What is Si: Definition and 150 Discussions

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.

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  1. Argonaut

    Mathematical Methods Book That Uses SI Units

    I'm looking for an undergraduate-level 'mathematical methods' or 'engineering mathematics' book that uses SI units for the purpose of self-study. I've had my eyes on Zill's Advanced Engineering Mathematics, but it seems to use US customary units. So ideally I'm looking for a book that covers...
  2. A

    A Why is there no saturation current for Si photoanode during OER?

    The Si/Ni photoanode was prepared by sputtering Ni on n-p homojunction Si substrate, and the electrolyte was KOH solution.
  3. Safinaz

    I Converting density unit ##MeV^4## to SI units

    How to transform density unit in natural units $MeV^4$ to SI units $kg/m^3$, Here's my trial: ##MeV^4 = (10^6)^4 ~ eV^4 = 10^{24} ~ eV^4 ##, ## eV = 1.6 * 10^{-19}~ kg~ m^2 / sec^2, ## ##MeV^4 = 10^{24} ~ 1.6^4 * 10^{-40} ~ kg^4 m^8 / sec^8 ## This is not simply ##kg/m^3##! Any help how to...
  4. lelouch_v1

    Donor and Acceptor Concentrations in a Si speciment

    At the left side we have the n-side of the junction, whereas at the right we have the p-side. I am a little confused over $N_D$ and $N_A$ at the n-side. Do the Ga atoms interact with the As ones, so we have $N_D = 2*10^{16} \text{cm}^{-3}$, or not, ans thus we have $N_A=2*10^{16} \text{cm}^{-3}$...
  5. V

    I Trouble understanding the SI definition of 1 second

    Homework Statement:: The SI definition of unit of time says the following. "The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." Relevant Equations:: None I know an...
  6. M

    Dot product: ##\vec{D} \cdot\vec{E}## in SI units

    I'm trying to calculate the electrostatic energy, and I'm wondering what happens when I dot the D-field and E-field, with Si-units V/m**2. This is my equation: D dot E = (-4x(epsilon) V/m**2)(-4x V/m**2) + (-12y(epsilon) V/m**2)(-12y V/m**2) Are the final Si-unit still V/m**2 or V**2/m**4?
  7. L

    Engineering Otto and Diesel cycles: Which PV diagram is for SI and CI?

    Writing about Engines and just muddled up diagrams and confused now. Which PV diagrams is for Compression Ignition and which one for Compression Ignition. I think one on left is CI and right SI. Cheers
  8. lakuzamalakuzam

    Statics example from Meriam ( SI improper )

    could not understand why it is partial constraints. Also, I don't understand how link works. Below here is what I am thinking i think it is totally wrong can someone explain what's wrong with my idea here.
  9. bagasme

    Why Isn't Horsepower an SI Unit, Despite Its Popularity?

    When we talk about engines on automotive industry (cars, motorbikes, trucks, etc.), we often refer the power produced by such engines by horsepower. But when we talk about electricity, the power is measured in kilowatts. Despite the former case is highly popular, why isn't horsepower considered...
  10. Z

    Si IGBT and SiC MOSFET comparison

    I am trying to understand the difference between an Si IGBT and SiC MOSFET. The device structure in question is the following: https://www.researchgate.net/figure/Comparison-between-Si-IGBT-and-SiC-MOSFET-modules-a-Cross-section-of-Trench-FS-Si-IGBT_fig6_318908365 It's hard to understand why...
  11. Z

    Building a CPU with Si JFETs: Fundamental Challenges?

    Would there be any fundamental reasoning (i.e. excluding cost, energy, and resources) that we could not build a modern day CPU using Si JFETs rather than Si CMOS? I realize the JFET device size would be huge, and the absences of enhancement mode JFETs may impact power consumption, but would...
  12. Z

    A Effective mass of the electron for Si

    https://ecee.colorado.edu/~bart/book/effmass.htm#short Looks like the effective mass for holes in Si can either be 0.57 or 0.81, according to the link above. Is there a temperature regime where one effective mass should be used instead of the other? Is anyone able to explain in layman's terms...
  13. P

    A When and Why Was the Definition of the Meter Changed?

