What is Si units: Definition and 53 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. B

    I Which units is this conversion factor for (molar volume)? 0.023901488

    I'm getting the wrong results when using an old, undocumented code and just realized there's a number lurking in it that I can't account for. It's: 0.023901488 and it is multiplied with molar volume and pressure. I have searched for a couple of hours but just can't figure out what the units...
  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. 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...
  5. Mr X

    B Fundamental Units and Supplementary Units — What are the differences?

    Fundamental units are defined as - each of a set of unrelated units of measurement, which are arbitrarily defined and from which other units are derived. while Supplementary units are - Supplementary units are the dimensionless units that are used along with the base units to form derived units...
  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. akardos

    I Replacing the Measurement standards (SI units)

    What might be better foundational units given the knowledge we now have and disregarding legacy, human-scale units. Perhaps setting some known constants to be the base unit of 1 in that measure. For example, the second, based on the unperturbed ground-state hyperfine transition frequency of the...
  8. 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...
  9. 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...
  10. 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
  11. 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...
  12. 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...
  13. 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...
  14. F

    Please clear my electromagnetism doubt about units

    It is a well known fact that, in electromagnetic units, strength of a shell and strength of current flowing through its boundary are same. See here. \begin{equation} \begin{matrix} \text{i.e.}\: i \text{(biot)} = \phi \text{(biot) } \end{matrix} \tag{1} \end{equation} (a) While converting to...
  15. 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}...
  16. 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?
  17. 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 *...
  18. 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...
  19. 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...
  20. 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.
  21. 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}}##...
  22. 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...
  23. 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...
  24. adjacent

    Newton's Discovery of Mass, Force & G in SI Units

    What did Newton use to determine the mass of an object(During those days) Even the SI unit for mass was established much time later. The SI Unit for force was also established much later. The G in F=GMm/d2 was discovered in Much later. How did Newton knew that a force of 1N would accelerate a...
  25. G

    How to use ballistic coefficient (SI units)?

    Reading in Russian sources I see they use ballistic coefficients such as these: BC = 3.0 kg/m^2 or alternatively BC = 3.0 m^2/kg That's quite different than the usual G1, G7, etc. Given a certain muzzle velocity, how do I use that to calculate velocity at some point downrange?
  26. X

    Voltage Vector/Scalar Components, SI Units, Direction and Polarity.

    All, please help me refine, restate, explain, understand, expand, add, remove, answer the below statements/ questions. Thanks in advance. 1.) Voltage is considered a scalar quantity. (J/C in SI Derived Units) which is a magnitude of energy per coulomb...no direction with this example. Some...
  27. S

    Help: From Ampere to Coulomb in SI units

    Hello, Could someone help me finish this train of thought? This is how we think in SI units: First, just because we like the value of 1 Ampere as it is now, we choose the force between two parallel conductors to be exactly 2×10^{-7}N= const ×\frac{1A×1A}{1m} Then, purely as choice, we decide...
  28. S

    Gaussian vs SI Units: Comparison & Advantages

    Hello, I was reading about a comparison betw. Gaussian and SI units and the author states "The main advantage of Gaussian units is that they make fundamental physical issues and theoretical relations involving electromagnetic phenomena more clear." Would someone know - is the advantage...
  29. J

    Do some scientists create equation that arent based on si units.

    The base units are meters kg and seconds, right? So are some equations meant to use a different set, like cm kg minutes or something. And is it always assumed to be si unless told otherwise?
  30. F

    Measuring SI Units on a Desert Island

    A few years ago, I was reading about a contest open to everyone. The question was something like : if you were on a desert island, with unlimited natural resources, and access to a book with the most accurate values of properties for various natural things (i.e. gravitational acceleration on...
  31. S

    Converting Between CGS and SI Units in Electromagnetism: A Complete Guide

    So I understand that converting between CGS and SI units is no easy task when you look at the general form of each equation in electromagnetism, as you can't simply relate the two systems given they have different ways of looking at what is a derived unit and what is fundamental. But once you've...
  32. D

    Help filling in a formula correctly? Not sure about SI units etc

    Homework Statement It is estimated that a particular diffuse hydrogen cloud has a particle density of 3e7 cm^-3, and the temperature is 150 K. Calculate Jeans mass. Homework Equations Jeans mass is given by: (5kT/GmewM(H))^(3/2) . (3/4∏p)^(1/2) where k = boltzmanns constant =...
  33. D

    Converting to SI units w/ significant digits & scientific notation

    Hi. I'm asked to convert 2 year into the SI Unit seconds: Known: 1 Hr = 3600 s 1 Day = 86400 s So using a calculator... 1 yr = 365*86,400 = 31,536,000 I want to express the answer in scientific notation: Because the lesser number on has 3 significant digits, the scientific...
  34. S

    Determine the density of a planet in SI units

    Homework Statement Determine the density of fictional planet in SI units from the information Homework Equations Planets mass 1.28x10^22 Kg divided by radius 1.12x10^3Km = 1.14x10^25 Kg/m3 The Attempt at a Solution My answer seems all wrong, the SI unit should be in Kg/m cubed. Is...
  35. X

    SI Units question in physics conceptual question

    Homework Statement If speed, v, of an object depends on time, t, according to the equation v = A + Bt + Ct^4, what are the SI units of A, B, C? Homework Equations v = A + Bt + Ct^4 The Attempt at a Solution For this, I've not known where to start, so I've be plugging in the variables...
  36. X

    SI Units Lengths: Defined by Light Speed

    After the discovery of the speed of light, it may have made a bit of sense to redefine the meter as \frac{1}{300,000,000} the distance light travels in a second. I understand that larger units like the light year may have been distorted a bit, and that conversions would be required for such...
  37. Y

    Natural Frequency Question SI Units

    i have a basic question about the natural frequency of a system. for a mass (M), spring (k constant) undamped system the natural frequerncy is: w_n=sqrt(k/M) the units of w_n according to a lot of resources i found on the internet & textbooks are [rad/sec], my question is why? if i use...
  38. E

    Inability to calculate kinetic energy if I use SI units

    Homework Statement If a 2000 lb car with a velocity of 88 ft/s has by my calculations a kinetic energy of 240 kJ. But for a 909kg car with a velocity of 26.73 m/s I calculate 324.7 kJ. Same velocity, same mass, different energy calculation. Homework Equations English units: Kinetic...
  39. B

    What are the SI units for measuring magnetic fields?

    Hi, I was curious as to what webers and teslas measure. As I read up on them I was able to speculate that teslas measured a magnetic fields dimensions and webers measured a magnetic fields strength; obviously though, this is a mere speculation: so please correct me if I am mistaken. Thank you
  40. U

    How to Convert CGSEm to SI Units?

    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?
  41. K

    Instantaneous Unit Conversion between CGS and SI units

    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...
  42. B

    Do I always have to convert to SI units?

    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...
  43. M

    Are SI Units Equivalent in Differential Equations?

    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...
  44. J

    SI units for Heat transfer equation

    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...
  45. F

    I-Beam Sizes: Standard Dimensions in SI Units

    Where can I get a list of standard sizes for I beams? (Preferably in SI units, just to save me time)
  46. M

    Finally a replacement for SI units

    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...
  47. A

    Convert British and SI Units Easily

    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.:yuck::yuck:for eg. changing lengths, pressure, weight etc...
  48. F

    Derivations of SI Units in my High-School Physics formulas.

    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...
  49. S

    Using CGS or SI Units: When to Choose?

    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.
  50. C

    How to Use SI Units in Equations: A Guide for Scientists

    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...