Questions tagged [units]
Units are standards of measurement used for different types of quantities.
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Are there any units of measurement (other than the ohm) that are represented by non-latin characters?
We have the prefix μ and the unit Ω, but other than that we seem to not use any other greek letters (or any non-latin symbols for that matter) for specifying units.
All other units – meters, seconds, ...
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1answer
61 views
How to define “unit spatial length” in general relativity?
Suppose I have a metric with time-independent $g_{\mu\nu}$. Suppose I have a small measuring rod at some spatial position and I define it to be of unit length. Will it still have unit length when it ...
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1answer
29 views
How does a proper motion displacement of $x$ arcseconds/year mean a star is moving at $x$ AU/year?
I trying to understand the author's answer to this question:
I don't understand the part underlined red. How does 3.6pc mean 3.6AU? It says a "distance of 3.6AU", but doesn't say what is ...
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0answers
7 views
Is it possible to extract real physical values from multiphase LBM simulation with Shan-Chen force?
Is it possible to extract real physical values from multiphase Lattice Boltzmann method simulation with Shan-Chen force?
SC force is of the form
$$\vec{F^{SC}} = -\psi(\vec{x}) G \sum_i w_i \psi(\vec{...
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0answers
21 views
Derived units described with multiple 'per's
In a dimensional analysis problem, a rate was given textually as so many "Milliliters per Kilograms per Day". This seems ambiguous, as one could either interpret it as
A:: Ml / (Kg / Day) = ...
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Units a/b/c Notation
I have come up a few times with the following notation:
10kWh per area per day = 10 kWh/A/day
Is this the same as: (10kWh/A)/day = 10kWh/(A $\cdot$ day) ?
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1answer
37 views
Trying to calculate slope from a centripetal force vs. speed squared graph. Can someone help me with the units? [closed]
So I calculated the slope and it was:
5.23 N/m2/s2
How exactly will the units work out?
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1answer
34 views
Meaning of a product of two physical units [duplicate]
I am having a hard time to understand the meaning of measurements that involve a product of physical units, like $10\, \rm N\cdot m$ ($10$ Newton-meters), $3.33\, \rm J\cdot s$ ($3.33$ Joules-second),...
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Why did the scientific community stop using hydrogen as the main measure for the atomic mass unit?
The only thing I could find is that the community switched form one hydrogen atom to 1/16 of Oxygen-16 (which they thought to be the only isotope that exists at time because they had no concept of ...
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1answer
57 views
Understanding the meaning of the integral of energy of an $\rm H$ atom
This is probably very basic but my notes are confusing and not clearly written so I would appreciate some help in trying to clarify the following points:
If we consider the expression $$\left\langle \...
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1answer
40 views
How to express the force of a GPU in Pound-hours?
I am training a neural network on a small cluster of GPU-equipped computers. Out of curiosity, I would like to express the force of the running GPU's in terms of the force required for a car to remain ...
0
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1answer
63 views
Hz units vs RPM units
I have seen Hz defined in different ways, if
$$\Omega = 1 \ \text{Hz} = 1\frac{\text{cycle}}{\text{s}} $$
Here $1 \ \text{cycle} =$ a full turn of the rotor $= 2\pi$
If I wish to express this a rpm:
...
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2answers
69 views
Converting energy from units of joules into units of wavenumber
If we have an expression for energy $E$ in Joules why is it that if we wish to convert the energy into wavenumber we divide the expression by $hc$?
I know that $$E = \frac{hc}{\lambda}$$
So surely ...
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0answers
23 views
Potential energy and atomic units in the Bohr model
I have a question regarding the final sentence written in the solution to part iii) (found below) for the question given below.
In the Bohr model of the hydrogen atom, the radius of the electron ...
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0answers
20 views
Formalism for Unit Consistency
I was thinking about how the propagation constant for Transmission Lines looks like this:
${\displaystyle \gamma =\alpha +i\beta}$ , and here the units for $\beta$ is $\frac{radians}{meter}$ and the ...
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2answers
76 views
How do scientists derive equations and formulas? [closed]
How do scientists derive equations?
For example, how do they know that:
Work done = Force × Displacement
And why is it always multiplication and not anything else? Why not addition?
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1answer
23 views
What is a $n_\text{eq} / \text{cm}^{-2}$?
What is "one neutron equivalent per centimeter squared"? I know it is a measure of accumulated radiation, or similar, but could not find precise definitions in Google.
