Questions tagged [si-units]
A set of internationally accepted units to aid in communication of measurements.
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Does the New SI spoil the nice Coulomb law in Gaussian units?
Now, after the redefinition of SI, the elementary charge $e$ and the reduced Planck constant $\hbar$ (and also $k_B$) are exact quantities in the SI units, as is the velocity of light $c$. The ...
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SI redefinition of the kilogram - what is one measuring? [duplicate]
I have been reading about the new SI units and specifically, want to get a better understanding of the definition of a kilogram. It was written that the kilogram will be defined in terms of Planck's ...
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3answers
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Will the volt, ampere, ohm or other electrical units change on May 20th, 2019? [duplicate]
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 ...
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0answers
40 views
MKS, why we need 7 base units? [duplicate]
What were the units of temperature in MKS system?
Why do we need 7 fundamental units?
I think we need only 2: kg, s. All other candela, mol, etc. Should be called derived ones.
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3answers
190 views
Redefinition of everything on May 20th, 2019 [closed]
A couple of issues:
So after May 20th, 2019, what exactly will be the defined value of $\hbar$?
What will be the defined number of elementary charges in a Coulomb?
Then $\mu_0$ and $\epsilon_0$ will ...
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2answers
80 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 ...
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2answers
38 views
Why have a conversion factor of $10 \:\rm mA / 10 \: mV$ instead of $1\:\rm A/1\:V$?
I’m working with a device that measures current by measuring voltage and then giving current with a conversion factor of $10 \:\rm mA / 10 \:mV$. Why write this instead of $1\:\rm A/1\:V$?
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46 views
In the SHM equation $F= -kx$, $k =mw^2$ why not use $mf^2$ where $f$ is frequency $w$ here comes out to be $1/s$ not $\text{rad}/s$?
The reason I am stating this is because on calculating the units of w(omega) I found is equal to s^-1 not regular the rad/s.
Proof:
...
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2answers
59 views
Has the kilogram changed?
In late 2018 the kilogram's definition switched from a physical object (the International Prototype Kilogram) to Planck's Constant; a grand move in my opinion.
However, I haven't heard of the ...
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Grasping systems of units
Perhaps it's that I've grown up with the SI system of units, but there's something about other unit systems that doesn't sit well with me. For example, Coulomb's law in Gaussian units can be written ...
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After the redefinition of the units of fundamental physical quantities in 2019, will an uncertainty incur in the Universal constants values?
I am specifically interested in the following constants because being a student these are some of the most common constants that I face:
$R$ (universal gas constant)
Stefan constant
Permeability of ...
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4answers
220 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. ...
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2answers
61 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 $7$ ( ...
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2answers
71 views
Dimensions of mass in terms of Length and Time
From Maxwell's Treaties on Electricity and Magnetism:
For acceleration due to attraction of a mass m at a distance r is by the Newtonian Law m/(r^2). Suppose this attraction to act fro a very small ...
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1answer
55 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 ...
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2answers
49 views
Factors of $c$ when giving masses in natural units?
I am starting a course on particle physics, and have been introduced to natural units. I am slightly confused, because we are using 'natural units', and yet masses are stated as, for instance, $139....
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2answers
141 views
Is there (or was there) a unit of electric current based on Avogadros number or Coulombs constant?
This has to do with the SI definition of the Ampere. Why the quantity $2*10^{-7} $ Newtons in particular? It would make more sense to define 1 Ampere = 1 mole of electron charge per second. Which ...
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102 views
How to convert J/mol-K to BTU/mol-R manually?
How to convert $\frac{J}{mol-K}$ to $\frac{BTU}{mol-R}$ manually?
Sorry for such a simple question, I am new to Physics and I'm getting confused when it comes to conversions of units. Please bear ...
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1answer
73 views
Correct form of Lorentz force equation [duplicate]
When I was reading the first volume of the theoretical minimum by Leonard Susskind, I encountered upon this:
But when I checked Griffiths and all my other electricity and magnetism books, it showed $...
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2answers
162 views
Why is the MKS unit of time the same as the CGS unit? [closed]
There are many system of units used in physics. In the CGS, the units are,
length : centimetre
mass : gram
time : second
And in the MKS system the units are,
length : metre
mass : kilogram
time : ...
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3answers
91 views
Is natural frequency of an LC circuit equal to angular frequency ? Why don't the units match?
When I was reading the L-C circuit in my textbook I came across the derivation of equations of instantaneous charge and current.
Which is no problem, but when I got to the derivation to current . ...
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3answers
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What was wrong with the old definition of temperature scale in kelvin?
Wikipedia's article on the recent change to the definition of the SI base units states, as the reason for changing the definition of the kelvin:
A report published in 2007 by the Consultative ...
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3answers
99 views
What is $\text{kg}\cdot \text{m}^2$ concretely? (multiplication of units)
I have problem to understand what $\text{kg}\cdot \text{m}^2$ (moment of inertia) is. So, for example a force does a work of $3\text{J}=3\text{Nm}$ means that the force can displace a weight of $3\...
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1answer
42 views
Is there a “standard” Newton?
Basic SI units have definitions through experiments that seems to imply a pretty obvious setup.
Is there a standard experiment for calibrating Newtons?
The definition is the force needed to cause a ...
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1answer
44 views
Units of Klein-Gordon equation
I'm looking at the units of the Klein-Gordon equation
$$u_{tt} - c^2\Delta u = -\frac{m^2c^2}{\hbar^2}u.
$$
Disregarding the units of $u$, which are the same everywhere and so cancel, I get $seconds^{-...
