Questions tagged [si-units]
A set of internationally accepted units to aid in communication of measurements.
94
questions
17
votes
12
answers
4k
views
Definition of a joule
I'm not getting the definition of a joule. From the definitions I've read if I apply one newton of force to any object, now matter how heavy/ how much mass it has, over one metre in a single direction ...
15
votes
3
answers
2k
views
Why was the Planck constant $h$ fixed to be exactly $6.62607015\times10^{−34}\text{Js}$ and not some other value?
So apparently in May, all of the SI base units were redefined to be relative to the Planck Constant $h$, instead of relying on physical objects like the Kilogram Prototype in a Paris Basement. Planck'...
12
votes
3
answers
815
views
If the candela is a base SI unit, why isn't the sone an SI unit at all?
Related: Why is the candela a base unit of the SI?
In the answers given in the previous question, the candela is included because lighting is important for humans. By the same argument, hearing is ...
9
votes
5
answers
3k
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,...
7
votes
2
answers
213
views
How much is the inherent quantum-mechanical uncertainty in the definition of the second?
Inspired by this other question.
The second is defined such that the electromagnetic radiation whose energy equals the hyperfine splitting of the ground-state of the Cs-133 atom has a frequency of ...
7
votes
5
answers
2k
views
Why is there no constant factor in the equation $E=mc^2$? [duplicate]
As I understand, the metric system starts with an arbitrary weight, and calls it $1\, \mathrm{kg}$, then the volume of this weight in water gives the meter. And then the energy is defined based on the ...
6
votes
1
answer
4k
views
What is an inverse femto barn?
I came across the use of the unit barn and inverse barn while reading about the operation of LHC. What is an inverse femtobarn? What does it tell about the experiment being described?
6
votes
1
answer
305
views
Why we only need three independent unit to describe physics?
From the article below, it is said that we can describe physics with only 3 independent units: length, mass, and time in cgs system/ velocity, action, and energy in natural units system. However, why ...
6
votes
1
answer
2k
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? ...
6
votes
3
answers
43k
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?
6
votes
2
answers
341
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 ...
5
votes
1
answer
581
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. ...
5
votes
2
answers
4k
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)....
5
votes
3
answers
858
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 ...
5
votes
2
answers
756
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 ...
5
votes
2
answers
4k
views
Asymmetric uncertainties
Inspired by these two question on tex.SX
Asymmetrical tolerancing
Asymmetric uncertainties with siunitx package
I'd like to ask for a nice explanation for these kind of uncertainties, like $10_{-2}^{...
4
votes
4
answers
1k
views
Why do (kilo)watt-hours double-count units per time?
A watt is defined as 1 joule per second and is therefore a measurement of energy transfer over a period of time.
A kilowatt-hour is a measurement of energy equal to one kilowatt sustained over a ...
4
votes
1
answer
746
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 ?
4
votes
3
answers
3k
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 ...
4
votes
1
answer
207
views
Has there been a big change in 1983 when the definition of the metre changed?
The metre was defined at the end of the $18^{th}$ century as the ten-millionth part of the quarter of the meridian (from the north pole to equator). Then, from $1983$ the definition changed for the ...
4
votes
1
answer
434
views
Confusion on the cgs, Heavside-Lorentz and SI system
I am very confused about three measuring systems: cgs, Heavside-Lorentz (never used) and SI ...
4
votes
1
answer
11k
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 ...
4
votes
1
answer
2k
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 ...
3
votes
2
answers
6k
views
What are the reasons for making the mole a base SI unit? [duplicate]
We have meter in the SI - we use that to measure length. Other length units like light years can be expressed in meters. But how often do we express amounts or quantities in moles? Mole is a number of ...
3
votes
1
answer
137
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 ...
3
votes
4
answers
2k
views
How does natural unit make sense? [duplicate]
Both the fundamental constants $\hbar$ and $c$ have dimensions. In particular, $[\hbar]=ML^2T^{-1}$ and $[c]=LT^{-1}$. But in natural units, we make them dimensionless constants of equal magnitude. ...