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2answers
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Why is light wavelength measured in nm?
Of course you can convert it into metres if you wished, but photon energy is measured as such:
$\Delta E=\frac{hc}{\lambda}$
In this scenario, Energy is measured in electron volts. But if we measured ...
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4answers
172 views
Multiplication allows for different units: why can't we multiply apples by apples?
In school, we are taught that addition must use the same units (we can't add 3 apples + 4 bananas). On the other hand, multiplication (and division) is allowed between quantities of different units, ...
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1answer
70 views
Lorentz transformation in system of unit [closed]
In a system of units in which the velocity of light $c =1$,
$$x' = 1.25x - 0.75t$$
$$y'= y$$
$$z' = z $$
$$t'=1.25t -0.75x$$
is a Lorentz transformation. Why is this so? That is, how can we ...
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1answer
35 views
Non-hypothetical problem of using rulers to measure lengths
Consider two factories on earth's Equator producing rulers of the same length. One factory produces rulers made of wood and the other one produces rulers made of aluminium. Both types of rulers are ...
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1answer
39 views
Units of angular frequency in a simple harmonic oscillator [closed]
The equation of a simple harmonic motion can be
$x=A \cos(\omega t)$.
$\omega$ therefore has units of $radians/sec$.
I was solving some problems when I found a statement on my notes
$x=\left(1+\...
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4answers
116 views
Angular velocity or frequency in an uniform circular motion? (for students of an high school withouts the calculus)
In an uniform circular motion, we know that the angular velocity is:
$$\omega=\frac{2\pi}{T}$$
and your measure is:
$$[\omega]=\frac{\text{rad}}{\text{s}}$$
i.e. revolutions per second (in Italian ...
1
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1answer
56 views
Measuring a speed in Kelvin $K$
Since the Kelvins are a temperature unit and temperature is basically molecule/atoms excitation (so they have a "speed of excitation"), could it be possible to measure a speed (let's say 10 ...
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3answers
495 views
What is the unit for electric flux?
When I was studying for competitive exams, I was told that the flux is unit of electric flux is V-m or Nm^2/C. But now in my UG I am being told that the unit of electric flux is C. When I the checked ...
5
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3answers
212 views
How to properly understand Gaussian Units?
I have trouble understanding Gaussian Units in relation to SI units: $1/c$ factors seem to appear almost at random, while other constant such as $\varepsilon _0$ or $4\pi$ disappear with seemingly ...
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2answers
87 views
Is equation $1/4πε_0$ in Coulombs law a scalar? Plus a question involving Jefimenko's equations [closed]
Is the equation of $1/4πε_0$ in Coulombs law a scalar? Also, does this equation equate to Coulombs constant?
In addition, why does Jefimenko use $H=1/4π...$ in his equations? What is the significance ...
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0answers
24 views
MOS-Capacitor Work function calculation. Units don't match. What am I missing?
I am currently doing some lab work and I am stuck at a single equation that doesn't make any sense to me. I am given the following formula for the work function of Silicon:
$$\Phi_{Si}=\chi_{Si}+\frac{...
0
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1answer
53 views
Why isn't magnetic flux a dimensionless quantity?
I know how magnetic flux is mathematically defined, and it clearly has dimensions of kg m^2 A^-1 S^-2, but I've read magnetic flux being described as "number of magnetic field lines passing ...
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0answers
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Why this expression for depletion width only worked for CGS unit?
An expression of contact potential was given as
$$\varphi_c=\frac{k_B T}{e} \ln \left[\frac{N_AN_D}{n_i^2}\right]$$
and depletion width
$$d=\left[\frac{\epsilon \varphi_c}{2\pi e} \frac{N_a+N_D}{N_A ...
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1answer
36 views
Planck constant in geometrized units? [closed]
How do I calculate the value of the Planck constant in geometrized units?
I cannot find its value anywhere.
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1answer
34 views
Generalised Unit Conversion
I would like to know the general method of converting between units. For example converting Joules to $\frac{g\cdot cm^2}{s^2}$
I assume it's possible because the SI unit of energy is $\frac{kg\cdot ...
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4answers
124 views
Why are we allowed to make algebraic calculation on units
Is there math proof that we can cancel out units in Physics? For example:
$\require{cancel}distance = \frac{meters}{\cancel{second}} * \cancel{second}$.
So we see that seconds cancel out and we left ...
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2answers
39 views
Exponent multiplication confusion
This might be one of the most beginner questions here but I really can't seem to wrap my head around it. How does the exponent change by one? Isn't it supposed to be the same but negative?
