# Questions tagged [si-units]

A set of internationally accepted units to aid in communication of measurements.

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### What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?

The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the ...
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In log-plots a quantity is plotted on a logarithmic scale. This got me thinking about what the logarithm of a unit actually is. Suppose I have something with length $L = 1 \:\mathrm{km}$. $\log L = \... 3answers 20k views ### Why is the ampere a base unit and not the coulomb? I always thought of current as the time derivative of charge,$\frac{dq}{dt}$. However, I found out recently that it is the ampere that is the base unit and not the coulomb. Why is this? It seems to ... 3answers 25k views ### Coulomb force in SI and cgs Coulomb force in SI is$ F = \frac{Q1*Q2}{4\pi\varepsilon R^{2}} $while in CGS$ F = \frac{Q1*Q2}{R^{2}} $why is it? I mean doesn't it any make difference in dimension? since$ \varepsilon $... 1answer 1k views ### What is a base unit in the new SI, and why is the ampere one of them? One question that comes up pretty much always in introductory electromagnetism courses is Why the base unit of electrical measurements is the ampere and not the coulomb, and the usual answer is that ... 2answers 2k views ### Why do we still not have an exact (constants-based) definition for a kilogram? I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It would seem this should be fairly important. Edit: ... 6answers 10k views ### Why is the mole/“amount of substance” a dimensional quantity? According to the BIPM and Wikipedia, "amount of substance" (as measured in moles) is one of the base quantities in our system of weights and measures. Why? I get why the mole is useful as a unit. In ... 3answers 11k views ### Why is a second equal to the duration of exactly 9,192,631,770 periods of radiations? Why is a second equal to the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom? Why is the number ... 1answer 563 views ### CGS Units for Magnetism Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra$c$go in SI units? 8answers 22k views ### Is there a symbol for “unitless”? I'm making a table where columns are labelled with the property and the units it's measured in: Length (m) |||| Force (N) |||| Safety Factor (unitless) ||| etc... I'd like not to write "unitless" ... 1answer 163 views ### What will be the uncertainty in$\mu_0$under the new SI scheme? As you may be aware, a new SI system is likely to be adopted in November 2018 (see https://www.nist.gov/si-redefinition/kilogram-introduction). Whilst the speed of light remains a fixed quantity and ... 0answers 84 views ### If two ends were a certain “length” apart were they therefore at rest (or at least rigid) to each other? [closed] Considering the definition of the SI unit of "length"  and [2 (" method a.")] I'm missing any requirements about the two "ends" of the required "path travelled by light" being "at rest to each ... 5answers 7k views ### Why is the prospective new kilogram standard a sphere? I can understand the choice of material, silicon 28, but why is it a sphere rather than (say) a cube? Article here I would have thought that a sphere would have been the hardest shape to machine ... 4answers 4k views ### Why is the candela a base unit of the SI? The candela is defined as The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency$540\cdot10^{12}$hertz and that has a radiant ... 4answers 901 views ### Why are electrical units (specifically, electrical current) considered a base unit? Note: this is NOT a question why current is the base unit as opposed to charge—that’s because measuring$1 \ \mathrm{ A }$through a wire is easier to measure in a lab than is$1 \ \mathrm{ C }$in ... 3answers 1k views ### Uncertainty of permittivity of vacuum Question: The value of permittivity of vacuum,$\epsilon_0$, is given with absolutely no uncertainty in NIST Why is this the case? More details: The permeability of vacuum can be given by $$\mu_0=... 2answers 956 views ### Definition of Ampere On Wikipedia it says: This force is used in the formal definition of the ampere, which states that it is "the constant current that will produce an attractive force of 2 × 10^{-7} newton per ... 2answers 3k views ### Is the second defined arbitrarily? [duplicate] According to the definition a second is 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 ... 6answers 4k views ### Why is the meter considered a basic SI unit if its definition depends on the second? The metre is the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299792458 of a second. – 17th CGPM (1983, Resolution 1, CR, 97), source The meter (or metre) is ... 2answers 5k views ### Why are scientists involved in the Avogadro Project using silicon-28 atoms instead of carbon-12? My question is, why use silicon-28 atoms to calculate the kilogram when you already have carbon-12 atoms defining the constant? Does the Avogadro Project intend to define the constant by replacing ... 5answers 11k views ### Why do we use the electron volt? Why do we use the electron volt? Why did it come to be the electron volt and not, say, just a prefix of the joule, like the nanojoule? Does the electron volt represent anything particular as far as ... 3answers 1k views ### Why aren't Faraday's law of induction and Maxwell-Ampere's law (without sources) symmetric? I was wondering why Faraday's law of induction and Maxwell-Ampere's law (without sources) are not totally symmetric in the sense that Maxwell-Ampere's law has a \epsilon_0 \mu_0 term on the right (... 4answers 3k views ### Why is the mol a fundamental physical quantity? [duplicate] I am starting to study physics in detail and as I read about physical quantities, I was puzzled why mol (amount of substance) is taken as a physical quantity. A physical quantity is any quantity ... 2answers 2k views ### Fundamental units Is it right that all units in physics can be defined in terms of only mass, length and time? Why is it so? Is there some principle that explains it or is it just observational fact? 4answers 1k views ### Why were the SI base quantities chosen as such? The reasons for choosing length, mass, time, temperature, and amount as base quantities look (at least to me) obvious. What I'm puzzling about is why current (as opposed to resistance, electromotive ... 