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Questions tagged [si-units]

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

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Physical quantity that can be expressed using multiple fundamental units [closed]

Any physical quantity can be represented as a product of powers of fundamental SI units.For example, Force has dimensions $[\text{kg}\ \text{m}\ \text{s}^{-2}]$ and has three fundamental units. ...
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Why are electrical units (specifically, electrical current) considered a base unit?

Note: this is NOT a question why current is the base unit as opposed to charge—that’s because measuring $1 \ \mathrm{ A }$ through a wire is easier to measure in a lab than is $1 \ \mathrm{ C }$ in ...
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1answer
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radius unit in Pressure vs radius graph

In the TOV equation of state of neutron star if we take central pressure and central energy density in Mev/fm^3, then in what will be the unit of radius in the graph of pressure vs radius. Usual we ...
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1answer
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Is the given unit conversion possible? [closed]

I have to convert from Joule to Newton than to a specific unit, but I don't know which two unit factors I have to choose (since I am not allowed to change $kg$) to get the final result: $$1\,{\rm J} ...
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0answers
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Why is ampere defined with $2 . 10^{-7}$? [duplicate]

The formal definition of the Ampere is that it is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one ...
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Why is the meter considered a basic SI unit if its definition depends on the second?

The metre is the length of the path travelled by light in vacuum during a time interval of 1  ⁄ 299792458 of a second. – 17th CGPM (1983, Resolution 1, CR, 97), source The meter (or metre) is ...
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1answer
70 views

Why do physical units have different prefixes than the ones used for counting? [closed]

For counting, we use thousand, million, billion, trillion etc. For physical quantities, thousand becomes Kilo, million is Mega and (short scale) billion is Giga. In both these systems, the consecutive ...
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Avogadro number: Can not understand this notation and approximation, help please! [duplicate]

Here is the Avogadro number found everywhere on the internet:$6.022140857(74)×10^{23}$. Now what does the $(74)$ mean? Also this number is approximated in lots of books and schools as $6.023×10^{23}$. ...
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2answers
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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 ...
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4answers
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Why is the mol a fundamental physical quantity? [duplicate]

I am starting to study physics in detail and as I read about physical quantities, I was puzzled why mol (amount of substance) is taken as a physical quantity. A physical quantity is any quantity ...
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1answer
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What does it mean by “an atom at rest”?

I was reading a document when an article about atomic clock passed by. There is statement that I don't understand The second is the duration of 9 192 631 770 periods of the radiation ...
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1answer
170 views

A unit that is not coherent?

"Derived units are defined as products of powers of the base units. When the product of powers includes no numerical factor other than one, the derived units are called coherent derived units" I know ...
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Why is kilo k but Mega M? [closed]

I was recently updating a piece of software with a pull-down menu for frequency units. It had Hz, kHz, MHz, GHz, and THz. Consistency is important in user interfaces, so I decided to update the table. ...
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2answers
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How measurements for defining a second were made?

As many of you know, the second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the ...
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1answer
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Is pressure the derivative of viscosity?

I noticed the units for viscosity are $\left(\frac{kg}{m \cdot s}\right)$ and the units for pressure are $\left(\frac{kg}{m \cdot s^2}\right)$. Are these related in any way? Is there another ...
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1answer
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What is the meaning for the unit kilogram-mole?

Recently, I came across the unit J/(kg-mole * K). What is the meaning of kg-mole? Please explain me the meaning behind it. Thanks in advance, Rram
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1answer
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How to properly define the physical dimension (unit) of a variable?

Let's say we have an equation like $$v=s/t.$$ I've come across multiple ways to define the units belonging to each variable: $(v/(\mathrm{m}/\mathrm{s}) = (s/\mathrm{m})/(t/(\mathrm{s}))$ $v_\mathrm{...
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1answer
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What's the motivation to define the unit of current as fundamental in SI system? [duplicate]

The charge in electromagnetism seemingly plays the same role like mass in the Newtonian mechanics.But why we define the current unit (Ampere) in SI system as the fundamental rather than charge unit?
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Can I plug in the SI unit variants of measurement for this formula?

Here, NASA gives a formula for aircraft lift: $$ L = \frac12 d\ v^2 s C_L $$ with $L =$ Lift, which must equal the airplane's weight in pounds $d =$ density of the air. This will change due to ...
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2answers
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Is $1m^{2}/s^{2}\cdot ºC$ appropriate to describe specific heat? [closed]

A student asserts that a suitable unit for specific heat is $1m^{2}/s^{2}\cdot ºC$. Is the correct? Why or why not? My solution: Specific heat units: $$\frac{J}{kg\cdot ºC}$$ Joule units: $$J=\...
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Nuclear Energy is measured in MeV, why not in Joules? [duplicate]

Why it is preffered to measure Nuclear Energy in MeV insted of Joules?
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1answer
232 views

Does a precise definition of the meter not involving light exist, so that variation of $c$ can be tested?

The second is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. The metre ...
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1answer
452 views

Units of Larmor frequency

I'm sorry if this is really basic, but what exactly is the unit of Larmor frequency? According to Wikipedia, the formula for Larmor frequency is: $$\omega = \frac{egB}{2m}$$ and with $g$ as a ...
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2answers
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Is there a standard, coded table of physical units?

