Physical quantity

Subscripts are used for two reasons, to simply attach a name to the quantity or associate it with another quantity, or represent a specific vector, matrix, or tensor component.

• Name reference: The quantity has a subscripted or superscripted single letter, group of letters, or complete word, to label what concept or entity they refer to, often to distinguish it from other quanities with the same main symbol. These subscripts or superscripts tend to be written in upright roman typeface rather than italic while the main symbol representing the quantity is in italic. For instance E_k or E_kinetic is usually used to denote kinetic energy and E_p or E_potential is usually used to denote potential energy.
• Quantity reference: The quantity has a subscripted or superscripted single letter, group of letters, or complete word, to parameterize what measurement/s they refer to. These subscripts or superscripts tend to be written in italic rather than upright roman typeface; the main symbol representing the quantity is in italic. For example c_p or c_pressure is heat capacity at the pressure given by the quantity in the subscript.

The type of subscript is expressed by its typeface: 'k' and 'p' are abbreviations of the words kinetic and potential, whereas p (italic) is the symbol for the physical quantity pressure rather than an abbreviation of the word.

• Indices: The use of indices is for mathematical formalism using index notation.

A scalar is a physical quantity that has magnitude but no direction. Symbols for physical quantities are usually chosen to be a single letter of the Latin or Greek alphabet, and are printed in italic type.

Vectors are physical quantities that possess both magnitude and direction. Symbols for physical quantities that are vectors are in bold type, underlined or with an arrow above. For example, if u is the speed of a particle, then the straightforward notations for its velocity are u, u, or ${\displaystyle {\vec {u}}\,\!}$.

Numerical quantities, even those denoted by letters, are usually printed in roman (upright) type, though sometimes in italic. Symbols for elementary functions (circular trigonometric, hyperbolic, logarithmic etc.), changes in a quantity like Δ in Δy or operators like d in dx, are also recommended to be printed in roman type.

Examples:

• Real numbers, such as 1 or √2,
• e, the base of natural logarithms,
• i, the imaginary unit,
• π for the ratio of a circle's circumference to its diameter, 3.14159265358979323846264338327950288...
• δx, Δy, dz, representing differences (finite or otherwise) in the quantities x, y and z
• sin α, sinh γ, log x

There is often a choice of unit, though SI units (including submultiples and multiples of the basic unit) are usually used in scientific contexts due to their ease of use, international familiarity and prescription. For example, a quantity of mass might be represented by the symbol m, and could be expressed in the units kilograms (kg), pounds (lb), or daltons (Da).

The notion of dimension of a physical quantity was introduced by Joseph Fourier in 1822.[1] By convention, physical quantities are organized in a dimensional system built upon base quantities, each of which is regarded as having its own dimension.

Important applied base units for space and time are below. Area and volume are thus of course derived from length, but included for completeness as they occur frequently in many derived quantities, in particular densities.

Quantity		SI unit	Dimensions
Description	Symbols
(Spatial) position (vector)	r, R, a, d	m	L
Angular position, angle of rotation (can be treated as vector or scalar)	θ, θ	rad	None
Area, cross-section	A, S, Ω	m2	L2
Vector area (Magnitude of surface area, directed normal to tangential plane of surface)	${\displaystyle \mathbf {A} \equiv A\mathbf {\hat {n}} ,\quad \mathbf {S} \equiv S\mathbf {\hat {n}} \,\!}$	m2	L2
Volume	τ, V	m3	L3

Important and convenient derived quantities such as densities, fluxes, flows, currents are associated with many quantities. Sometimes different terms such as current density and flux density, rate, frequency and current, are used interchangeably in the same context, sometimes they are used uniqueley.

To clarify these effective template derived quantities, we let q be any quantity within some scope of context (not necessarily base quantities) and present in the table below some of the most commonly used symbols where applicable, their definitions, usage, SI units and SI dimensions – where [q] denotes the dimension of q.

For time derivatives, specific, molar, and flux densities of quantities there is no one symbol, nomenclature depends on subject, though time derivatives can be generally written using overdot notation. For generality we use q_m, q_n, and F respectively. No symbol is necessarily required for the gradient of a scalar field, since only the nabla/del operator ∇ or grad needs to be written. For spatial density, current, current density and flux, the notations are common from one context to another, differing only by a change in subscripts.

For current density, ${\displaystyle \mathbf {\hat {t}} }$ is a unit vector in the direction of flow, i.e. tangent to a flowline. Notice the dot product with the unit normal for a surface, since the amount of current passing through the surface is reduced when the current is not normal to the area. Only the current passing perpendicular to the surface contributes to the current passing through the surface, no current passes in the (tangential) plane of the surface.

The calculus notations below can be used synonymously.

If X is a n-variable function ${\displaystyle X\equiv X\left(x_{1},x_{2}\cdots x_{n}\right)}$, then:

Differential The differential n-space volume element is ${\displaystyle \mathrm {d} ^{n}x\equiv \mathrm {d} V_{n}\equiv \mathrm {d} x_{1}\mathrm {d} x_{2}\cdots \mathrm {d} x_{n}}$,

Integral: The multiple integral of X over the n-space volume is ${\displaystyle \int X\mathrm {d} ^{n}x\equiv \int X\mathrm {d} V_{n}\equiv \int \cdots \int \int X\mathrm {d} x_{1}\mathrm {d} x_{2}\cdots \mathrm {d} x_{n}\,\!}$.