    The wording of the definition of the meter has apparently changed recently. I'm wondering about the motivation for the change. The current definition is: The older definition (1983, I think) was Does anyone know when and why the change was made?
  14. R

    Find suitable SI units to work with a Vibration Sensor

    We are working on an IOT project in which we are trying to learn about different vibrations of using Wireless Vibration Sensor According to the manual, the wireless vibration sensor is giving the output while testing on CNC machine device 0.75, 0.71,0.69 whose SI units are in 'g'(which is...
  15. jybe

    How Do You Convert and Round Measurements to SI Units with Significant Figures?

    Homework Statement If I am asked to give my answer in acceptable SI units and to 3 significant figures, how would I express my answer? Homework Equations Answer: 589883.4263 J The Attempt at a Solution My instinct would be to put this in KJ, but I don't know if that's an "acceptable SI...
  16. L

    SI Units: Is the combination of SI units still an SI unit?

    Homework Statement Is N/m^2 a si unit I know Newton is and meters but togeather are there or is m^2 Homework Equations The Attempt at a Solution N/m^2 m^2
  17. enter

    Why does America still use imperial units in science?

    America is mostly leading in science, especially in astronomy, yet they still use imperial units. (I am not talking about the great people doing the science.) Everyone else is using SI and the metric system however they insist on using miles, yards, pounds, feet, etc., and continue on being a...
  18. P

    I SI definition of second: what day/year is the reference year

    The SI definition of a second was originally based on 1/86 400 of a day - but the Earth's rotation is a) unsteady and b) changing so it was changed to a defined number of cycles of an atomic clock. Was there a specific day for which the atomic clock was calibrated? Put another way, why were 9...
  19. D

    What SI Gear Standards Do You Use?

    I am an American author, working on a revision for my Theory of Machines textbook. In the past, I used only US Customary units, but now see a need to introduce SI units. This is particularly a problem with regard to gear standards. An internet search shows that there are dozens of SI-based gear...
  20. A

    Why there is no unit for memory recognised by SI

    As we all know that there is 7 fundamental units and rest can be delivered for physical Quantities.why there is no any define unit for memory element.Now these days all around us we can see electronic systems which have memory element and memory element store data to measures data we have...
  21. D

    SI vs USC Units in Engineering

    It is well known that most of the world use the SI system of units while the USA and a couple of other countries continue to use US Customary units (ft-lb-s = FPS or in-lb-s = IPS). The question of which is better is very much a matter of personal preference, but I'd like to avoid that issue...
  22. M

    Dimension of the electric charge in CGS and in SI?

    The Bohr magneton is (see e.g. Wikipedia) in SI units: $$\mu_B=\frac{e\hbar}{2m_e}$$ and in CGS units: $$\mu_B=\frac{e\hbar}{2m_ec}$$ So the dimension of the electric charge in SI, ##[q_{SI}]##, is related to the dimension of the electric charge in CGS, ##[q_{CGS}]##, by...
  23. F

    I Why am I getting two different results in emu and SI unit?

    I am computing force between two magnetic poles each of one unit pole (in emu) and situated one centimeter apart. In electromagnetic units: ##F_{dyne}=\dfrac{p^2}{r_{cm}^2}=\dfrac{1^2}{1^2}=1 dyne## where ##p## is pole strength in emu In SI units: ##F_{N}=k_A \dfrac{P^2}{r_m^2}=10^{-7}...
  24. DrClaude

    Dimensionless Units: SI Reform for Clarity & Precision

    The topic has been discussed before at PF, in particular @haruspex's Insight Can angles be assigned a dimension? Nature has now published an editorial on the subject: SI units need reform to avoid confusion
  25. Nim

    I Why Is There No SI Unit For Information?

