Questions tagged [si-units]

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

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What is the quantity that is measured in "millijoules per cubic radian"?

Two years back, I asked a similar question, but it was closed as needing details of clarity and then was also deleted. However, I can give a thorough explanation of the translational units and their ...
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Are there any significant integer constants that are not unitless? [closed]

When it comes to meaningful integer constants, the only ones I can come to think of (except zero) are unitless, for example 1 (the multiplicative identity), 2 (the base of the binary system), 10 (the ...
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Why isn't the SI unit for temperature selected so the universal gas constant is 1?

This is similar to, but not quite a duplicate, of Why isn't temperature measured in joules per mole (J/mol)? , which underspecifies Joule/mol rather than "mean disordered translational ...
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What are cubic gallons? [closed]

I was working on my physics homework and thge answer came out to gallons^3, and the textbook said I had the right answer, so I'm trying to figre out what exactly a cubic gallon is? does anyone have a ...
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Convert $\rm Lux$ to $\rm W/m^2$

I use four LED matrix (UV, green, red, blue) to irradiate a solar panel with an area of S at a distance. The matrix is located at a distance l from the solar panel. I want to plot the volt-ampere ...
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Why there is no unit of energy? [duplicate]

In the SI System (https://en.wikipedia.org/wiki/International_System_of_Units) there are 7 units, but no unit of energy, though this is surely the fundamental unit of physics. Yet the Joule is usually ...
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The definition of Time and second

From the book "University of Physics 15th edition", in chapter 1, they talk about the fundamental units. They stated that the definition of unit of time is based on an atomic clock, where 1s ...
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Can the SI kg be realized without superconducting josephson junctions?

This question is motivated by me trying to finish my answer to this question. In the 2019 SI redefinition the kg was redefined in terms of Planck's constant (and the second and meter): $$ 1 \text{ kg}=...
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What technology is needed for an individual to reproduce the current SI meter and kilogram from scratch?

With the 2019 redefinition of the SI base units, I wonder what kind of technology is needed to reproduce the meter and kilogram in practice from scratch with a tolerance of ±0.1 mm and ±0.1 g? ...
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Why is $(\text{kg} \cdot \text{m}^2)/\text{s}$ written as "Joule second" when there is no joule present, and time is divided and not multiplied?

The Joule is $(\text{kg} \cdot \text{m}^2)/\text{s}^2$, which is not contained in $(\text{kg} \cdot \text{m}^2)/\text{s}$, the Joule second. So, why does the unit name contain "Joule"? Also, ...
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CGS of resistance

What's the CGS unit of resistance? Is it the same as that in SI system, i.e. ohm (Ω)? I googled but found no explicit answer.
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What is a joule? I find the definition confusing

This is the definition on Wikipedia: It is equal to the amount of work done when a force of 1 newton displaces a body through a distance of 1 metre in the direction of the force applied. I take that ...
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Why are there two different units for charge; the coulomb and elementary charge? [closed]

Why are their two different units for $q$ charges?
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Notation used to denote the unit $\rm mm^3$

For example, to denote the volume of a cube with the sides $1mm$ we write $1mm^3$. But I think it should be written as $1 (mm)^3$ because the prefix $m$ denote the factor $10^{-3}$ hence $$[1mm^3=10^{-...
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Thermometers and the Celsius scale

Why is the Celsius scale not useful for making extremely accurate temperature measurements? We know that $\delta L/L=\alpha \delta T$. By setting $\delta T$ to $1°$C we can find how much mercury or ...
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Do squared units grow in size or shrink in size if the value can be represented by a different unit?

I was doing a very simple equation, $\frac{1}{2}kx^{2}$, when I realized that, if I represented the distance in centimeters, $x^2$ would grow in size (because x is 10 centimeters), but if I ...
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Conversion factors derivation

Unit conversion chart Queries regarding MKS to CGS system for the following formulas : Force $=$ Newton (MKS), Dynes (CGS) $$\mathrm {1\ N = 10^5 \ dynes}$$ Work $=$ Joule (MKS), Ergs (CGS) $$\...
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Why is 1 newton defined as 1 $\rm kg · m/s^2$?