3
votes
2
answers
471
views
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 ...
2
votes
0
answers
49
views
Can a value of "length, in meters" be attributed to a pair of ends which are rigid (but not at rest) to each other? [duplicate]
The definition of the SI base unit "metre" [1] doesn't seem to rule out explicitly that a certain value of "length, in meters" could be attributed to a pair of ends which are rigid to each other, but ...
2
votes
2
answers
922
views
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 ...
2
votes
4
answers
1k
views
Why was the original definition of meter abandoned?
The meter was originally intended to represent $1$ ten-millionth($10^{-7}$) of the distance from pole to equator of the Earth along a meridian of longitude.
The definition was later discarded. Now, ...
2
votes
2
answers
421
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 ...
2
votes
2
answers
43k
views
What is the difference between emu and esu?
My textbook contains the following two statements:
In the CGS system the unit of charge is electrostatic unit of charge (E.S.U). It is also called Stat Coulomb (StatC).
In the CGS system, the unit ...
1
vote
3
answers
447
views
Messed up units!
In the article, Environment-assisted quantum transport, $\gamma$ is a constant equal to $2\pi kT/\hbar*E_{R}/(\hbar\omega_{c})$ where $T$ is the temperature, $k$ is the Boltzmann constant. Supposedly,...
1
vote
4
answers
210
views
How do you interpret the radian in physics? [duplicate]
When calculating $\sin x$, $x$ needs to be radian to calculate it.
so for example when solving Uniform Circular motion, $x(t)$, $y(t)$ can be expressed
$$x(t)=R\cos(ωt) [m] $$ $$y(t)=R\sin(ωt) [m]$$
...
1
vote
1
answer
3k
views
A question about natural/geometrized units
I had a question about the following document- Natural units
I understand the conversion factors. But if you look at the tables, they take an SI unit, say 1 kg, convert it into geometrized units, say ...
1
vote
0
answers
74
views
Converting $\rm W/m2/sr/nm$ to $\rm kR/nm$ for any wavelength
I have atmospheric spectra given in W/m2/sr/nm units and wavelength in nm. I want to convert W/m2/sr/nm to kilo Rayleigh/nm (kR/nm) for any wavelength.
I tried the following method from this paper: ...
1
vote
2
answers
14k
views
Unit confusion - How Pressure and Volume combines to Joule
Imagine some helium in a cylinder with an initial volume of $1$ liter
ant initial pressure of $1$ atm. Somehow the helium is made to expand
to a final volume of $3$ liters, in such a way that its ...
0
votes
2
answers
8k
views
Convert cubic feet per second to feet per second
Is there a way to convert cubic feet per second to feet per second. Or in general volumetric flow to velocity?
I want to know the time taken by water to travel from point A to point B. I have the ...
0
votes
2
answers
3k
views
Classification of plane angle as fundamental or derived
I recently started with my high school studies and the chapter I am stuck at is units and measurements.
I was told about two types of physical quantities and my teacher gave me the following ...
0
votes
2
answers
334
views
About the dimension of the SI units vector space
We know that the set of fundamental and derived physical units can be structured as a vector space over the rational numbers. In the International System of Units the dimension of this space is $7$ ( ...
0
votes
1
answer
141
views
What is the mass of $N_A$ atoms of carbon-12?
With the recent redefinition of the kilogram, what is the mass of $N_A$ (Avogadro's constant) of carbon-12 atoms? $N_A$ was defined as exactly 6.02214076×$10^{23}$ atoms. Then how close would the ...
0
votes
2
answers
119
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 ...
0
votes
6
answers
972
views
Standard Definition of speed of light and metre
The speed of light is the speed at which lightwaves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per ...
-3
votes
3
answers
375
views
Why and who has established that $1\, cal \equiv4.186\, J$?
At one time, it was mistakenly believed that an object contained a certain amount of 'heat fluid' that could flow from one body to another.
of 'caloric fluid' that could flow from one body to another.
...