laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a proportional but opposite force on that system. 1. The pushing or pulling force developed by an aircraft engine or a rocket engine. 2. The force exerted in any direction by a fluid jet or by a powered screw, as, the thrust of an antitorque rotor. 3. (symbol F). Specifically, in rocketry, F = mv where m is propellant mass flow and v is exhaust velocity relative to the vehicle. Also called momentum thrust. Microbial Formation Specific Impulse by Weight Radiant Energy Radiant Energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux (or power) with respect to time Irradiance Irradiance and Radiant Emittance are radiometry terms for the power per unit area of electromagnetic radiation at a surface. "Irradiance" is used when the electromagnetic radiation is incident on the surface. "Radiant emmitance" (or "radiant exitance") is used when the radiation is emerging from the surface. Enthalpy Static enthalpy per unit mass. The specific enthalpy of a working mass is a property of that mass used in thermodynamics, defined as h=u+p . v where u is the specific internal energy, p is the pressure, and v is specific volume. In other words, h = H / m where m is the mass of the system. The SI unit for specific enthalpy is joules per kilogram. [Wikipedia] Mole Fraction In chemistry, the mole fraction of a component in a mixture is the relative proportion of molecules belonging to the component to those in the mixture, by number of molecules. It is one way of measuring concentration. Heat Capacity Ratio The heat capacity ratio, or ratio of specific heats, is the ratio of the heat capacity at constant pressure (C_P) to heat capacity at constant volume (C_V). For an ideal gas, the heat capacity is constant with temperature (Θ). Accordingly we can express the enthalpy as H = C_P*Θ and the internal energy as U = C_V*Θ. Thus, it can also be said that the heat capacity ratio is the ratio between enthalpy and internal energy Reynolds Number The Reynolds number (Re) is a dimensionless number defined as the ratio of inertial forces to viscous forces and, consequently, it quantifies the relative importance of these two types of forces for given flow conditions. Dynamic Pressure Dynamic Pressure (indicated with q, or Q, and sometimes called velocity pressure) is the quantity defined by: q = 1/2 * ρv^2 where (using SI units): q = dynamic pressure in pascals ρ = fluid density in kg/m3 (e.g. density of air) v = fluid velocity in m/s q Heat Energy transferred by a thermal process. Heat can be measured in terms of the dynamical units of energy, as the erg, joule, etc., or in terms of the amount of energy required to produce a definite thermal change in some substance, as, for example, the energy required per degree to raise the temperature of a unit mass of water at some temperature ( calorie, Btu). Molecular Viscosity Signal Detection Threshold Turbidity Turbidity is the cloudiness or haziness of a fluid, or of air, caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. Turbidity in open water is often caused by phytoplankton and the measurement of turbidity is a key test of water quality. The higher the turbidity, the higher the risk of the drinkers developing gastrointestinal diseases, especially for immune-compromised people, because contaminants like virus or bacteria can become attached to the suspended solid. The suspended solids interfere with water disinfection with chlorine because the particles act as shields for the virus and bacteria. Similarly suspended solids can protect bacteria from UV sterilisation of water. Fluids can contain suspended solid matter consisting of particles of many different sizes. While some suspended material will be large enough and heavy enough to settle rapidly to the bottom container if a liquid sample is left to stand (the settleable solids), very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid. Information Entropy Planck System of Quantities Atmospheric Pressure The pressure exerted at a point due to the presence of an atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. [Wikipedia] Electric Power Electric power is the rate at which electrical energy is transferred by an electric circuit. In the simple case of direct current circuits, electric power can be calculated as the product of the potential difference in the circuit (V) and the amount of current flowing in the circuit (I): P = VI where P is the power V is the potential difference I is the current. However, in general electric power is calculated by taking the integral of the vector cross-product of the electrical and magnetic fields over a specified area. Currency Asset An Asset is an economic resource owned by a business or company. Simply stated, assets are things of value that can be readily converted into cash (although cash itself is also considered an asset). Gain A general term used to denote an increase in signal power or signal strength in transmission from one point to another. Gain is usually expressed in decibels and is widely used to denote transducer gain. An increase or amplification. In radar there are two general usages of the term: (a) antenna gain, or gain factor, is the ratio of the power transmitted along the beam axis to that of an isotropic radiator transmitting the same total power; (b) receiver gain, or video gain, is the amplification given a signal by the receiver. Strain Energy Density u Area Time Temperature Potential Energy Energy possessed by a body by virtue of its position in a gravity field in contrast with kinetic energy, that possessed by virtue of its motion. Gyromagnetic Ratio γ Radiant Emmitance Irradiance and Radiant Emittance are radiometry terms for the power per unit area of electromagnetic radiation at a surface. "Irradiance" is used when the electromagnetic radiation is incident on the surface. "Radiant emmitance" (or "radiant exitance") is used when the