Skip to main content

Questions tagged [si-units]

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

Filter by
Sorted by
Tagged with
1 vote
2 answers
87 views

Can all physical quantities and units be derived from the seven base SI units? [closed]

I understood that from cesium that we derived the unit for second and procedurally with other universal constants we derived the other SI units. Can all other “physical” units be derived from the ...
Arden Tsang's user avatar
0 votes
2 answers
49 views

Complete unit of vectors and their scalar counterparts [closed]

I'm aware that we use m/s as the SI unit for both speed and velocity. My question then is, we use units to define the magnitude of the vector (velocity), why not add units for the direction too? For a ...
PANAV SINGH's user avatar
2 votes
3 answers
588 views

How to convert units when calculating a dimensionless quantity?

For instance, consider calculating this quantity: $$ P= \frac{1}{H^4~~ } \times \left(\frac{1}{k_0}\right)^{0.2} ~~(1), $$ where $H$ and $k_0$ are constants in units of GeV and Mpc, respectively. How ...
Dr. phy's user avatar
  • 395
-1 votes
1 answer
88 views

Why did we choose Coulomb's constant $k$ as $9\cdot10^9 \rm Nm^2/C^2$ while define unit of charge instead of any other number?

I understand that while defining charge, Coulomb had to choose any arbitrary value of $k$ to describe unit of charge. But, why did we chose $9\cdot10^9 \rm Nm^2/C^2$ as the value of $k$, but not any ...
Singer Asvj's user avatar
0 votes
1 answer
93 views

Unit problem: $\rm N/cm$ or $\rm N/m$

Is that really wrong to solve a simple Hooke’s law equation which gave Force in N and displacement in cm to find out the spring constant in N/cm? The problem I was attempting to solve did not specify ...
Alvin's user avatar
  • 19
1 vote
0 answers
59 views

Mass of the Prototype $\rm kg$

With the new SI kg based upon Planck's constant $h$, is there any plan to eventually measure the former IPK using a Watt Balance, and determine how far its mass deviates from the new definition of 1 ...
RC_23's user avatar
  • 9,508
0 votes
2 answers
83 views

What is the key difference between SI and MKS units?

In MKS as well SI system, the length, mass and time are measured in the units meter, kilogram and second, respectively. Then how the SI units are different from MKS units? What are the key ...
San26's user avatar
  • 1
0 votes
0 answers
22 views

What is the value of the ideal gas constant in J/(mol*°C)? [duplicate]

I've seen in some sites that it is the same as in J/(mol*K), is that true? $8.314 \frac{J}{mol×K}=8.314 \frac{J}{mol×°C}$
Malcolm's user avatar
0 votes
1 answer
66 views

Applicability of the $\frac{1}{4\pi}$ pre-factor for Coulomb Attraction in Nanocrystals

I am currently working on a thesis and stumbled upon a seemingly formal issue which I was not able to resolve myself. In short The classical coulomb energy between two charges (let's say, electrons ...
Alex Schmitz's user avatar
3 votes
1 answer
282 views

What are natural units?

I need to make a presentation on natural units. My professor asked me to visualize a world where $c$ and $\hbar$ are actually equal to unity. Like, what are the consequences? I also want to know the ...
Shrishti's user avatar
0 votes
1 answer
96 views

Convert Coulomb's law in CGS units to SI units

I recently translated the appendix to an electromagnetics text from 1945 into English. Now the client is asking me to update the formulas (I studied electrical engineering). The formulas use CGS units ...
garbo999's user avatar
  • 103
1 vote
1 answer
54 views

Fundamental and derived physical quantities

I read that fundamental physical quantities are independent of each other but, if we write length = velocity x time, then length depends on the time interval so how come it is independent?
Manish's user avatar
  • 51
3 votes
2 answers
69 views

Why is the general unit for energy (in terms of energy bills) $\rm kWh$? [closed]

Why is the unit for energy (in terms of energy bills) expressed as $$\text{time} \cdot \frac{\text{energy}}{\text{time}}$$ rather than just energy? Wouldn't it be better to express it in megajoules (...
flakpm's user avatar
  • 33
4 votes
5 answers
294 views

Where does $4\pi$ come from in electromagnetism?