From my limited understanding, one newton is defined as the amount of force that gives a mass of 1 kilogram an acceleration of 1 meter per second squared. What I don't understand is why it corresponds ...
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How do I distinguish $\rm m$ for "meter" and $m$ for "mass"?

For example, an object in height 10 meters has 60 Joules of potential energy. PE = mgh 60 J = m⋅(9.8 m/s²)(10m) 60 J = m⋅(98 m²/s²) m ≈ 0.6 How can I distinguish it?
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Why does the SI unit use $\rm kg$ as it's unit of mass? [closed]

Since the kilogram has a prefix in the the list of SI units would it be wise to redefine the mass of the platinum iridium cylinder at the international bureau of weights and measures as 1g rather than ...
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What does spectral flux density mean "per wavelength"?

If I understand correctly the term "spectral flux density" describes radiant flux for a given wavelength, right? Like the given wavelength is the $\nu$ in: $$F(\mathbf{x},t;\nu)=\oint_\Omega ...
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Understanding the units in an expression

I am reading an article, and I saw this expression in Eq.5: $$\dot{u} \sim \frac{\psi}{L} \sim (\frac{H^{-1}}{L})H\psi\sim (\frac{40~\rm{kpc}}{L})(\frac{\psi}{b})H$$ Where $u$ is the velocity, $L$ is ...
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Why aren't liters a SI Unit? And why isn't there a unit for volume?

In the International System of Units, there is the second for time, the metre for length, the kilogram for mass, the ampere for electric current, the kelvin for temperature, the mole for density, and ...
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Why are Amperes and moles both base units?

Current and "amount of substance" are base units The SI system treats both electric current and "amount of substance" as a quantity that is measured in "fundamental" or &...
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What's with all these products as units? [closed]

I've been studying physics for a few years now and I'm just fascinated by the number of useful quantities that are actually products. To name a few: Electric potential is the product between energy ...
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What is the gravitational constant in 5D?

I am trying to find the energy density for a given spacetime using Einstein's Equation $G_{\mu\nu}=\kappa T_{\mu\nu}$. I am trying to do this in 5D and with restored SI units, but I am having trouble ...
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In the FLRW solutions for the EFE, what is $T_{00}$ (in SI)?

I am attempting to create a Mathematica solver for the Einstein Field Equations (EFE) that will work for SI and Natural Units and any sign convention. It works for Natural Units, but when I try ...
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Why are there just 3 main units ($L$,$T$,$M$) in physics?

Most physics books define physical units in terms of length, time and mass. Some books add temperature. And yes, the SI unit system has 7 base units, but some are clearly redundant. Why are exactly ...
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Why does everyone keep using torsion balances for measuring the gravitational constant?

Most of the experiments aimed at measuring the gravitational constant use very complex setups involving suspended balls (Cavendish-like experiments). This is however not exactly the ideal setup – and ...
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Has anyone charged an object with 1 coulomb? Why was such a ridiculously large charge chosen as the unit of charge?

The fact that two balls charged with 1 coulomb each would repel/attract each other from a distance of 1 metre with a force sufficient to lift the Seawise Giant would suggest me otherwise, but has ...
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How do we actually build the new kilogram?

Once upon a time the kilogram was a piece of metal preserved in a vault located in a basement. If I needed to build a high precision balance all I needed to do was tuning the the value of my kilograms ...
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Could you have a quartic or square degree Celsius or other degree on a temperature scale raised to any power?

Could you have a quartic or square degree Celsius or other degree on a temperature scale raised to any power? The units of the Stefan-Boltzmann constant are watts per square meter per quartic kelvin, ...
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Is the "space of physical quantities" a field of transcendence degree $6$ or $7$ over the rationals / real numbers?

Excuse my naive question and please let me explain it: In everyday life we experience 3 spatial "dimensions" + time etc. Usually the 3 dimensions are represented by a coordinate system and ...
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Deriving Hartree from fundamental atomic units leads to error?

