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A physical quantity (or "physical magnitude") is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.^{[1]}
An extensive quantity is equal to the sum of that quantity for all of its constituent subsystems; examples include volume, mass, and electric charge. For instance, if an object has mass m_{1} and another has mass m_{2} then a system simply comprising those two objects will have a mass of m_{1} + m_{2}.
An intensive quantity is independent of the extent of the system; quantities such as temperature, pressure, and density are examples. To illustrate, if two objects having a given temperature are combined, together they still have the same temperature (not twice the temperature).
There are also physical quantities that can be classified as neither extensive nor intensive, for example an extensive quantity with a nonlinear operator applied, such as the square of volume.^{[2]}
General: Symbols for quantities should be chosen according to the international recommendations from ISO/IEC 80000, the IUPAP red book and the IUPAC green book. For example, the recommended symbol for the physical quantity 'mass' is m, and the recommended symbol for the quantity 'charge' is Q.
Subscripts and indices
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.
Scalars: 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: Symbols for physical quantities that are vectors are in bold type, underlined or with an arrow above. If, e.g., u is the speed of a particle, then the straightforward notation for its velocity is u, u, or \vec{u}\,\!.
Numbers and elementary functions
Numerical quantities, even those denoted by letters, are usually printed in roman (upright) type, though sometimes can be 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.
Units
Most physical quantities include a unit, but not all - some are dimensionless. Neither the name of a physical quantity, nor the symbol used to denote it, implies a particular choice of unit, though SI units are usually preferred and assumed today due to their ease of use and all-round applicability. 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).
Dimensions
The notion of physical dimension of a physical quantity was introduced by Joseph Fourier in 1822.^{[3]} By convention, physical quantities are organized in a dimensional system built upon base quantities, each of which is regarded as having its own dimension.
Base Quantities are those quantities on the basis of which other quantities can be expressed. The seven base quantities of the International System of Quantities (ISQ) and their corresponding SI units and dimensions are listed in the following table. Other conventions may have a different number of fundamental units (e.g. the CGS and MKS systems of units).
The last two angular units; plane angle and solid angle are subsidiary units used in the SI, but treated dimensionless. The subsidiary units are used for convenience to differentiate between a truly dimensionless quantity (pure number) and an angle, which are different measurements.
Important applied base units for space and time are below. Area and volume are of course derived from length, but included for completeness as they occur frequently in many derived quantities, in particular densities.
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] is 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, \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 X \equiv X \left ( x_1, x_2 \cdots x_n \right ) \,\!, then:
No common symbol for n-space density, here ρ_{n} is used.
(length, area, volume or higher dimensions)
q=\int q_\lambda \mathrm{d} \lambda q=\int q_\nu \mathrm{d} \nu
[q][T] (q_{ν})
Transport mechanics, nuclear physics/particle physics: q = \iiint F \mathrm{d} A \mathrm{d} t
Vector field: \Phi_F = \iint_S \mathbf{F} \cdot \mathrm{d} \mathbf{A} \,\!
section/ surface boundary
q is a scalar: \mathbf{m} = \mathbf{r} q \,\! q is a vector: \mathbf{m} = \mathbf{r} \times \mathbf{q} \,\!
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, particular 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 depended on our measurement scheme, coordinate system and metric system.
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