Questions tagged [si-units]
A set of internationally accepted units to aid in communication of measurements.
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1answer
52 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
84 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
44 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 . ...
4
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3answers
94 views
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 ...
2
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3answers
98 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 ...
0
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1answer
26 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
44 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 ...
2
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2answers
131 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 ...
1
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1answer
58 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
36 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)...
5
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2answers
94 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 ...
0
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1answer
64 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
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1answer
151 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
69 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
140 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
101 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 ...
7
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1answer
105 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 ...
1
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1answer
38 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
62 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
131 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
81 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
52 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
61 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
90 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
467 views
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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0answers
38 views
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
96 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
95 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
323 views
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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2answers
1k 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 ...
3
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4answers
1k views
How do we recover units of force from units of gravitational potential?
The gravitational potential $G_\text{pot}$ has units of energy per unit mass:
$$
\bigg[\rm\frac{J}{kg}\bigg] = \bigg[\rm\frac{kg\cdot m^2}{s^2\cdot kg}\bigg] = \bigg[\rm\frac{m^2 }{s^2}\bigg].
$$
...
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2answers
67 views
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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3answers
2k views
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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1answer
146 views
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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2answers
670 views
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 ...
2
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0answers
35 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 ...
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1answer
47 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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0answers
24 views
How do we know the standard of units of measurements do not change? [duplicate]
Throughout human history we have used various scales to represent things like distance (length of someones foot, yard), time duration (time rotation of earth on it's axis for a day etc). When ...
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0answers
57 views
What is the unit of a concentration in a formula for the reverse saturation current in a diode?
I've the following formula:
What are the units of $N_D$, $N_A$ and $n_i$?
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1answer
128 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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1answer
132 views
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
52 views
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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0answers
64 views
Terminology regarding the units [duplicate]
Is "m⁄s²" called a unit (singular)? Or should it be called "units" because it is composed of basic units?
Would you say that "m" and "s²" are components of this unit? Theses components being ...
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0answers
10 views
How would you replicate the measurements and calculations originally used by Delambre, Mechain et al to establish the length of the meter?
Meter was then defined as one ten-millionth of the distance between the equator and the North Pole. In practice, the researchers measured the distance between dunkerque and Barcelona, "then reported ...
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0answers
105 views
Why is the meter not redifined to match the time light travels in 1/300,000,000 sec? [duplicate]
I know the lightspeed is 299,792,458 m/s and in fact the meter has been defined in such a way that lightspeed has this value by definition. Now I wonder, why hasn't that been rounded of to 300 million?...
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2answers
370 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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3answers
74 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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1answer
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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 ...