radiation is emerging from the surface. Weight 1. The force with which a body is attracted toward an astronomical body. 2. The product of the mass of a body and the acceleration acting on a body. In a dynamic situation, the weight can be a multiple of that under resting conditions. Weight also varies on other planets in accordance with their gravity. CGS-ESU System of Quantities The electrostatic system of units is used to measure electrical quantities of electric charge, current, and voltage within the centimeter gram second (or "CGS") metric system of units. In electrostatic units, electric charge is derived from Coulomb's Law (expressing the force exerted between two charged particles separated by a distance) by solving for electric charge and setting the constant of proportionality (k_s) equal to unity. Thus, in electrostatic units, the dimensionality of electric charge is derived from the base CGS quantities of length, mass, and time. Mass Amount of Substance Video Frame Rate RF-Power Signal Strength In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in voltage per length or signal power received by a reference antenna. High-powered transmissions, such as those used in broadcasting, are expressed in dB-millivolts per metre (dBmV/m). [Wikipedia] Vorticity In the simplest sense, vorticity is the tendency for elements of a fluid to "spin." More formally, vorticity can be related to the amount of "circulation" or "rotation" (or more strictly, the local angular rate of rotation) in a fluid. The average vorticity in a small region of fluid flow is equal to the circulation C around the boundary of the small region, divided by the area A of the small region. Mathematically, vorticity is a vector field and is defined as the curl of the velocity field. ω CGS System of Quantities CGS-Gauss System of Quantities Mach Number Mach number (Ma) is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance: M = V_o/V_s where M is the Mach number V_o is the velocity of the object relative to the medium and V_s is the velocity of sound in the medium The Mach number is commonly used both with objects traveling at high speed in a fluid, and with high-speed fluid flows inside channels such as nozzles, diffusers or wind tunnels. As it is defined as a ratio of two speeds, it is a dimensionless number. [Wikipedia] Serum or Plasma Level Second Moment of Area The second moment of area is a property of a physical object that can be used to predict its resistance to bending and deflection. The deflection of an object under load depends not only on the load, but also on the geometry of the object's cross-section. Polar moment of inertia The polar moment of inertia is a quantity used to predict an object's ability to resist torsion, in objects (or segments of objects) with an invariant circular cross-section and no significant warping or out-of-plane deformation. It is used to calculate the angular displacement of an object subjected to a torque. It is analogous to the area moment of inertia, which characterizes an object's ability to resist bending. Kinetic Energy Internal Energy Number Density In physics, astronomy, and chemistry, number density (symbol: n) is a kind of quantity used to describe the degree of concentration of countable objects (atoms, molecules, dust particles, galaxies, etc.) in the three-dimensional physical space. n Thermal Energy Length US Customary System of Quantities Liquid Volume Dry Volume Molecular Mass The molecular mass, or molecular weight of a chemical compound is the mass of one molecule of that compound, relative to the unified atomic mass unit, u. Molecular mass should not be confused with molar mass, which is the mass of one mole of a substance. M Friction Friction is the force of two surfaces In contact, or the force of a medium acting on a moving object (i.e. air on an aircraft). When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy. Length Force Length Temperature Time Speed Speed is the magnitude of velocity. Radiosity Radiosity is the total emitted and reflected radiation leaving a surface. Kinetic Energy The energy which a body possesses as a consequence of its motion, defined as one-half the product of its mass m and the square of its speed v, 1/2 mv^2. The kinetic energy per unit volume of a fluid parcel is the 1/2 p v2 , where p is the density and v the speed of the parcel. See potential energy. For relativistic speeds the kinetic energy is given by Ek = mc^2 - m0c^2 where c is the velocity of light in a vacuum, m0 is the rest mass, and m is the moving mass. Tension Tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression. Force Magnitude Thermal Efficiency Thermal efficiency is a dimensionless performance measure of a thermal device such as an internal combustion engine, a boiler, or a furnace. The input to the device is heat, or the heat-content of a fuel that is consumed. The desired output is mechanical work, or heat, or possibly both. Strain In any branch of science dealing with materials and their behaviour, strain is the geometrical expression of deformation caused by the action of stress on a physical body. Strain is calculated by first assuming a change between two body states: the beginning state and the final state. Then the difference in placement of two points in this body in those two states expresses the numerical value of strain. Strain therefore expresses itself as a change in size and/or shape. [Wikipedia] Angular Frequency Angular frequency is a scalar measure of rotation rate. It is the magnitude of the vector quantity angular velocity. ω Atomic Charge TBD In computer operations, (a) the largest quantity which can be stored, processed, or transferred; (b) the largest number of digits or characters which may regularly be processed; (c) the upper and lower limits of the quantities which may be processed. In other contexts, the amount of material that can be stored, such as fuel or food. Capacity