Questions tagged [si-units]
A set of internationally accepted units to aid in communication of measurements.
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Does dimensional consistency imply the same units?
Can two units having the same dimensions always be used interchangeably? For example $s^{-1}$ and $\frac{rad}{s}$ have the same physical dimension, does that mean we can measure frequency $\nu$ with ...
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Is Joule equivalent to Joule/radian? [duplicate]
The work a constant torque does when it moves a particle through an angel $\theta$ is given by $\tau \theta$. The work is measured in Joule, so the torque should be measured Joule/radian, but the ...
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Translating plasma units into SI units
I am trying to compute the characteristic lengths and frequencies in a plasma, namely the electron plasma frequency and electron inertial length. From the NRL plasma formulary
$$\omega_{pe}\ [rad/s] = ...
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Relationship between bel and decibel
Bel is a unit of $log_{10}$ of ratio of two quantities.
1 Bel = $\log_{10}\frac{P_1}{P_2}$
On Wikipedia it says: 1 decibel = $\frac{1}{10}$ bel
According to this definition then,
1dB = $\frac{1}{10}$ $...
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Converting from Planck units to Hartree units
I am trying to include an analytical formula into a numerical program. In Planck units, the expression looks like this: $\frac{A}{B}$, where $A$ is in $m$ while $B$ is in $eV^{-1}$.
If we go to SI ...
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Difference between - & [closed]
I’m taking Introductions to Physics book. In the book on the chapter “Physics & Mathematics” there was one question where we were supposed to find the magnitude of work done from given vectors ...
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What was the exact triple point of water in the pre-1954 definition of Celsius and in the post-2019 definition of Celsius?
In 1954 the definition of Celsius was changed in terms of absolute zero and the triple point of water. And the triple point was chosen to be exactly 0.01C. Since there is no physical reason for the ...
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Confusion regarding SI-units of Stiffness $K$ [duplicate]
I know that stiffness,$K$ is equal to product of moment of inertia and square of the natural frequency
$$ K=J\omega_n{}^2$$
Where $K$ = stiffness, $J$ = moment of inertia and $\omega_n$ = natural ...
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Definition of proportionality constant in Coulomb's law
My textbook, which was written before 2019, says:
$k$ [Coulomb's constant] is defined in terms of $c$ to be precisely $k=(10^{-7}\ {\rm N\cdot s^2/C^2)}c^2$
$\implies\displaystyle\frac{1}{4\pi\...
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Melting point of water [duplicate]
Wikipedia says about melting point that:
The melting point of ice at 1 atmosphere of pressure is very close to 0 °C (32 °F; 273 K); this is also known as the ice point.
But why it is very close and ...
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Is Coulomb written/found on any object/product as its unit?
Today I had physics class and the unit Coulomb made me wonder as to why it was even invented when I don't see it being used anywhere. When I asked where it may be shown just like bulbs have x Watts ...
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Why isn't the charge of an electron taken to be "$-1.6 \times 10^{-19}$ coulombs" [closed]
Even though we refer to electrons being negatively charged, why is it that we don't write the charge as "-1.6 × 10^-19 coulombs"?
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Gravitomagnetic Field vs Distance
If we look at what the Wikipedia says about gravitomagnetic fields $B$ generated by rotating planets / stars we have:
https://en.wikipedia.org/wiki/Lense%E2%80%93Thirring_precession#...
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Why can the Ampere not be defined as the flow of $n$ Coulomb in $n$ seconds? [closed]
1 Ampere is defined as the flow of 1 Coulomb of charge in one second. However, I do not understand why it cannot be defined as the flow of n Coulomb of charge in n seconds. This definition is ...
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Is kilo-Kelvin ($\rm kK$) avoided as it would be confusing?
When describing the colour of light globes, you see temperatures like 2700 K or 6000 K.
The surface of the sun is around 5500 K.
This could be written as 2.7 kilo-Kelvin or 2.7 kK (or 6 kK, 5.5 kK ...