3answers 251 views ### Why are there 1 / 1.602176634 \times 10^{-19} electrons in a coulomb? Why that exact number of electrons in one coulomb? who decided it? there is nothing wrong with the number, it just seems slightly messy. Why didn't the scientific community just settle on an easier ... 2answers 4k views ### Relating milliampere-hours to watt-hours for batteries I've seen many batteries that are measured in milliampere hours (mAh), while others are measured in watt hours (wh). How can I convert them between each other so that I can actually compare them? It's ... 2answers 251 views ### What is c in the Lorentz force expression? The usual Lorentz force expression I am familiar with is this:$$\vec F=q(\vec E+\vec v \times \vec B)$$I have seen some other versions lately that include an extra factor 1/c:$$\vec F=q\left(\... 3answers 55k views ### What is the use of this formula 1 Tesla = 1 Newton/Ampere/Meter? [closed] What does Newtons/Ampere/Meter stand for? From this formula: 1 Tesla = 1 Newtons/Ampere/meter what can it be used for? To do what? Ampere/meter Is the same unit as a field's intensity H? Or what is ... 9answers 17k views ### Why are “degrees” and “bytes” not considered base units From Wikipedia: The SI base units and their physical quantities are the metre for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for ... 12answers 13k views ### Do all equations have identical units on the left- and right-hand sides? Do all equations have $$\text{left hand side unit} = \text{right hand side unit}$$ for example, $$\text{velocity (m/s)} = \text{distance (m) / time (s)},$$ or is there an equation that has different ... 3answers 2k views ### Is there any truth to interpreting definition of a second as corresponding to oscillations? As far as I understand the definition of a second, the Cs-133 atom has two hyperfine ground states (which I don't really understand what they are but it's not really important), with a specific energy ... 2answers 3k views ### Why is the length of the Kelvin unit of temperature equal to that of the Celsius unit? [duplicate] The Celsius unit is arbitrarily defined, based on the boiling and freezing point of water. Is it a coincidence, then, that the SI unit of temperature Kelvin, which is used in all natural equations, ... 1answer 5k views ### Why is charge not taken as a fundamental unit? [duplicate] According to the definition of electric current, it appears to be a derived quantity. Charge on the other hand seems more fundamental than electric current. Then why is current taken as fundamental ... 3answers 20k views ### What is the relationship between the magnetic units oersted and tesla? How are the units oersted and tesla related? For example, how would you express$20\:\mathrm{Oe}$in tesla? 3answers 1k views ### Why wasn't the meter defined using a round-number fraction (like 1/300 000 000) of the distance travelled by light in 1 second? We know that 1 meter is the distance travelled by light in vacuum within a time interval of 1/299,792,458 second. My question is why we didn't take a simpler number like 1/300,000.000 or why not just ... 1answer 1k views ### “Natural units” of mass Gravitational attraction is given by$\frac{GMm}{r^2}$while attraction due to electric charge is given by$\frac{q_1 q_2}{r^2}$. Why does gravity need a constant while electric charge doesn't? ... 1answer 539 views ### There are plans to develop a better definition of a “second”. How does the current definition fall short? The current definition of a second is stated here and I found a presentation on the BIPM site which discusses plans to change to a "better" definition of a second. You can find the presentation here. ... 2answers 2k views ### Usage of singular or plural SI base units when written in both symbol as well as name [closed] I have multiple doubts related to the usage of singular or plural SI base units when written in both symbol as well as name. I have framed this question under two parts, namely, Part (a) and Part (b).... 2answers 2k views ### Why is the Fermi coupling constant always expressed in units of$(\hbar c)^3$? Everywhere I've looked so far (such as NIST) the Fermi coupling constant$G_F$is always expressed as $$\frac{G_F}{(\hbar c)^3} = 1.166 364(5) \times 10^{-5} \textrm{ GeV}^{-2}$$ never as just ... 2answers 87 views ### What allows us to treat physical units in algebra? I have been thinking about this problem: $$Speed = \frac{Distance}{Time}$$ Following this, is makes sense that the units of speed is m/s. However, I do not follow why we are able to divide units to ... 5answers 2k views ### Is anything actually 1 meter long (or 1kg of weight)? I believe that no real objects are actually (exactly) 1 meter long, since for something to be 1.00000000... meters long, we would have to have the ability to measure with infinite precision. Obviously,... 2answers 199 views ### Will the SI units need redefining ever again? Up until recently, there were obvious problems with the SI definitions of fundamental units, like bits rubbing off the kilogram prototypes (or mercury vapour absorption), and the water used for the ... 3answers 123 views ### How to find the corrsponding expression after working with natural units$\hbar=c=1$? If one does long calculations in natural units how does one find the right expression in let's say SI units in the end? I know that natural units make the calculations easier and also help to show ... 2answers 600 views ### Watt (Kibble) balance and the kilogram - how does the dependence on$g$get eliminated? The standard${kg}$is now in the process of being redefined by the watt balance (rather than the lump of metal in Paris) A watt balance is very simple, you measure the force needed to support a mass ... 1answer 729 views ### Why does the vacuum permeability have the value of$\pi$in it? The vacuum permeability, or the capability of the vacuum to permit magnetic field lines, contains the value of$\pi$. Why? What does this have to do with the ratio of a circle's circumference to its ... 1answer 591 views ### How large is the information collected from an inverse femtobarn of collisions? I ran into this while looking at measures of humongous amounts of data. How does the information (data) collected in an inverse femtobarn exposure compare to a gigabyte of data ? 1answer 58 views ### How bright is the night sky in$\frac{\mathrm{W}}{\mathrm{m}^2}\$?

I'm looking at the Wikipedia page on sky brightness, which gives the answer in "S10" units. I've looked at the definition of S10 and I'm still completely at a loss as to how to convert that to SI ...