I am trying to find how many newton meters are in a Hartree by using the following definition in terms of other physical constants: $$E_h = \frac{\hbar^2}{m_ea_0^2} $$ The values of the other physical ...
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Problem with units in quantum mechanics [duplicate]

I was reading a booklet on formalism in quantum mechanics. Therein, I learnt that the wavefunction $\psi(\mathbf{r})$ is but just one way of representing the state of system which is a vector in ...
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How is time and space defined? [duplicate]

It occurred to me while discussing a different question on this exchange that in order to understand space and time we must first agree on how it's defined in terms of measurement. My current ...
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How to write the unit of molar gas constant correctly?

Is it $8.314J{mol}^{-1}K^{-1}$ or $8.314JK^{-1}{mol}^{-1}$ or $8.314mol^{-1}K^{-1}J$ or $8.314K^{-1}mol^{-1}J$ or $8.314mol^{-1}JK^{-1}$?
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Why am I getting different values here by using different units? [closed]

Problem 1: At $27^{\circ}C$ and $\frac{77}{76}atm$ pressure, how many moles of methane gas are present in $20L$ methane? Using SI units: $$n=\frac{PV}{RT}$$ $$n=\frac{101325\times\frac{77}{76}\times ...
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Why is the charge density of a rod in $\rm C/m$ and not in $\rm C/m^2$?

Is it because we consider the rod to be a 1-dimensional straight line? Or is there any other reason? I know it sounds like a stupid question but I am confused in this regard.
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How to convert $\rm cm^{-1}$ to $\rm eV/Å^2$? [closed]

As in the title, how to convert $\rm cm^{-1}$ to $\rm eV/Å^2$? Å stands for angstrom.
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If 1 gram force on Earth is 0.0098 N, what would it be on Mars or others?

I know that this is a very simple question, but I am not really sure about this. If 1 gram force on Earth is $0.001 \;\text{kg} \times 9.8 \;\text{m}\,\text{s}^{-2}$, what would it be on Mars or any ...
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Units in the multipole expansion

In the Wikipedia page (https://en.wikipedia.org/wiki/Multipole_expansion#Expansion_in_Cartesian_coordinates) the expression for the multipole expansion gives the dipole term as $$V(r)=\frac{1}{4\pi\...
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Should it be 1.5K instead of 1.5°C?

So everyone talks about how global warming should be limitted to 1.5°C. However, in my understanding the following conversion should be used: $${\displaystyle \left\{t\right\}_{\mathrm {^{\circ }C} }=\...
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Units of Linear Speed in a Circular Motion

During Uniform Circular Motion, the linear speed of the particle is defined as the radius times the angular speed. $$ v = r\omega $$ The units of linear speed is meters/second (m/s). But the units of $...
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Confusion on molecular dynamics (MD) simulation units leading to absurd acceleration values

I am trying to code up a very simple MD simulation in order to learn more about it. I am using the Leonard-Jones potential, expressed as $ V=4\epsilon (\frac{\sigma}{r}^{12}-\frac{\sigma}{r}^{6}) $ ...
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Silver atom magnetic moment factor proportional to Spin in Stern-Gerlach experiment in Sakurai's text [duplicate]

In Sakurai's Modern Quantum Mechanics, when he trying to derive the magnetic moment of the silver atom, he simply the whole atom have the same moment as the 47th electron's spin magnetic moment and ...
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What's the meaning of unit of moment $\mathrm{N\cdot m}$? How do you explain it? [duplicate]

I know the meaning of units with division like $\mathrm{m/s}$ or $\mathrm{m/s^2}$ etc. they make sense, like $2\ \mathrm{m/s}$ is like the car pass $2\ \mathrm m$ in $1$ second, you know what I mean? ...
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What is the name for 100 litres? [closed]

1 litre = 1 litre 10 litres = decalitre 100 litres = ? 1000 litres = kilolitre Is there a scale for the naming like there is for data?
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Unit of angular velocity in oscillatory motion

Suppose an angular oscillatory motion. In the function below, $\alpha$ and $\alpha_o$ are angles measured in radian, $\omega$ is circular frequency ($2\pi/T$) measured in [radian/s] and $t$ is time. $$...
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Units in Bohr's model of an atom

In the Bohr model the values of the orbital kinetic moment and of the allowed radii are quantized according to : $$mv_nr_n = n \hbar, \, r_n = \frac{n^2 \hbar^2}{me^2}$$ By combining the two, i get : $...
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