Defining equation (physics)

In physics, defining equations are equations that define new quantities in terms of base quantities. This article uses the current SI system of units, not natural or characteristic units. L = p r , {displaystyle L=pr,,!} J ⋅ A = I , {displaystyle mathbf {J} cdot mathbf {A} =I,,!} where dA means a differential area element (see also surface integral).where dA = ndA is the differential vector area. L = r × p {displaystyle mathbf {L} =mathbf {r} imes mathbf {p} ,!} J i n i d A = d I {displaystyle J_{i}n_{i}mathrm {d} A=mathrm {d} I,!} In physics, defining equations are equations that define new quantities in terms of base quantities. This article uses the current SI system of units, not natural or characteristic units. Physical quantities and units follow the same hierarchy; chosen base quantities have defined base units, from these any other quantities may be derived and have corresponding derived units. Defining quantities is analogous to mixing colours, and could be classified a similar way, although this is not standard. Primary colours are to base quantities; as secondary (or tertiary etc.) colours are to derived quantities. Mixing colours is analogous to combining quantities using mathematical operations. But colours could be for light or paint, and analogously the system of units could be one of many forms: such as SI (now most common), CGS, Gaussian, old imperial units, a specific form of natural units or even arbitrarily defined units characteristic to the physical system in consideration (characteristic units). The choice of a base system of quantities and units is arbitrary; but once chosen it must be adhered to throughout all analysis which follows for consistency. It makes no sense to mix up different systems of units. Choosing a system of units, one system out of the SI, CGS etc., is like choosing whether use paint or light colours.