We know that in Coulomb's law, the constant $k_e$ is $\frac{1}{4\pi \epsilon_0}$. Where does $4\pi$ come from? Why is it related to the force between two charges?
Toboraton's user avatar
  • 109
0 votes
2 answers
162 views

Are there any non-SI base units? [closed]

Are there any base units that are not in the SI system? If so why aren't they? Edit: By this I don’t mean other systems that you can convert to (ie. Feet, Ounces, Hours, etc) I mean new base units ...
Gabriel Tellez's user avatar
0 votes
1 answer
84 views

Can unit prefixes be interpreted as algebraic stand-ins?

I am currently a little confused about whether unit prefixes can be interpreted algebraically For example: $$4km = 4\times 10^{3} m$$ Is it incorrect to say that $k$ is simply an algebraic stand in ...
cookiecainsy's user avatar
1 vote
0 answers
52 views

Units for the Calculus of Variations [duplicate]

Just a quick question regarding the units for a quantity. I just started reading a QFT textbook, and it starts out with a little bit of Calculus of Variations. Specifically, there is a result that ...
Hobson Carion's user avatar
6 votes
2 answers
326 views

Radian unit mystery (damped oscillator)

I would be extremely grateful for any help that anyone could offer here. I am interested in solving the optical bloch equations for the excited state population Rabi oscillations with damping due to ...
HB123's user avatar
  • 61
0 votes
3 answers
94 views

The SI-unit of the cosmological constant (vacuum energy) is $\frac{1}{m^2}$. What does that have to do with Energy?

I just don't get how Energy is measured in $\frac{1}{m^2}$. Wasn't it measured in Joules? (source is https://en.wikipedia.org/wiki/Cosmological_constant#Equation)
Marvas's user avatar
  • 19
0 votes
6 answers
165 views

How to interpret units of measurement like $\text{kg m/s}^2$?

Let's first take an example. I understand that if a car has $v=20 \,\text{m/s}$ this means that every second it moves $20 \,\text{m}$. But how should I interpret units that are multiplied like $\text{...
costantino corsi's user avatar
4 votes
1 answer
217 views

Has there been a big change in 1983 when the definition of the metre changed?

The metre was defined at the end of the $18^{th}$ century as the ten-millionth part of the quarter of the meridian (from the north pole to equator). Then, from $1983$ the definition changed for the ...
niobium's user avatar
  • 608
0 votes
2 answers
171 views

Reason Why 1 Coulomb was redefined in SI unit system?

Since current can be calculated as $I = q/t$, the original definition of 1 Coulomb in SI unit system was that it is such amount of charge which flows through wire cross-section having current of $1 A$ ...
Agnius Vasiliauskas's user avatar
0 votes
1 answer
176 views

How to convert quantities between SI units and a natural unit system?

Let's say I'm working in a natural unit system defined by a set of physical constants set to dimensionless numbers. How can I convert quantities between that natural unit system and a more ...
MattHusz's user avatar
  • 239
0 votes
1 answer
39 views

Abbreviations for energy and torque

What are correct abbreviations for torque and energy in US Customary and S.I. systems? Both include distance and force. Do we write N-m and lbf-ft for both torque and energy? I always heard (years ago)...
Bill's user avatar
  • 79
7 votes
6 answers
547 views

Why is the mole an SI unit?

In the SI unit system you have stuff like mass and length which are pretty fundamental. But the mole seems to not be as fundamental as the others. I think we can express it in terms of the other ...
Euler-Masceroni's user avatar
2 votes
1 answer
57 views

Right notation for SI units/dimension in polynomial function?

I integrated a jerk-function three times (acceleration, velocity, position) to get the resulting function $s(t)$ for the position. I am not sure how to use the SI units or dimension in the function. ...
user avatar
0 votes
0 answers
38 views

How to convert $\rm W/m2/sr/nm$ to $\rm kR/nm$ [duplicate]

How do I convert spectral radiance $\rm W/m2/sr/nm$ to $\rm kR/nm$? I tried the following method from this paper: https://opg.optica.org/ao/abstract.cfm?URI=ao-13-9-2160
Aadarsh's user avatar
  • 11
0 votes
1 answer
76 views

Validity of equation for $E \times B$ drift in SI and Gaussian unit systems

I derive $E \times B$ drift $\vec v_{E \times B} = \dfrac{\vec E \times \vec B}{B^2}$ and in SI I use V/m and T for the fields. I do not use Gaussian (cgs) units often, but I thought this equation ...
Paweł J's user avatar
1 vote
0 answers
83 views

Converting $\rm W/m2/sr/nm$ to $\rm kR/nm$ for any wavelength

I have atmospheric spectra given in W/m2/sr/nm units and wavelength in nm. I want to convert W/m2/sr/nm to kilo Rayleigh/nm (kR/nm) for any wavelength. I tried the following method from this paper: ...
Aadarsh's user avatar
  • 11
4 votes
2 answers
91 views

What was defined first and how? The ampere or the vacuum permeability?

