The International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the metric system. It is the only system of measurement with an official status in nearly every country in the world. It comprises a coherent system of units of measurement starting with seven base units, which are the second (the unit of time with the symbol s), metre (length, m), kilogram (mass, kg), ampere (electric current, A), kelvin (thermodynamic temperature, K), mole (amount of substance, mol), and candela (luminous intensity, cd). The system allows for an unlimited number of additional units, called derived units, which can always be represented as products of powers of the base units. Twenty-two derived units have been provided with special names and symbols. The seven base units and the 22 derived units with special names and symbols may be used in combination to express other derived units, which are adopted to facilitate measurement of diverse quantities. The SI also provides twenty prefixes to the unit names and unit symbols that may be used when specifying power-of-ten (i.e. decimal) multiples and sub-multiples of SI units. The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves.
Since 2019, the magnitudes of all SI units have been defined by declaring exact numerical values for seven defining constants when expressed in terms of their SI units. These defining constants are the speed of light in vacuum, c, the hyperfine transition frequency of caesium ΔνCs, the Planck constant h, the elementary charge e, the Boltzmann constant k, the Avogadro constant NA, and the luminous efficacy Kcd. The nature of the defining constants ranges from fundamental constants of nature such as c to the purely technical constant Kcd. Prior to 2019, h, e, k, and NA were not defined a priori but were rather very precisely measured quantities. In 2019, their values were fixed by definition to their best estimates at the time, ensuring continuity with previous definitions of the base units. One consequence of the redefinition of the SI is that the distinction between the base units and derived units is in principle not needed, since any unit can be constructed directly from the seven defining constants.The current way of defining the SI is a result of a decades-long move towards increasingly abstract and idealised formulation in which the realisations of the units are separated conceptually from the definitions. A consequence is that as science and technologies develop, new and superior realisations may be introduced without the need to redefine the unit. One problem with artefacts is that they can be lost, damaged, or changed; another is that they introduce uncertainties that cannot be reduced by advancements in science and technology. The last artefact used by the SI was the International Prototype of the Kilogram, a cylinder of platinum-iridium.
The original motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units) and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements. The system was published in 1960 as a result of an initiative that began in 1948, so it is based on the metre–kilogram–second system of units (MKS) rather than any variant of the CGS.
Hello,
I have the problem, from Sze' book, tunneling current, what is m*?
I want to konw the value of it.
The m* has many reference, how do I select?
My interested device is diode that was doped with Boron and Phosphorus for p-n junction.
Thank very much.
Hi,
Looking for the SI Notation practices for Direct Current and Voltage Direct Current. We commonly use DC and VDC, but I wonder if that's correct per SI.
I've looked through several guides and pdf's of SI Notation, and haven't yet been able to find the answer. Any help you can...
Homework Statement
Got a charge of 2CGSEm need to find it in SI
Homework Equations
1C=2.998*10^{9}CGSE
The Attempt at a Solution
Just a plain question, I get q\approx6.66*10^{-10}
Is that correct?
Homework Statement
Convert the following as full (decimal) numbers with standard (base) SI units:
86.6 m
The Attempt at a Solution
I thought I could just change it to 8.66x10^1 then just add the 10^-3 because of the milli to make it 8.66x10^-2. But I'm wrong so could someone help me?
In doing some research, I cam across an equation for instantaneous radiated power, with the CGS units "erg/(sec rad cm)", rad being radians (not rad the unit for radiation exposure). Try as I might, I'm not able to come up with a way to convert it to the SI units for watts. Does anyone know...
Hi, I'm trying to calculate the minimum mass of a star, where
M=PV³/2πG
The values I've been given are V=400km s^-1 and P=34.4 hours.
Does this mean that I have to convert these values to SI units, i.e. give the values in terms of metres and seconds rather than km and hours? This would...
Hope you can give me a hand here guys.
I'm finding the whole carrier concentration thing a bit confusing!
Si doped with say 10^17 P (/cm3). At 300K, calculate n, p, E_F (Fermi energy).
Now I know the relations such as n=(N_c)exp[(E_c-E_F)/kT], N_c given
I just can't figure out...
Maybe some of you are familiar with Lindsey Vonn, a prodigy alpine skier, often considered the best skier US has. She was features in Sports Illustrated magazine and she made the cover.
The shoot for the cover can be seen here...
Hi all,
I would like to know if any of you know about anything the equivalence of SI units for differential equations? For example, for the equation
E=mc2 SI units for RHS must equal LHS. I am wondering if this would apply to differential equations?
I recently came across a journal paper with...
Homework Statement
I am trying to get the units to match up on both sides. \frac{\partial T}{\partial t} = \frac{\nabla ^{2} T}{k} = \frac{1}{K}(\frac{\partial ^2 T}{\partial x^2} + \frac{\partial ^2 T}{\partial y^2} + \frac{\partial ^2 T}{\partial z^2}}), where k is the thermal resistivity...
Si -> Silicidation, Ge -> Germanidation, III-V (i.e. GaAs) -> ?
hi,
i'm new in this forums and not sure whether to post this under electrical engineering or chemical engineering, since it's a material based question regarding an electron device.
Here goes:
For Si MOS devices utilising...
Does anyone knows what this unit "a.u" means? I found it on many graphs, where "a.u" represents the unit of |E|^2 (E represents electromagnetic field). How do I convert "a.u" to SI unit?
1 Mol = SI Unit - What the...!?
This is ridiculous. How can 1 Mol be considered an SI-BASE-UNIT although it is simply a factor
1 Mol = 6.022 * 10²³
Every attempt to explain this thorough misconception is naturally predestined to fail! I'm not looking for an explanation, neither going to...
hi..i'm an undergraduate student.i'm working on my final year project. I've been looking for the permittivity and permeability of ErAs, SI or any metamaterial but i find it difficult to get.
i hope this forum could help me (n hope this forum is still active..hehe)
THANX! o:)
Why is the SI unit for acceleration \frac{m}{s^2}(meters per second squared) when it is actually \frac{m}{\frac{s}{s}} (meters per second per second). Isn't the part concerning the seconds different? Wouldn't this give you different answers sometimes, or does that usually never get in the way.