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1answer
114 views
How did Coulomb arrive at value of electron charge?
Charge of one electron is known to be as $1.6$ x $10^{-19}$ C or alternative 1 Coulomb contains charge of $6.24$ x $10^{18}$ electrons. I am just wondering if these numbers are arbitrarily chosen or ...
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2answers
141 views
Does the death of Kilogram ($kg$) affect us in any means in our day to day life? [closed]
Recently, the sleek cylinder of platinum-iridium metal has been discarded and the kilogram is set to be redefined along with ampere for electricity and Kelvin for temperature. Hereafter the Kilogram ...
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1answer
495 views
Can a physical quantity be of different dimensions depending of the system of measurement?
When comparing the Wikipedia articles on the International System of Units, the Planck unit system, and the geometrized unit system one question arises: can a physical quantity be of different ...
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1answer
39 views
Finding the unit of Pressure/Volume
Me and my friend are trying to find the exact unit of Power=Pressure/Volume. We know that the unit has to be W(J/s) so we try from the equation above:
(Kgm²/s³)=(N/A)/V
The right side will be:
(Kgm/s²)...
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2answers
181 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 ...
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1answer
75 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 ...
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1answer
154 views
Is absolute zero still 0 Kelvin?
Following the recent decision to change the definition of SI units, I understand that Kelvin is no longer defined in terms of the number 1/273.16. Does that mean that absolute zero is no longer ...
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1answer
100 views
What will happen to the International Prototype Kilogram? [closed]
The kilogram has been redefined in a way that does not refer to the International Prototype Kilogram. That kind of makes the International Prototype Kilogram useless.
So what will happen to it? Can I ...
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1answer
249 views
A new definition of mass using Planck constant
Tonight in a italian television news channel well known internationally I have heard that almost certainly the definition of mass that we currently know will be obtained by means of the constant ...
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3answers
111 views
$\rm kg$ is to symbol, as kilogram is to unit as mass is to what?
I'm sure this has a very obvious answer but I can't even work out how to properly phrase the question.
How do you collectively define what mass, temperature, and area actually are?
For example, $\rm ...
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1answer
136 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 ...
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1answer
44 views
Converting $\mathrm{ps/nm}$ to $\mathrm{ps}^2$
I have a dataset in the unit $\mathrm{ps/nm}$ for many different $\lambda$ which I want to convert to $\mathrm{ps}^2$.
I guess I can assume that I only deal with Gaussian bandwidths such that $1\ \...
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1answer
116 views
How is the division of physical quantities into base quantities and derived quantities a matter of convention?
Several physics textbooks and even the SI-The International System of Units Brochure(8th-edition) says that, ''The division of quantities into base quantities and derived quantities is a matter of ...
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4answers
497 views
What exactly is a hyperfine level as mentioned in the definition of a second?
I have a quite simple question: What exactly is a hyperfine level (or a hyperfine structure)?
Maybe my question sounds (or is) stupid $-$ I am not an expert in physics $-$ but I didn't find any ...
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3answers
179 views
What does the unit “kcal/mol” mean?
I recently encountered units of energy written as $``\mathrm{kJ}/\mathrm{mol}"$ or $`` \mathrm{kcal} / \mathrm{mol} "$. I'm aware of the meanings of $`` \mathrm{kJ} "$ and $`` \mathrm{mol} " , $ ...
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1answer
53 views
Why do the Electric and Magnetic Fields have Different Units?
I just finished my first upper-division E&M course and have a question. Why do the electric field and the magnetic field have different units if they are essentially rotations of each other? ...
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1answer
63 views
Is quantum physics a reason for the measurement differences of the International Prototype Kilogram
I've recently learned that there is an official Kilogram (IPK) kept in Paris which is used to calibrate, directly or indirectly, all weight measurement tools. I've also learned that each time the IPK ...
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1answer
118 views
Is there an MKS unit system in which electric and magnetic fields have the same measurement unit?
I'd like to know if there's an MKS unit system in which electric and magnetic fields have the same measurement unit. I couldn't find anything like that on the internet. Does that system exist? Do ...
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1answer
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Is it a coincidence that 1 liter water has a mass of 1 kg while 1 cubic meter has 1000 liters of water?
1 Cubic Meter has 1000 liters of liquid. Meter is a unit independent of Kilogram.
Then why does 1 liter of water at max density (4°C) have a mass of 1 Kg?Is it a mere coincidence?
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Geometrized units in Wald
I am trying to follow a derivation in this paper from Wald
Specifically, at the end of the paper, just under Eq. 4.7 there is given the equation
$$ Q/m \leq 2 B_0 m$$
where $Q$ is the system ...
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1answer
108 views
Why is the constant of proportionality in Gauss's law exactly $1/\epsilon_0$
$\epsilon_0$ is epsilon naught, or permittivity of free space.
Let me preface this by saying that I've just started to learn about electromagnetism. When I first saw Coulomb's law, I was incredibly ...
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2answers
105 views
Is the International System of Units complete?
Are there any known (measurable continuous) physical quantities, which are neither base quantities of SI, nor are derivable from the base quantities?
In other words, are there any quantities which ...
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9answers
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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 ...
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2answers
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What actually is 1 Coulomb ? Is it number of electrons or the amount of force?
(Pardon my English).
My question is very simple (so simple that it might get the "on hold" honour).
Anyways, since I've just started my highschool, only to land in the beautiful world of electricity ...
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1answer
2k views
How to convert bars to meters head?
I am trying to convert 1 bar to meters to come up with a conversion factor. Thus,
1 bar = x meters head
What is x?
From my computations I got something like ...
