Questions tagged [si-units]

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

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How to interpret resistivity and its unit?

I was wondering which interpretation could we find for the resistivity, what image correspond to the concept. Furthermore, how to interpret its unit $\Omega \:\rm m$ . Why is it more logical than a $ ...
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What is the unit for work done?

My textbook's equation for work done is: work done = force * distance So this means that the unit should be Nm. However, when I researched on Google, a lot of people were saying that the unit is J....
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Are the 7 fundamental SI units able to differentiate between all elementary particles?

More specifically, can the 7 base SI units express qualities like quark strangeness (of quarks) and quark color? How do these SI units differentiate between different quarks (charm, up, top...)?
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Why can't we define fundamental unit of mass? [duplicate]

In my physics textbook of class $11^{th}$ The kilogram was defined as :- mass of the platinum-iridium standard cylinder kept at Sevre's France But this isn't a proper and scientific definition ...
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1answer
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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 ...
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89 views

How is the Length of a Meter Physically Measured

I have two parts to this question. First, I understand that the meter is defined as the distance light travels in 1/299,792,458 seconds. But how is this distance actually measured? The second is ...
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1answer
218 views

Why was the fraction 1/31,556,925.9747, in the 1956-1968 definition of the second in terms of ephemeris time, chosen?

The recent question Why are leap seconds needed so often? pulled up some interesting details about the definition of the second, and I'd like to have some of them confirmed explicitly. I'm ...
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Why are the international prototype kilogram and its copies kept under nested bell jars?

I'm watching a YouTube video about the American standard kilogram by the Veritasium channel and noticed something that I'd seen before but never questioned. The standards are kept under two, nested ...
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1answer
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Understanding the statcoulomb conceptually

I've just learned about the statcoulomb, which is basically a way to express charge when we didn't have units of charge (I think), with the definition $$1 \mathrm{statC} = 1 \mathrm{dyne}^{1/2} \...
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2answers
919 views

Complete list of fundamental properties [closed]

What are all of the fundamental properties, that is all measurable quantities which are not derived from anything else? Many quantities are derived e.g. area is length squared, velocity is length per ...
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90 views

Is energy always measured in units of mass × length$^2$/time$^2$ in physics?

Is it always $ M\,L^2/T^2$? Is special relativity different from general relativity regarding the units of energy?
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Units of the metric tensor or how to get the unit right for the line element

In this answer it is stated that the metric tensor elements have no physical unit, i.e. $[g_{\mu\nu}] = 1$. What is the convention to get the physical unit of the line element $ds = g_{\mu\nu}dx^\mu ...
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Is there a SI unit for space-time?

Space and time are routinely combined into space-time nowadays, which implies that the SI meter and second should be combined into a single SI unit such as [meter-second]. So far, I haven't come ...
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Are there any physical quantities whose units are defined using n-th roots, logs, sins, etc. of SI units?

All the SI derived units I know are defined as products of SI units raised to an integer power (e.g. the coulomb is measured in $\text{s}\cdot\text{A}$, the pascal in $\text{kg}\cdot\text{m}^{-1}\cdot\...
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Why is the unit for resistivity sometimes typeset as $\rm m\Omega\text{-}cm$, with a hyphen instead of a dot or a space?

Quote from published paper: and moderately low electrical resistivities ($\rho<5\:\rm m\Omega\text{-}cm$) Why is this $\Omega\text{-} \rm cm$? Resistivity is volume normalized, and markup is ...
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1answer
279 views

Why is $k$ taken as 1 in the derivation of $F=kma$? [duplicate]

In the derivation of F=ma, when we reach the point F=kma, we take k=1. Why can't we take 'k' as some other value?
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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 ...
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Is $V/m$ equal to $N/C$? [closed]

Is $V/m$ equal to $N/C$? I searched a lot but I can't find it anywhere V=Volt m=meters N=Newton C=Coulomb
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Definition of Joule?

The Wikipedia definition of Joule is: the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of ...
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1answer
277 views

Are ‘fundamental measures’ a thing?

The question I want to ask is: What measures are needed to describe the physical world and what are the fundamental ones of those, in the proper sense of the word fundamental? But that might be too ...
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1answer
153 views

Is Planck angular-frequency equal to 1/Planck-time or $2\pi$/Planck-time?

On wikipedia https://en.wikipedia.org/wiki/Planck_units it states that the angular-frequency is equal to the inverse of the planck time. $$ \begin{align} \omega_p=\frac{1}{t_p}=\sqrt{\frac{c^5}{\hbar ...
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Why we need Watts in electricity if power is done by the Amps?

If: I = V / R And a light bulb needs X amount of electrons flowing per second (I=amps) to create light. Why do we use watts on the first place if all the light bulb cares is the number of ...
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What $\oint\vec{B}\cdot d\vec{l}=51.5\mu T\cdot m$ mean?