    Why is the quantity "information" missing from the SI system? Also, if they did add it, do you think it would be added as a base unit or dimensionless derived unit?
  26. I

    Need help with gravitational acceleration SI units

    Please ignore strikeout (not sure why post is doing that) I just watched a video from Caltech (Video link at bottom of post) It says: Displacement = Distance Fallen in first second of time in [m] * Time2 so Time2 has no si unit as it is just a ratio right? Instant or Average Velocity = 2 *...
  27. AdrianMachin

    Multiplying epsilon naught by a length quantity

    Homework Statement Note that this formula (##C=4 \pi \epsilon_0 R##) and the others we have derived for capacitance involve the constant multiplied by a quantity that has the dimensions of a length. Homework Equations ##\epsilon_0## has the following units in SI: $$\frac {C^2} {N \cdot m^2}$$...
  28. T

    I Is there dipoles in mono crystaline Si?

    How to model the electromagnetic behavior of mono-Si in room temperature? Shall we consider electron/hole only? Or shall we also take dipoles into account? If so, how are these dipoles generate? THX! Z.J.
  29. K

    A Xray NanoDiffraction of Si and SiGe

    Hello I am doing research on kinematic and dynamic scattering of xrays on a crystals. I am attempting to simulate the diffraction patterns of a silicon substrate and I have already simulated two other layers of a Silicon Quantum Well and SiGe from which the hetero structure was composed of. In...
  30. Const@ntine

    B Solving SI Units with ln(n): A High School Puzzler

    I tried with Google but I couldn't find anything, so here goes: When I "use ln on a quantity" (I don't really know how to phrase it in english, as we just have a verb for it), say, I have n = 0.00149 kg/m*s, and I put it into the ln, so now I have ln(0.00149 kg/m*s) what happens to the SI Units...
  31. K

    I Xray NanoDiffraction of Si and SiGe

    Hello I have been struggling with my research on xray diffraction of crystals and had a few questions about crystallography, rocking curves(omega scan), θ-2θ scans and diffraction patterns. Let me know if you need more explanation. Thank you! -What is the difference between a rocking...
  32. nomadreid

    I Cgs or SI in quantum field theory?

    I have an acquaintance who maintains that in quantum field theory, primarily the cgs system is used. OK, I know it's not really important, but I was under the impression that everyone had switched to SI. (My book on quantum field theory has very few actual quantities with units outside of GeV...
  33. adamaero

    Ground state energy for exciton in Si

    Homework Statement Estimate the ground state energy (eV) for an exciton in Si. εSi = 12 ε = 1.0359×10−10 Effective masses me* = 0.26me mh* = 0.36me effective mass = 0.15me Values of h 6.626×10−34 J⋅s 4.136×10−15 eV⋅s Values of ħ Units 1.055×10−34 J⋅s 6.582×10−16 eV⋅s Homework Equations E1 =...
  34. D

    Can Teflon Tape Withstand the Attack of Etching Solutions?

    Im trying to etch 525um (100) p-type silicon, and i intend to use HF+HNO3+CH3COOH for high etching rate since i need to etch 525um of Si. does anyone know what ratio should i use to obtain high etching rate? I am trying to look for a mask that is able to withstand the attack of the etching...
  35. RabbitWho

    Expressing measurements in SI units

    Homework Statement I am given some measurements and I am supposed to express them as standard SI units. I can't find any examples of this done online or extra practice exercises to help me. I am probably googling the wrong words. I have been out of school for quite a while and I don't know what...
  36. S

    Electroplating Cu on Cu sputtered Si Substrate (Lithography)

    I have been trying to plate Copper on Copper sputtered Electronic grade Silicon Substrate which has been patterned and developed using UV photolithography. The patterns measure about 50-100microns in width and are about 1600microns long. I have tried plating 10micrometer thick Cu using a stock...
  37. S

    I How to Convert Cgs to SI Units for Flux and Energy Values?