Quantity	Typical symbols	Definition	Meaning, usage	Dimension
Quantity	q	q	Amount of a property	[q]
Rate of change of quantity, Time derivative	${\displaystyle {\dot {q}}\,\!}$	${\displaystyle {\dot {q}}\equiv {\frac {\mathrm {d} q}{\mathrm {d} t}}}$	Rate of change of property with respect to time	[q]T−1
Quantity spatial density	ρ = volume density (n = 3), σ = surface density (n = 2), λ = linear density (n = 1) No common symbol for n-space density, here ρn is used.	${\displaystyle q=\int \rho _{n}\mathrm {d} V_{n}}$	Amount of property per unit n-space (length, area, volume or higher dimensions)	[q]L−n
Specific quantity	qm	${\displaystyle q_{m}={\frac {\mathrm {d} q}{\mathrm {d} m}}\,\!}$	Amount of property per unit mass	[q]M−1
Molar quantity	qn	${\displaystyle q_{n}={\frac {\mathrm {d} q}{\mathrm {d} n}}\,\!}$	Amount of property per mole of substance	[q]N−1
Quantity gradient (if q is a scalar field).		${\displaystyle \nabla q}$	Rate of change of property with respect to position	[q]L−1
Spectral quantity (for EM waves)	qv, qν, qλ	Two definitions are used, for frequency and wavelength: ${\displaystyle q=\int q_{\lambda }\mathrm {d} \lambda }$ ${\displaystyle q=\int q_{\nu }\mathrm {d} \nu }$	Amount of property per unit wavelength or frequency.	[q]L−1 (qλ) [q]T (qν)
Flux, flow (synonymous)	ΦF, F	Two definitions are used; Transport mechanics, nuclear physics/particle physics: ${\displaystyle q=\iiint F\mathrm {d} A\mathrm {d} t}$ Vector field: ${\displaystyle \Phi _{F}=\iint _{S}\mathbf {F} \cdot \mathrm {d} \mathbf {A} }$	Flow of a property though a cross-section/surface boundary.	[q]T−1L−2, [F]L2
Flux density	F	${\displaystyle \mathbf {F} \cdot \mathbf {\hat {n}} ={\frac {\mathrm {d} \Phi _{F}}{\mathrm {d} A}}\,\!}$	Flow of a property though a cross-section/surface boundary per unit cross-section/surface area	[F]
Current	i, I	${\displaystyle I={\frac {\mathrm {d} q}{\mathrm {d} t}}}$	Rate of flow of property through a cross section / surface boundary	[q]T−1
Current density (sometimes called flux density in transport mechanics)	j, J	${\displaystyle I=\iint \mathbf {J} \cdot \mathrm {d} \mathbf {S} }$	Rate of flow of property per unit cross-section/surface area	[q]T−1L−2
Moment of quantity	m, M	Two definitions can be used; q is a scalar: ${\displaystyle \mathbf {m} =\mathbf {r} q}$ q is a vector: ${\displaystyle \mathbf {m} =\mathbf {r} \times \mathbf {q} }$	Quantity at position r has a moment about a point or axes, often relates to tendency of rotation or potential energy.	[q]L

The meaning of the term physical quantity is generally well understood (everyone understands what is meant by the frequency of a periodic phenomenon, or the resistance of an electric wire). The term physical quantity does not imply a physically invariant quantity. Length for example is a physical quantity, yet it is variant under coordinate change in special and general relativity. The notion of physical quantities is so basic and intuitive in the realm of science, that it does not need to be explicitly spelled out or even mentioned. It is universally understood that scientists will (more often than not) deal with quantitative data, as opposed to qualitative data. Explicit mention and discussion of physical quantities is not part of any standard science program, and is more suited for a philosophy of science or philosophy program.

The notion of physical quantities is seldom used in physics, nor is it part of the standard physics vernacular. The idea is often misleading, as its name implies "a quantity that can be physically measured", yet is often incorrectly used to mean a physical invariant. Due to the rich complexity of physics, many different fields possess different physical invariants. There is no known physical invariant sacred in all possible fields of physics. Energy, space, momentum, torque, position, and length (just to name a few) are all found to be experimentally variant in some particular scale and system. Additionally, the notion that it is possible to measure "physical quantities" comes into question, particularly in quantum field theory and normalization techniques. As infinities are produced by the theory, the actual “measurements” made are not really those of the physical universe (as we cannot measure infinities), they are those of the renormalization scheme which is expressly dependent on our measurement scheme, coordinate system and metric system.

• DEVLIB project in C# Language and Delphi Language
• PhysicalQuantities project in C# Language at CodePlex
• PhysicalMeasure C# library project in C# Language at CodePlex
• Ethica Measures project in C# Language at CodePlex
• EngineerJS online calculation and scripting tool supporting physical quantities.