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SI unit

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.

SI (International System of unit)

In 1960, the SI unit was defined at international conference in Paris, which is based on the MKSA unit. Japan also obeys to this decision.
MKSA unit is the very old basic unit, which had been used for the physics internationally.

SI is constructed from this MKSA; MKSA has four basic units, [m] (meter) for length, weight [kg] kilogram, the time is [s] (second), and [A] (ampere) for the electric current flow.
SI unit based on these four units, and [K] (Kelvin) for the temperature, [mol] for the amount of Substance and [cd] (candela) for the luminous intensity are employed as "basic unit".

Additionally, there are two semi-base units, [rad] (radian) for the angle and [sr] (steradian) for "solid angle" (three dimensional equivalent angle) are recognised to use for supplementary units.

Other all units in SI system are "derived Units" constructed from above basic units.
For example, the unit of the force is [N] (Newton), this unit can be derived from basic unit, [N] = [kg·m/s2].

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!

The [A] (Ampere) is defined for the electric unit. Why not [V] (Volt)?

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.

Please find below table, which shows electric/electronic units and their symbols.

SI derrived units
Measure of
unit symbol
by other symbols
electric voltage
Volt V J/C m2·kg·s-3·A-1
electric charge
Coulomb C A·s s·A
(electric) power
Watt W J/s m2·kg·s-3
magnetic flux
Weber Wb V·s m2·kg·s-2·A-1
magnetic flux density
Tesla T Wb/m2 kg·s-2·A-1
Ohm Ω
V/A m2·kg·s-3·A-2
Hertz Hz -- s-1
Henry H Wb/A m2·kg·s-2·A-2
Farad F C/V m-2·kg-1·s4·A2
Siemens S A/V m-2·kg-1·s3·A2

From these derived units, we can construct other derived units. For example, permeability (the degree of magnetisation of a material in response to a Magnetic field) is μ [H/m] (Henry/meter).

As you can easily understand, these units stand on the name of scientists. In Japan, Tesla is not known well, but Nikola Tesla is the "Father of alternative current".

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)".

In truth, [dB] (decibel) is not SI, but it seems to be recognised to use together with other SI.
Why this unit is recognised to use? This unit has not a dimension, and to begin with, this unit is defined by the logarithmic ratio (of cause we cannot calculate by the simple multiply or division), so I think this is not an in terms of "physical unit".

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.

As far as I heard, [bit] will be changed to the [Shannon] ( but I am not sure, sorry!), it is name of the great computer scientist, who was dead several years ago.
(Information theory had been born by his hand. Additionally, he Claude Shannon, Alan M. Turing and John von Neumann are fathers of the computer. Furthermore, a sampling theorem is very important for the current digital audio technology, correctly this theory is "Nyquist-Shannon's sampling theorem".)


In passing, here I write about the prefix. Prefixes are used for N power of 10th.
Please find below table. Here, "deci" in this table, only [dB] is the case which uses the deci for electronics or acoustics.

We normally use the 3n power of 10th, so deci or centi are not used except for the ratio, like as [dB].

You may say, "centi is used for the length", but [cm] is a not SI unit.
Please do not confuse, SI unit does NOT mean Metric system.

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.

prefixes of 10n
true meaning
peta P 1015
tera T 1012
giga G 109
maga M 106
kilo k 103
deci d 10-1
milli m 10-3
micro μ 10-6
nano n 10-9
pico p 10-12
femto f 10-15
ato a 10-18

Here, I do not write about Exa, but you may not use if possible, because Exa is written "E", which is sometimes misunderstood to exp, as we often use "exp" in the world of electronics.

Physical constant

There are constant values, like as the speed of light in the vacuum space is the constant.
I sometimes use these values in my article, so here I wrote below table content.

Physical constant
μ0 permeability in vacuum
(mu zero)
4π × 10-7 [H/m]
ε0 permittivity (dielecric constant)
(epsilon zero)
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]

Reduced Planck constant (so called Dirac's constant) is sometimes used, which is the h/2π, but written by h with bar and called "h bar". But I cannot write here because no suitable font ;)

What is Planck's constant? The energy of a photon is written by
  E = h·ν     here, ν is the frequency of light.

Here, let us return to the story of the SI unit, I would like to talk about the dimension. Basically, physical equation has to be the same dimension for the left and right of the equation. This is very natural of cause, because if they are different, we cannot know the equation shows what value it is.

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

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Last update 30th/May/2004