I've been looking up the history and evolution of the seven base units and am currently checking out the ampere. What I've found is that 1A is defined as the current in a wire which would experience a ...
SpectraXCD's user avatar
1 vote
2 answers
103 views

Want to know about permittivity

Why is the speed of light $c$ used to determine the value of permittivity in free space?
shanto khan's user avatar
2 votes
6 answers
975 views

Does a squared unit require squaring the value?

I would like to know if an acceleration number would remain squared in $$ v=v_{o}+at $$ Such as 1.35 m/s^2, for example, would end as $$ v=v_{o}+(1.35^2)t $$ or simply as $$ v=v_{o}+(1.35)t $$ Thank ...
jvno's user avatar
  • 39
1 vote
2 answers
54 views

How to typeset "$\rm kVp$" (kilovolt-peak) in CT physics?

I'm not a radiation physicist. Kilovolt-peak appears to be used as a unit, with descriptions such as "80 kVp". Should this be typeset as a single unit written "Vp", similar to e.g. ...
Anna's user avatar
  • 191
0 votes
1 answer
96 views

What is the value of $e^2$ in CGS units? [closed]

Seems yo me that units of the square power of electron charge $e^2$ are in the SI units system Coulombs squared, $[C^2]$. But what are the corresponding units of $e^2$ in CGS units system? I am not ...
Markoul11's user avatar
  • 4,170
0 votes
1 answer
64 views

Kilogram is SI unit but not gram at the same time meter is SI unit and not kilometer. What decides whether a unit should be considered as SI or not?

centi, milli,kilo,micro etc all are prefixes used in SI system of measurement. So in centimeter both the prefix-centi and meter belong to SI system but centimeter is not SI unit. If it is not SI unit ...
Shinnaaan's user avatar
  • 1,357
0 votes
1 answer
78 views

What is the dimension of wavefunction in spherical polar coordinates?

I understand that the dimension of the wavefunction in Cartesian coordinates is $L^{-3/2}$ since the norm squared of the wavefunction is the probability density, where probability is a dimensionless ...
Dainsleif's user avatar
1 vote
0 answers
49 views

Is there something special about the 7 fundamental quantities or are they just convention? [closed]

I mean could we do as good with any fewer (or more) fundamental quantities, or is seven really a magical number when it comes to them?
NotAScientist's user avatar
3 votes
2 answers
133 views

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 ...
Jack's user avatar
  • 959
1 vote
2 answers
211 views

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 ...
Jack's user avatar
  • 959
0 votes
0 answers
29 views

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] = ...
Zelkin's user avatar
  • 1
3 votes
8 answers
1k views

Relationship between bel and decibel

Bel is a unit of $\log_{10}$ of ratio of two quantities. $$1\,\text{bel} = \log_{10}\frac{P_1}{P_2}$$ On Wikipedia it says: $1\,\text{decibel} = \frac{1}{10}\,\text{bel}$. According to this definition ...
Dinesh Katoch's user avatar
0 votes
1 answer
69 views

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 ...
Samael's user avatar
  • 43
0 votes
1 answer
67 views

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 ...
user avatar
-1 votes
1 answer
79 views

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 ...
elechris's user avatar
0 votes
0 answers
48 views

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 ...
DSP_CS's user avatar
  • 111
1 vote
2 answers
76 views

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\...
Shoes's user avatar
  • 100
1 vote
0 answers
66 views

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 ...
Harjot Dhillon's user avatar
1 vote
2 answers
81 views

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 ...
Moon_Hawk77's user avatar
0 votes
2 answers
168 views

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"?
Darth Nandan's user avatar
0 votes
1 answer
85 views

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#...
Phil Bouchard's user avatar

1
2 3 4 5
13