Originally Posted by **werehk** Are there only "time", "length" and "mass" dimensions in physics?
Are there any examples for which the dimensions are difficult to determine in terms of the three dimensions?
What is the dimensions of magnetic flux in terms of the variables above? |

You ask a very interesting question, one related to my own research.

The standard view of Physics is that time, length, and mass are the basic units of Nature to which all other quantities have units that can be derived from them. (Well, at least for Introductory Mechanics.) Must we insist on using that system? I see no reason why we can't change it, but we would have to have three other "base" units to take their place. The difficulty in changing these is that the physical interpretation of the different quantities that Physics uses can change slightly. For example, the unit m/s is easily seen to physically be a velocity and we can get a visual picture of what this means. But if we take the units of Joules and momentum (call this unit "p") what the heck does a J/p mean? It is equivalent to m/s, but the interpretation is a lot more difficult to see.

Having said all this, there are other basic units that are used. When we start talking about Electromagnetic units the typical SI unit systems split into a vareity of different unit systems, all of which have some specific reason for existing (usually to make the equations neater.) The MKS system adds the Ampere (A) as a base unit and the system is called MKSA. Don't be confused though: most textbooks use the Coulomb (C) as a base unit, not the Ampere, which is C/s. But as far as I know there is no "MKSC" unit system officially in use.

The CGS system is where the real confusion starts. There are at least

four of them in use and unfortunately when you look at a textbook or professional paper, they rarely tell you what system they are using. These systems typically set certain constants of nature to have a value of 1 (and sometimes are defined then to be unitless.) For example, the Heaviside-Lorentz system of units sets $\displaystyle c = \hbar = 1$ to remove the "excess" constants from the equations.

For the record, magnetic flux in the MKSA system is measured in Webers (Wb). This can be expressed in base units as $\displaystyle kg / A m s^2$, or using Coulombs, kg/Cms.

-Dan