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What exactly is kg? [closed]
Kilogram literally makes no sense. We say that we are 10 kilogram by using the weighing scale but according to its definition, it is the amount of matter contained in an object. So how much matter is ...
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Notation: units with negative exponents
I'm not sure if this belongs on Meta or here but:
In many scientific journals, books, and posts on this site and others, I see the negative exponent convention used for units, e.g.
$\mathrm{N} \,\...
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What is stopping optical clocks from redefining the second?
Optical clocks, based on optical transitions either in cold atomic lattices or trapped ions, have been shown to up to one million times better accuracy/precision compared to the cesium microwave ...
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Defining the second by an alien civilization [closed]
(The above question could be phrased better, so feel free to suggest.)
Like many, I was imprecisely told that the second is 9192631770 oscillations of caesium valence electron (or smth along those ...
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How does dimension work?
So, I know dimensions are the powers to which the base quantity is raised. For ex: we denote acceleration as LT^-1… But how does it work?
If i go to a different country, they may measure length in ...
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Why do we measure plane angle in radians and solid angle in radians and steradians respectively rather than degrees? [duplicate]
Recently, I learnt about physical quantities. When i got to know about plane angle and solid angle, i had a doubt that even though they are just angles, why do we measure it in radians or steradians ...
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Value of Inverse of Thermal Voltage
I am trying to solve the Poisson-Boltzmann Equations and have come about the constant
$$
\beta = \frac{e}{K_BT}
$$
which Wikipedia denotes as the inverse of Thermal voltage. Now this constant is ...
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Why do we not use SI units for optical rotation of substances in solution?
This question is with regards to a polarimetric approach in analysing the concentration of a substance in solution such as sugar in water.
I have been trying to find an answer to it through a little ...
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On constants that maintain proportionality [closed]
I do not consider my knowledge on mechanics higher than average, and average is perhaps as good as it gets; however, I ask this question to better my knowledge on the way physics has been modeled. I ...
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Thermal expansion: why in Celsius and not in Kelvin?
When dealing with thermal expansion problems I've encountered a situation where I must calculate the change in length of an iron bridge 2km long if the amplitude of temperature changes by 40ºC, ...
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Is the unit $m^2$ for area size ambiguous? [closed]
In normal case, we use $m^2$ to represent the size of an area - the product of two distance whose corresponding quantities are perpendicular. But it can also be simply the square of one distance, such ...
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Units of Irradiance
I am studying the black body curve currently and am confused about the unit for irradiance. Why is it watts per unit area per unit wavelength working out to be power per unit volume? More specifically ...
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Why this form of Lorentz force law in Gaussian units?
In Ashcroft and Mermin's first chapter, equation (1.13), they write the Lorentz force law as (due only to a magnetic field)
$$\textbf{F} = \frac{-e}{c} \textbf{v} \times \textbf{H}$$
rather than as
$$\...
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Definition of an electron volt
One electron volt is defined as the kinetic energy of a electron in a potential of $1$ volt.
Hence, by conservation of energy, electric potential energy = kinetic energy:
$$ q V = \mathrm{K.E.} $$
As ...
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Electric field definition
In my book it says that "electric field" is defined as the "force per unit charge". What does the "unit charge" mean? My initial thoughts was that it meant 1 Coulumb, but ...
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Why are the seconds/minutes/hours units are universally used, while other measures have several units? [closed]
There are several units to measure length like meter, foot, and rod. The same is true for weight. While time seem to have a universal unit which is the second. Was there other measures of time that ...
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Does gravity change local space and time units?
Gravity is the curvature of spacetime. Therefore, gravity affects spatial and temporal dimensions in some parts of space. We know that measuring the speed of light in the observer's local inertial ...
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What does stand behind the formula: "watt = ampere * volt"?
I know the "watt = ampere * volt" formula, but I don't understand what it means.
So, let's start from the beginning.
Coulomb answers the question: how many electrons do you have (in a ...
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Understanding the units of cosmic string number density
I am reading this old paper: https://arxiv.org/pdf/1309.6637.pdf and trying to work out the units in equation 63. It gives the number density of cosmic strings in the radiation era as
$$
\frac{n(\ell,...