0
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1answer
68 views
Units of Poynting vector after Fourier transform
Suppose the electric and magnetic fields $e(x)$ and $h(x)$ with units $\left[\frac{V}{m}\right]$ and $\left[\frac{A}{m}\right]$ and their Fourier transforms $E(k)$ and $H(k)$ with units $\left[V\right]...
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4answers
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Why is 1000 micrometer not a correct representation of a prefix?
Why is 1000 micrometer not a correct representation of a prefix?
I ask because I recently took an entrance exam with a multiple-choice question which went along the lines of
which of the following is ...
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1answer
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Units of energy and frequency
I read a paper about using femtosecond laser to process an atom. The units used in the paper bothered me. I have four questions.
(1) Why the unit of energy is cm$^{-1}$?
(2) It seems that the author ...
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0answers
21 views
How can I determine the value of the electric permittivity my arbritrary unit system?
The unit system
Working with electromagnetic fields, I define a unit system where the unit of length is $l_0$, and the unit of speed is $v_0$. In other words, I introduce a system of semi-normalized ...
0
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1answer
30 views
The units of unified atomic mass
There is something I don't understand about the units of unified atomic mass.
I have seen on Khan Academy that $1$ unified atomic mass ($u$) is equal to $1.6605\times 10^{-27}$ kg.
But after 2 videos ...
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1answer
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Some questions in Newtonian gravitational force derivation
I came across a good website that explains how Newton derived his formula of gravitational force click here
Why is $\frac{k}{4\pi ^2}=\frac{c}{M}$ and $f=(kmM)/r^2$ not $(4\pi^2)/r^2\sqrt{MmcC}$? ...
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1answer
52 views
Unit Analysis Question: Possible to Solve [closed]
A nautical mile is the distance traveled in moving through one minute of latitude. A ship which is sailing at a speed of five knots travels five nautical miles in one hour. Two hundred years ago, the ...
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1answer
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Why does a right-circular cylinder helps reduce surface area of the former International Prototype of Kilograms
I read on Wikipedia that right-circular cylinder shape helps reduce surface area of the former IPK, but could not find an explanation as to why. So how does such shape helps reduce its surface area? ...
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0answers
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What does the derived unit “square coulomb” quantify?
As a square meter (m$^2$) quantifies area and not length, what does a square coulomb (C$^2$) quantify?
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1answer
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Units of $\hat{a}$ and $\hat{a}^\dagger$ in discrete vs continuous $k$ and normalization
Consider the quantization of the electromagnetic field. In the discrete case, given in Wikipedia, the operators $\hat{a}$ and $\hat{a}^\dagger$ are dimensionless $$[\hat{a}]=[\hat{a}^\dagger] = 1,$$ ...
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0answers
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What does luminosity unit “Rayleigh” mean?
I read some articles which adopted "Rayleigh" as luminosity unit, what does it means? How bright is "1 Rayleigh"? How could I convert it to normal unit such as photon$\cdot$cm$^{-2}...
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1answer
47 views
Why not to multiply with $g$ when doing sums in the unit $\rm kgf$? [closed]
My book told me a thing to keep in mind while doing upthrust sums in $\rm kgf$. It's stated as follows: (See point number 2)
Can you please explain why not to multiply with $g$?
Also I am giving ...
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6answers
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Is Nm the same unit of torque as mN?
A couple of days ago, I noticed that the torque unit used by my teachers is $mN$, and while reading on the internet it came to my notice that in all textbooks the official unit is $Nm$.
I asked one ...
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0answers
150 views
When discussing discrepancy and its relationship to uncertainty, how much is acceptable?
I am new to physics and am learning about discrepancy and uncertainty. The below question is a Check Your Understanding Question, DISCLAIMER: NOT HOMEWORK. The way I went about solving the problem was ...
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1answer
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Lorentz Force Dimension Analysis [closed]
A charged particle in the presence of charges and currents experiences
a force due to electric (E) and magnetic (B) fields. Is
is described by the Lorentz force:
$$F = e (E + v × B )$$
where e is ...
0
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4answers
152 views
Units in curvilinear (e.g. polar) coordinate systems [closed]
Let’s say that someone measures the distance from a rocket to the earth's surface to be equal to $h=10[m]$ (meters) at time $t=5[s]$ (seconds) (in a Cartesian coordinate system) - so our vector with ...
