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A closely related system is the International System of Electric and Magnetic Units, [18] which has a different unit of mass so that the formula for 𝜆′ is invalid. The unit of mass was chosen to remove powers of ten from contexts in which they were considered to be objectionable (e.g., P = VI and F = qE). Inevitably, the powers of ten ...
Ratio of flow velocity to the local speed of sound unitless: 1: Magnetic flux: Φ: Measure of magnetism, taking account of the strength and the extent of a magnetic field: weber (Wb) L 2 M T −2 I −1: scalar Mass fraction: x: Mass of a substance as a fraction of the total mass kg/kg 1: intensive (Mass) Density (or volume density) ρ: Mass ...
Since all gases have the same volume per mole at a given temperature and pressure far from their points of liquefaction and solidification (see Perfect gas), and air is about 1 / 5 oxygen (molecular mass 32) and 4 / 5 nitrogen (molecular mass 28), the density of any near-perfect gas relative to air can be obtained to a good ...
The definition of velocity above satisfies this requirement since it implies that v 1 /v 2 = (d 1 /d 2)/(t 1 /t 2); thus if the ratios of distances and times are determined, then so is the ratio of velocities. A definition of a unit of a physical quantity is a statement that determines the ratio of any instance of the quantity to the unit.
A physical quantity can be expressed as a value, which is the algebraic multiplication of a numerical value and a unit of measurement. For example, the physical quantity mass, symbol m, can be quantified as m=n kg, where n is the numerical value and kg is the unit symbol (for kilogram). Quantities that are vectors have, besides numerical value ...
The value of a physical quantity Z is expressed as the product of a numerical value {Z} (a pure number) and a unit [Z]: = {} [] For example, let be "2 metres"; then, {} = is the numerical value and [] = is the unit. Conversely, the numerical value expressed in an arbitrary unit can be obtained as:
SI derived units are units of measurement derived from the seven SI base units specified by the International System of Units (SI). They can be expressed as a product (or ratio) of one or more of the base units, possibly scaled by an appropriate power of exponentiation (see: Buckingham π theorem).
The base units and the derived units formed as the product of powers of the base units with a numerical factor of one form a coherent system of units. Every physical quantity has exactly one coherent SI unit. For example, 1 m/s = 1 m / (1 s) is the coherent derived unit for velocity.