I've been stuck on this practice exam question. I'm supposed to find current through a loop using Ampere's law. But instead of $\mu_0$ it says $\mu$ and it's made me very confused because I can't get ...
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1answer
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Why is the kilogram defined using Earth's gravity? [closed]

Since there are variations of $g$ depending on location on Earth's surface, why not use a reproducible lab experiment using a vertical axis centrifugal balance, and say that one kg is defined by ...
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1answer
162 views

Why aren't Faraday's law of induction and Maxwell-Ampere's law symmetric? [duplicate]

I don't see Faraday's law of induction and Maxwell-Ampere's law are totally symmetric in the sense that Maxwell-Ampere's law has a factor of $ϵ_0μ_0$: \begin{align} \nabla\times\mathbf E&=-\frac{\...
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Really weird question about unit analysis

Hi guys this is quite an interesting question I have because there appears to be two correct answers. There is an equation described by: $$ Pt = mC_\mathrm{mass}T+m_\mathrm{water}C_\mathrm{water}T+...
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Why the reference for kilogram has to be round in the Avogadro Project? [duplicate]

The question popped into my mind after reading this post and the corresponding Wikipedia article. From what I've understood (please correct me if I'm wrong), they wanted to redefine the Kilogram. So ...
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354 views

Why must the kilogram standard be based on a kilogram mass object?

Inspired by the accepted answer to a question about the Avogadro Project, why must an object used to define a new standard for the kilogram have a mass of one full kilogram? If a smaller mass were ...
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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 ...
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1answer
288 views

What determines whether a unit is a base unit? [duplicate]

I've had base units explained to me as units that cannot be broken down into a combination of other units. The Ampere is considered a base unit, however it can be written as coulombs per second. Is ...
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58 views

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: $$F_{dyne}=\dfrac{p^2}{r_{cm}^2}=\dfrac{1^2}{1^2}=1 dyne$$ ...
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Why aren't SI units squared or cubed, e.g. cubic centimetres?

For example when we look at cm^3: the multiplier is then (10^-2)^3 so why don't we write (cm)^3 instead?
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Why is Quantum Spin measured in JouleSeconds?

To my knowledge, the unit of quantum spin is h-bar on two. This has units of Joules times seconds divided by radians. The radians part makes sense to me, the Joules Seconds part, does not. Why is ...
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808 views

Why frequency is a SI-derived unit?

Frequency is a derived SI unit but its unit is 1/s. It uses only time once and no other fundamental quantities. So why is not included in SI base unit?
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Is displacement vector fundamental or derived quantity?

We know that we have 7 fundamental quantities (all scalars) and length is one of them. I classify velocity as a derived quantity. What about a position displacement vector? How do I classify ...
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112 views

In a force model, $F = ma$, how to understand the units?

For instance, in an aerodynamic force model, the force terms can be lift terms, drag terms, both of which have translational velocities as factors in their models; but, velocities are in units of, say,...
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Proper reference for the International System of Units

I would like to cite a relevant document that guides usage of SI units. It seems like currently there are two nearly identical literature sources: 8th edition of SI Brochure issued by BIPM, 2006; ...
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Why is unit of pressure (psi) used to determine things like bite force of an animal?

Whether in tv documentaries or journals, whenever they talk about an animal's bite force, it's measured in PSI anytime imperial units are used (ex: National Geographic, NIH Journal). Many even seem to ...
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What is the name and symbol of the derived unit for one thousand metres? [closed]

I'm still getting confused of what a derived unit is. I used the internet to find an answer but I'm still getting confused.
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Is the fact that 100 kPa equals about 1 atmosphere accidental?

Typical atmosphere near sea level, in ambient conditions is around 100,000 pascals. But the pascal, as the unit, is not defined through Earth atmospheric pressure. It's defined as one newton per ...
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Magnetic charge and magnetic fine structure constant in SI units

What are the definitions and values of the elementary magnetic charge and the magnetic fine structure constant in SI units? What are the relations with the elementary electric charge and the electric ...
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What does the $m/s^2$ in acceleration mean?

I'm just a little bit confused about the $m/s^2$ in acceleration. If an object is accelerating at $10m/s^2$, does it mean that every second, it speeds up at $10m/s$?
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287 views

What are the units for the C parameter in the formula for surface excess concentration?

I came across this with a lecturer while discussing the Gibbs-Adsorption Isotherm. The lecturer insisted that the unit for the concentration parameter in the the formula surface excess concentration ...
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1answer
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How do the radian have a unit? [duplicate]

The radian is defined as the ratio of the circumference and the radius. Both are measured in meters. So there should not be a unit for that. But we use 'rad' as the unit of the radian value. The ...
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223 views

How do I convert natural/Planck units to SI? [closed]

I am looking at a paper which has set: $\beta=1/kT=1$ where k is the Boltzman constant. It uses natural planck units ie $m=\hbar=k=1$. Does it make sense to extract a temperature $T$ and mass $m$ ...
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Why is a Coulomb not a fundamental unit? [duplicate]

Why is a coulomb not a fundamental unit but an ampere is considering that a coulomb is more ' fundamental '?
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How to interpret units such as $m^{-2}$? [closed]

I'm doing a project of mangroves and found this chart http://www.habitat.noaa.gov/coastalbluecarbon.html It lists how much carbon mangroves and a few other ecosystems absorb but the chart uses m^-2/yr^...
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How can we define the kilogram in terms of Planck's constant?

Scientists are going to redefine the kilogram by using the value of Planck's constant. What is the physics behind this would-be definition of a kilogram? How is Planck's constant related to kilogram? ...
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Redefining the kilogram using Planck's constant instead of the density of water among other examples

The kilogram is in the process of being redefined in terms of Planck's constant so as to eliminate its dependence on a physical artefact. Since the length and temperature units are already precisely ...
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Why are the electric and magnetic constants where they are?

$ε_0$, the electric permittivity and $μ_0$, the magnetic magnetic permeability were introduced in Coulomb's Constant and Ampere's Constant in order to make units and magnitudes match, in Coulomb's ...