What do we mean by a ‘dimension’? The distance between two points, say Exeter and London, can be measured in many units : metres, miles, yards, chains, cubits, barleycorns, parsecs . . . however the quantity distance is a measure of length, no matter what units we use to measure it in. We say that this quantity has dimensions of length. Square brackets [] are often used to denote dimensions, so we can say that
In the metric system, we have 7 basic dimensional quantities :
The dimensions of all other quantities can be derived from this basic set. In fluid dynamics, only the first three are really important, although the temperature might appear in problems involving heat transfer, or the current for magnetohydrodynamical problems.