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Frequency of harmonic oscillator potential
Consider, a particle is moving in a harmonic oscillator potential :
$V=\frac{1}{2}m\omega^2x^2$.
The force on the particle will be :
$F=-m\omega^2x$.
What is the unit of $\omega$ here ? Is it $Hz$ or $...
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How is the SI unit for mass defined (as of 2019)? [duplicate]
So, the SI unit of mass is defined by taking an exact value for Planck's constant, which is units of J$\cdot$s = kg m$^2$ s$^{-1}$, and meter and second already defined in ways I understand. I don't ...
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Einstein coefficient unit
Einstein-A coefficient of diatomic molecule can be written as (W. A. Brown, 1970):
[unit]
A= 1/sec (transition probability, Einstein coefficient)
h-bar= Joule*Second (planck constant)
Re=Coulomb*...
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Unit of $v$ in $v=rω$ [duplicate]
unit of velocity is m/sec but in $v=rω$
unit of $r$ is metre and unit of $ω$ is radian/sec so unit of $v$ should be radian*metre/sec but it is m/sec how
I know this is dimensionally correct but I dont ...
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How accurately do we know how long a meter is?
A meter is, by definition, a specific fraction (1/299792458) of the distance light travels in a vacuum in one second. How accurately do we know how long this is? In principle, it's exactly defined, ...
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What does the qualifier NOMINAL exactly refer to?
This question may not specifically belong to the physics domain, and rather perhaps to some engineering one, yet I couldn't find a better place to ask it either, trying my chance here...
From time to ...
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How do you go from watts to lumens?
let's say you have a light source and using a solar panel or photoelectric diode you can absorb all the emitted light, which would produce some amount of power outputted by the solar panel. How would ...
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Question about unit for intensity
The picture is from one physics textbook about treating sun as a blackbody and compare the electromagnetic radiation spectrum. Note the unit used for Intensity in the picture shown, it's Watts / (...
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How to reinvent measurement units?
Imagine you're in deserted island. You will eventually need to know how much there is of something or how long is some thing.
Is there a way to get all main measurement units (kg, m, °C, m$^3$, etc.) ...
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Why use units of $\rm 1/Hz$ instead of $\rm s$?
The Wikipedia page for Planck's constant frequently includes the constant in units $\text{J/Hz}$ or $\text{J} \times \text{Hz}^{-1}$. Is there a reason these units are used instead of $\text{J} \times ...
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Intuitive understanding of the unit $kg/s^3$ — the unit of sound volume
I know that decibels are used to measure volume of sound, and they are basically a logarithm of $kg/s^3$. The best explanation that I have for the unit of $kg/s^3$ is that it is an alternate way of ...
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Smaller Unit that a Second - Sexagesimal
Most of us seem to measure time HH:mm:ss and then drop into SI prefixes of seconds.
Naturally, this results in a weird swap from base 24 (12 if you're American), to base 60, to base 10 as you progress ...
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How much does the International Prototype Kilogramme weigh?
From [some date in] 1889 to 20 May 2019 the International Prototype Kilogramme in Paris (called the ur-kilogram by German-influenced writers) weighed $1.000\,000\,00kg$. It seems very hard to find out ...
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Compressibility of Solids
This is an extension of the question asked here, with the same setup.
Question 1
In 3D compressibility is:
$$\beta = -\frac{1}{V} \left (\frac{\partial V}{\partial P} \right )_T$$
In 1D ...
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Why the specific factor of $2 * 10^{-7}$ is used while defining 1 ampere?
while defining an ampere through force between two current-carrying wires, I noticed that they considered the force between two wires as $2 \times 10^{-7}$ forming a definition as follows:-
"One ...
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Is Joule / kg a dimensionless quantity?
If (given that Mass * constant = Energy, as in E = mc²), Joule is convertible to kg, just with a very small conversion factor, does this mean 1 Joule / kg doesn't have any unit and just equals that ...
