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In physics and metrology, units are standards for measurement of physical quantities that need clear definitions to be useful. Reproducibility of experimental results is central to the scientific method. To facilitate this we need standards, and to get convenient measures of the standards we need a system of units. Scientific systems of units are a formalization of the concept of weights and measures, initially developed for commercial purposes.

Different systems of units are based on different choices of a set of fundamental units. The most widely used system of units is the international system, or SI system, of units derived from the seven SI base units. All other SI units can be derived from these base units.

Other systems of units that have been used for various purposes include:

the centimeter-gram-second system of units
the metre-tonne-second system of units
the Planck units also known as natural units
U.S. customary unitscustomary units (more commonly known in the US as English units or standard units are the non-metric units of measurement that are presently used in the United States, in some cases alongside the metric system of units. This system of units is similar to
Imperial units
Chinese unit
Geometrized units

1 Units as dimensions

Any value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Q is written as the product of a unit [Q] and a numerical factor:

The multiplication sign is usually left out, just as it is left out between variables in scientific notation of formulas. In formulas the unit [Q] can be treated as if it was a kind of physical dimensionAbstract algebra Algebra Linear algebra Dimension (from Latin "measured out") is, in essence, the number of degrees of freedom available for movement in a space. In common usage, the dimensions of an object are the measurements that define its shape and s: see dimensional analysisDimensional analysis is a mathematical tool often applied in physics, chemistry, and engineering to simplify a problem by reducing the number of variables to the smallest number of "essential" parameters. Systems which share these parameters are called si for more on this treatment.

A distinction should be made between units and standards. A unit is fixed by its definition, and is independent of physical conditions such as temperature. By contrast, a standard is a physical realization of a unit, and realizes that unit only under certain physical conditions. For example, the metre is a unit, while a metal bar is a standard. One metre is the same length regardless of temperature, but a metal bar will be one metre long only at a certain temperature.

2 Basic and derived units

For most quantities a unit is absolutely necessary to communicate values of that physical quantity. Try for example to tell someone the value of a length without the use of a unit. That is not possible because you can't verbally describe a length.

But not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as combinations of units of other quantities. Thus only a small set of units is required. These units are taken as the basic units. Other units are derived units. Derived units are a matter of convenience, as they can be expressed in terms of basic units. Which units are considered basic is a matter of choice.

The basic units of SI are actually not the smallest set. Smaller sets have been defined. There are sets in which the electricIn physics, an electric field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity. Definition and derivation The mathematical definition of the electric field is developed as follows. Coulomb's law gives the and magnetic field have the same unit. This is based on physical laws that show that electric and magnetic field are actually different manifestations of the same phenomenon. In some fields of science such systems of units are highly favored over the SI system.

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