We need "units" for any measurements. For example, in order to define the length, there are quite many units: meter, feet, mile, pc (Parsec), AU (Astronomical unit), Å (angstrom), Yukawa, etc, etc, etc ···
It is not useful as there are too much units, so we "earthmen" naturally want to unify and standardize all these units ON THE EARTH ··· The answer is "SI unit".
Unfortunately, USA and UK people seem to disagree ··· They still use the "mile", "feet" and "inch", even though the Japanese were stopped to use our own units, historically compelled by the USA.
The SI is unfortunately not put to full use in the USA, where selling "Quater pounder" is no concern of any regulating body of the government.
Understandably, the SI might be too French to go with french frys.
In my opinion, Anglo-Saxons do not want to be "earthmen", rather than Anglo-Saxons :)
SI unit has a set of basic units and derived units. Derived units can be constructed ONLY by the multiplying and division, which is defined by the recognized physical law and definition. Only these units can be said "coherent".
I show here some of SI units those are used for electronics, and I used in my other page.
In old days, [kg·f] (or [kg·w]. kilogram-force / kilogram-weight) had been used for the force, it may be easily understood for us, but this is NOT good for physics.
Why it should be [N]? Because of "coherence".
Of cause these are the same dimension, because they show the force, the same physics. Force is the multiplying of the acceleration and the mass, so we can construct it:
F=m·a [kg·m/s2] (Newton's law of motion).
However, we should notice that 1[N]=1[m/s2]×1[kg], which means no coefficients need except for the "1", if we use the SI unit [N].
This is just the essence of coherent unit. You may easily imagine [kg·f], but it is useless for physics, because it is not coherent, not to mention about the English units :-)
In other words, in order to understand the deep meaning of the science, you must use the coherent units, SI.
Otherwise, you cannot, NEVER!
When current flows, ordinal circuit do some "job", it brings heat or something, but if no flow with some voltages, it does not work (no energy has been lost).
Therefore, the [V] is not basic unit, but it is a derived unit.
Electric current unit [A] is defined as "When current flows parallel cables at interval of 1[m], 1[A] is defined when the force of 2·10-7[N] has been occurred for each of these cables."
The voltage is defined that the energy divides electric charge. Electric charge is defined by time by current. In other words, the current means how much charge was flowed whilst one second.
Electric charge can be charged in the capacitor, and you might have learned the equation:
Q = C·V
And you also know what the mechanical work. Mechanical work is defined by the motion of a body against a constant force, which can be explained moreeasier, how log distance you had moved the Mass. In other words, it is energy.
You need the force to bring up the Mass, so the force [N] multiplied by distance [m] is the energy. In the SI unit, the unit of this energy (or mechanical work) is [J] (joule), and here its dimension is as follows:
J = N·m = m2·kg/s2.
||by other symbols
|magnetic flux density||Tesla||T||Wb/m2||kg·s-2·A-1|
Sometimes we can find his name in the Pseudo science article, but he was a Real Scientist. He believed "Aether", however, please notice that he invented the principle of an induction motor in 1882, it was before the "Relativity theory (A. Einstein, 1905)".
Probably, the reason is this unit is quite useful.
Anyway, basically the [dB] is defined by the ratio of power level, but not a voltage or such a thing. But we can get the ration of voltage in [dB] by the calculation.
In the first place, decibel means deci (1/10 of metric system) Bell (the name of telephone inventor), so of cause it must be based on the power, because if it is based on voltage, it should be 1/20 Bell ;)
For the high-frequency usage, especially radio communication technology, we sometimes use the derived unit using [dB], like as [dBm], [dBd], [dBi], [dBm/Hz] and so on.
These units are not SI, but I do not know how it will be described in future.
I also sometimes use the non-SI unit in my article, like as [μF] for the capacitance, but if I calculate with this unit, I always convert it to the 10-6 in stead of μF.
This is not so difficlt to change, but if you forget it, the coherence of unit is disapperars.
|μ0||permeability in vacuum
|4π × 10-7 [H/m]|
|ε0||permittivity (dielecric constant)
|1/(4π·c2) × 107 [F/m]|
The speed of light in vacuum
||2.99792458 × 108 [m/s]|
||Gravitational constant||6.6725985 ×10-11 [N·m2/kg2]|
|h||Planck's constant||6.62617 × 10-34 [J·s]|
|q (or 'e')||The charge of an electron||1.6021 × 10-19 [C]|
|k||Boltzmann's constant||1.38066 × 10-23 [J/K]|
I already wrote about the E = h·ν , the dimension of frequency ν should be [1/s], and Energy is [J], so the Planck's constant should be [J·s].
In Japan, there is a very famous Pseudo Scientist, his name is "Seike", and his equations are always very complex and difficult to understand.
However, we sometimes found that his equation had different dimension for its left and right :-)
Abstract in English
Copy right Katsu
Last update 30th/May/2004