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Questions tagged [si-units]

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

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2answers
45 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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133 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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65 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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31 views

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
319 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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75 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
286 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
178 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
54 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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272 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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357 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
90 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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1answer
490 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
291 views

Is natural frequency of an LC circuit equal to angular frequency? Why don't the units match?

When I was reading the LC circuit in my textbook I came across the derivation of equations of instantaneous charge and current. 7.8 LC Oscillations We know that a capacitor and an inductor ...
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3answers
112 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 ...
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3answers
100 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
43 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
69 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
206 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
143 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
3k 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
42 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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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
81 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
196 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
117 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
329 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
127 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
163 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
45 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
248 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
1k 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
354 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
60 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
66 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
157 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
9k 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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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
127 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
112 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
17k views

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
2k views

What actually is 1 coulomb? Is it number of electrons or the amount of force?

I've just started my highschool, only to land in the beautiful world of electricity and magnetism, I have many queries and dilemmas, so I want some guidance. Now on to the question. I've just read ...
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1answer
4k 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 ...
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4answers
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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
88 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
9k 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
163 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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947 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 ...
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1answer
58 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 ...