    Hi I have a value that is 10.7 mJy, that I need to convert to SI units. I thought it would be 1.7*10^(-2)*10^(-26) but that might be wrong? Then I have a flux value, kappa, of 2 cm^2.g^(-1) that needs to be converted to meters and kilogram. I thought it was 2*10^(-7) m2/kg, but that might be...
  38. A

    Calculating Equations in SI Units: Boltzmann Constant & More

    Excuse me, If I want to calculate one equation in SI units. Which value of Boltzmann constant I should deal with. Also, the electron temperature it can be used in Kev or it should be in ev. Also, the nm must be converted to meter or it can be used in nanometer.
  39. 1729

    Simplifying SI Units: What Quantity Does JNW⁻¹kg⁻¹ Denote?

    Homework Statement What physical quantity does ##\mathrm{J\cdot N\cdot W^{-1}\cdot kg^{-1}}## denote? Homework Equations You can simplify using mechanics equations or ones from electrodynamics. Be creative, I guess. The Attempt at a Solution ##\mathrm{J\cdot N\cdot W^{-1}\cdot kg^{-1}}##...
  40. A

    Why Is the Conversion Between CGS and SI Units in Electromagnetism Confusing?

    Dear friends, As you know, in CGS units we have vacuum permeability as unity (1). so H=B and we use Oersted for H. on the other hand, in SI, permeability is (4 x pi x 0.0000001) and we use Ampere per meter for H. unfortunately after trying for couple hours, I couldn't derive the algebra of...

    Why c is divided while converting cgs into SI system?

    In conversion of formula from cgs into SI, usually c, the velocity of light is divided. Why it is so?
  42. F

    Ellipsometry of thicker SiO2 film over bulk Si substrate

    Dear All, I hope you are good. But I am stuck in a measurement. I use JWoollam Ellipsometer and WVASE 32 software to perform ellipsometric measurements of a wafer which contains Silicon as substrate and SiO2 8 um as film over it. I performed a number of measurements on different points of the...
  43. at94official

    Converting 1,602 ×10^−19 C to SI Unit Prefixes

    Just simple questions and I need clarifications why is that your answer please. 1,602 ×10^−19 C write to SI Unit Prefixes
  44. CKaiL

    Photoelectric Equation - SI unit question

    Homework Statement I am trying to plot a graph of stopping voltage against frequency The given equation is eV0 = hf - w0 So i need to change it to the form of y = mx + C in order to plot this graph V0 = h*f/e - w0/e V0 = h/e * f - w0/e -- > V0 = y , m(gradient) = h/e , x= f and -w0/e = C (y...
  45. Marvin

    Linear loss coefficient in Si waveguide

    What is the value of linear loss coefficient in Si waveguide for light of 1550 nm?
  46. E

    Calculate Si cohesive energy using VASP

    Hello, actually I am a newbie in solid state physics computational and I am starting using VASP. I am practicing to do calculation with bulk Si. I want to calculate the cohesive energy of Si. As I know that cohesive energy equals to the difference between bulk energy/atom and single atom...
  47. R

    What Do Lbf, Lb, and Lbm Really Mean in American Units?

    Ok i am too confused with these american units. I have a couple of questions about them Q1) Is lbf a force. Is lbm a mass. If yh does it mean 1 lbm = 32.174 lbf. Q2) Is lb equal to lbm or lbf. Q3) Is lbs equal to lb or lbm or lbf. Q4) Let's say 1 lbm = 32.174 lbf. and from many sources i...
  48. N

    Fermi Energy in Intrinsic Si, Ge, and GaAs

    Homework Statement Using the values of the density of states effective masses me* and mh* in table 5.1, find the position of the Fermi energy in intrinsic Si, Ge, and GaAs with respect to the middle of the bandgap (Eg/2). Table 5.1 shows the following density of states effective masses me*/me...
  49. M

    Difficulty working with SI and derived SI conversions

    The physics textbook I'm working with doesn't seem to give me the required background for working with and understanding SI and derived units. Here is an example of something i am struggling with: In studying fluid resistance, the book starts by displaying 2 equations. 1 is for fluid...
  50. M

    Resources or learning how SI units work?

    Hi, I'm looking for an in-depth guide/tutorial about how to use and understand SI notation. As it stands, I've realized it's a source of confusion in physics for me because i never took the time to properly learn. If anyone knows of any resources which would help me to understand it would be...