A set of internationally accepted units to aid in communication of measurements.
A set of internationally accepted units to aid in communication of measurements.
Some common SI units are
The kilogram
The joule
The metre
The second
The Ampere
Questions tagged [si-units]
657 questions
157
votes
8 answers
Is $\pi^2 \approx g$ a coincidence?
In spite of their different dimensions, the numerical values of $\pi^2$ and $g$ in SI units are surprisingly similar,
$$\frac{\pi^2}{g}\approx 1.00642$$
After some searching, I thought that this fact isn't a coincidence, but an inevitable result of…
nalzok
- 1,365
78
votes
9 answers
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 temperature, the candela for luminous intensity,…
Wais Kamal
- 939
78
votes
5 answers
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 fact, my question isn't really about the mole at…
Tim Pederick
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70
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3 answers
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 me that charge can exist without current, but…
Greg
- 1,790
70
votes
2 answers
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 fundamental constants of nature than were possible when…
Emilio Pisanty
- 137,480
67
votes
4 answers
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 of periods so complicated?
It could be any…
A. Vats
- 797
59
votes
11 answers
What is the logarithm of a kilometer? Is it a dimensionless number?
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 $L = 1 \:\mathrm{km}$.
$\log L = \log \mathrm{km}$
It seems that the unit of $\log…
Statec
- 725
52
votes
6 answers
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 anyone ever charged an object with 1 coulomb of net…
moonblink
- 1,159
47
votes
5 answers
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 accurately.
user56903
41
votes
3 answers
Will the volt, ampere, ohm or other electrical units change on May 20th, 2019?
When watching a video by Veritasium about the SI units redefinition (5:29), a claim that the volt and unit of resistance (presumably the ohm) will change by about 1 part in 10 million caught my attention:
[...] I should point out that a volt will…
user231851
41
votes
2 answers
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 the idea of carbon-12 and putting silicon-28 in its…
Captaine Code
- 629
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- 12
39
votes
8 answers
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" on several columns...and I'm quite surprised I…
Ben
- 1,001
37
votes
3 answers
Why does the metric system use "kilogram" as a base SI unit?
SI system uses all (that I know) measurement basic units as 1 (single) instance: meter, second, Ampère, etc, except for the KILOgram. It already defined with 1000 multiplier (kilo).
It prevents from using usual multiplier prefixes: mega, giga, tera,…
Sasha
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37
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6 answers
Is Nm the same unit of torque as mN?
A couple of days ago, I noticed that the torque unit used by my teachers is $mN$, and while reading on the internet it came to my notice that in all textbooks the official unit is $Nm$.
I asked one teacher about it and he insisted that I'm wrong,…
35
votes
5 answers
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 the mathematics goes? I am guessing that it does,…
Benjamin Rogers-Newsome
- 1,740