I am a programmer in the medical/biosignals area, and I want to represent physical units in a database table. Ideally, I would like to have a code that uniquely identifies a given physical quantity. ...
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1answer
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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 ...
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Definition of the Kilogram

I have been reading about how the SI kilogram has been defined throughout history. From what I understand, the kilogram was initially defined in terms of the mass of a specific volume of water at a ...
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Units in gravitational constant

I was reading on the internet and I found that the gravitational constant is roughly $6.674 \times 10^{-11}~\mathrm{ m^3~ kg^{-1} ~s^{-2}}.$ I also found that it is equal to $6.674\times 10^{−11}~\...
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How do you convert 4180 $\rm{J \cdot kg^{-1} K^{-1} to~ J\cdot kg^{-1} C^{-1}}$?

I'm stuck because you can't simply -273.15 because it is the inverse of (temperature -273.15)
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1answer
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Why isn't kilogram written as square kilogram ($\mathrm{kg^2}$) but meter is? [closed]

I was wondering why some notations can be written with square and some can't.
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1answer
113 views

Where does the constant of Planck $6.6\times10^{−34}$ J ⋅ s come from?

I've read that this number is made out of the mass or mass equivalent of any atomic particle equal to ($9.10939 \times 10^{-31}$ kg or electron mass) x ($2.42631 \times 10^{-12}$ m or electron Compton ...
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1answer
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Help with conversion from SI Units to General Relativity Units [closed]

The Book im reading about general relativity says that energy unit in GR are kg, so until i know the units of energy in the SI are Joules (J) $$ 1 J = 1 kg \frac{m^2}{...
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Why are there seven SI base units?

I have been pondering this thought recently: why are there seven SI base units? Is there any relationship between this and any more fundamental physical fact or phenomenon? Is there a absolute ...
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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 ...
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1answer
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Is the definition of the meter arbitrary? [duplicate]

From Wikipedia, the definition of the meter is The meter is defined as the distance traveled by light in a vacuum in 1/299792458 seconds. Why is this number of seconds chosen? Is there a ...
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1answer
51 views

Order used when listing units

When writing out measurements, what order should be used when listing the units? e.g., Why is newton-metre (N m) used instead of metre-newton (m N)? I encourage you to include resources/links/etc.
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What are the units of the half life in exponential decay?

I have tried to look up how to determine the units of the half life of an exponential decay, but I keep reading its 'time'. I am well aware it's time. I'm not aware of the specific units of time, is ...
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1answer
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SI unit for the speed of light through a vacuum [closed]

What is the SI unit for the speed of light through a vacuum? For example for mass it is kilograms and for force it is $\rm kg\, m\, s^{-2}$.
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Over the definition of Joule in relation to a vector quantity [duplicate]

I was thinking about the unit of measure of the Energy (as well as Heat and Work) which is Joule $$\text{J} = \frac{\text{kg}\cdot \text{m}^2}{\text{s}^2} = \text{N}\cdot \text{m}$$ But again, ...
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Mass and weight

Physical quantity of mass is kilo gram and weight is Newton but generally we say my weight is some x kg why so why does this confusion if I was not wrong weight=mass*acceleration due to gravitation ...
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2answers
834 views

Convert formula from CGS to SI

I'd like to convert this formula \begin{equation} l^2 =\frac{c\hbar}{eH} \end{equation} where $l$ is a length, and $H$ is in oersted, to SI units. I am pretty sure it uses CGS, since Oe is mentioned ...
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Is there an easy way to remember which formulas use Kelvin vs which ones use Celsius?

I know that the ones with change in temperature don't matter since the increments are the same, but I'm not sure if I can memorize which ones require $K$ and which ones require $^{\circ}C$. Is it ...
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2answers
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Why are there two different SI units of magnetic charge?

Taking a look at the wiki on magnetic charge, there are two different forms of the Dirac Quantization Condition in SI units: the Weber form $$\frac{q_e q_m}{2 \pi \hbar} \in \mathbb{Z}$$ and the ...
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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 ...
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3answers
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How do we know the precise speed of light in a vacuum? [duplicate]

From Wikipedia - "Its exact value is 299792458 metres per second" I'm wondering how we can be so precise as to the speed of light given that we cannot create a true vacuum on Earth and as I ...
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1answer
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Relationship between platinun and iridium in the kilogram

Kilogram, is the basic unit of mass in the International System of Units ( SI) , and your employer is defined as the mass of the international prototype is composed of an alloy of platinum and iridium,...
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1answer
771 views

Do pounds convert to kilograms or newtons?

I'm currently writing an essay titled "What Will Replace the Space Shuttle?" I'm having no problem finding sources of information, however most of my sources are North American in origin. (The space ...
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8answers
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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 ...
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3answers
208 views

Is there a system of units that replaces time with light-distance?

I would like to start this by saying that my motivation for asking is that I find relativity very difficult to deal with using the SI system. It strikes me that the problem with this system is the ...
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6answers
659 views

What exactly is a “volt”?

What exactly is a volt? So I Studied the chapter "electricity" in the month of April and got introduced to the concept of "volt". The concept was too unclear for me so I tried to ask some questions ...
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1answer
907 views

What's the difference between a realization, a representation and an implementation in metrology?

In a recent answer, a metrologist casually used the terms 'realization' and 'implementation' of an SI unit as if they were different, which looks very strange to the untrained eye. Some further ...