I'm doing a lab report on optical absorption of semiconductors. As such I would like to verify the values of the bandgap Eg which I have calculated through experiment. I tried searching Google but I got a lot of results to technical papers which didn't help at all. I would like to know if...
Al and Si?? flux??
to creat a film of alloy Al and Ti we use to flux of each element and a substrat.
if the flux of Al is double of that of Ti, then we obtain an alloy on the substrat AlTi or what?
If
The second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
And
The meter is defined as 1⁄299 792 458 of a light-second.
Then
Why can't we...
I've got a sample containing 8 substances: P, S, C and Si (+4 metals) which i need to analyse. I was planning on using aqua regia to dissolve the metals, remove the 4 substances that aren't metals (and deal with them later ^^) and then use atomic absorbtion spectroscopy to measure the metals'...
Finally after man(and woman)fully resisting the urge to go fully metric for 30years the UK's boffins have produced an alternative system of units, even more brilliant then the venerable Firkin/Fortnight/Furlong system.
The velocity of a sheep in a vacuum is 26.22 kilobrontosauruses per...
Hi guys, I need some help converting between units. I hope you can help. :)
The candela is defined as "the luminous intensity of a light source producing single-frequency light at a frequency of 540 terahertz (THz) with a power of 1/683 watt per steradian".
Now, one watt is equal to...
hey can anyone give a calculator(like any html link) or something of that sort which can convert units, like from british system to SI units and vice versa. looking at the tables everytime is sooooo boring and i am fed up of it.for eg. changing lengths, pressure, weight etc. etc.:-p:-p
I am trying to understand 3D Si dispersion relations and reciprocal lattice vectors. My confusion is that when I look at dispersion relations the wave vector typically is normalized from 0 to 1 by a/2pi. I thought the edge of the first BZ was pi/a. Is this correct or is it 2pi/a for a diamond...
I was bored so I make fun with derivations of SI units of many Physics formulas.
Note, this should just be called "introductory physics", that means NO calculus (but rate of change designated by the delta symbol) and motion is in one or two directions only.
This is high school physics level in...
Homework Statement
Ok, this isn't hard, I'm just thrown off by the question. From the book: "The quantity called mass density is mass per unit volume of a substance. Express the following in SI units:"
a. Aluminum, 2.7*10^-3 kg/cm^3
b. Alcohol, .81g/cm^3
Homework Equations...
I have a simple question.
I understand the quantitative differences between cgs and SI units, but when will I be expected to use one over the other? For example, do physicists prefer one set of units?
Thanks for the help.
Time-oriented geometrized units→Second and its derivates in terms of 7 SI base units
I expressed second in terms of meter and kilogram. For doing this I used:
For expressing second in meters:
1s¹ = c = 299792458 m
For expressing second in kilograms:
1s¹ = c³/G =...
Of all the evil things that I could be stuck on, its not even bad arithmetic this time around. No, I have been stumped by units.
Does anybody know what the si symbol 'sc' stands for? Is that even a symbol?
Hi Everyone,
I think I posted this message to the wrong forum, so second try... I hope this is the right one? Could someone please explain to me how I know when to use SI units to solve a problem and how to enter these units into an equation - are there any rules of thumb? If I could get a...
I am doing my homework and i come across a question.
What is the SI unit for refractive index?All my revision books did not have the answer.Or perhaps the book is wrong by any chance?
Hi,
I understand that the conduction band in si is 6 fold degenerate but why do they form 6 ellipsoids of constant energy? i.e 2 for each axes. Are they just simply fermi surfaces?? How are they constructed? Thanks
What happens to large (greater than 100 micrometer) fuel droplets (however few) in the SI engine cylinder?
Do they move up along with the piston or do they get deposited on the piston itself?
Hi,
We know that the SI unit for mass is the kg, and that for force is the Newton, where 1 N = 1 Kg x the acceleration due to gravity.
On the other hand, the English unit for mass is the slug, and that for force is the pound, and again we have a similar relationship based on Newton's...
in SI engines fuel/air mixture is compressed. The disadvantage is that one can't achieve high compression ratios. I was wondering if it was possible to compress fuel/air mixture to the maximum possible ratio without fear of ignition and compress a pure air mixture to very high ratios 25:1 and...
Question:
If Si1 represents the wavefunction of a Px orbital in the hydrogen atom, and Si2 represents the wavefunction of a Py orbital in the same hydrogen atom. Si1 and Si2 are both normalized wavefunctions.
a) what is the value of integral:Si1*Si2*dt. ?
b) what is the value of...
I know that the typical values of Doppler coefficient over core life are about -1 *10^-5 Deltak/k/°F at BOL and -1.5 * 10^-5 Deltak/k/°F at EOL.
What are the value in SI units?
Are they the same, but in Kelvin?
Thanks,
Ken
This is the SI unit for luminous intensity. The definition relates to blackbody radiation emitted at a certain temperature for a certain material (so I guess it ISN'T blackbody!), platinum, I think.
Except... I don't understand the necessity for the introduction of this unit.
Isn't...
Please, help me to calculate the band structure of Si using the pseudopotential method. I will appreciate if you send me a simple program of calculation in any programming language very much and will be very grateful for any link or reference. The problem is than I've read the pile of books and...
Apparently it has been decided that the new unit for beauty will be the milliHelen.
One milliHelen is said to be the beauty required to launch one ship.
Tee Hee