Questions tagged [dimensional-analysis]
Dimensional analysis means to obtain results by analyzing the units in question, etc. DO NOT USE THIS TAG if your question is about degrees of freedom or spatial dimensions.
677
questions
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42 views
Physical quantity of energy and it's dimension in the article
I am reading an article about Permutation glass by Mobolaji Williams and not sure about the unit (value/dimension) of energy on Fig.3. If $k_BT_c$ is measured in Joules, then I am not totally sure ...
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2answers
80 views
Does “phase” in physics always have to expressed in radians or degrees?
According to this post
What is a phase of a wave and a phase difference?
The phase of the wave is the quantity inside the brackets of the sin-function, and it is an angle measured either in degrees ...
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5answers
100 views
Units of variable acceleration
If we have a function, which describes, how a displacement in space along a line varies as a function of time: e.g.: $s(t)=vt$, its units are meters because $[v]=\frac{\text{meters}}{\text{seconds}}$ ...
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0answers
46 views
Dimension of the candela unit: What does J stand for? [migrated]
The J symbol can represent the unit of energy but it's also the symbol for the dimension of the candela (or luminous intensity).
For the energy unit, it clearly comes from the family name of the ...
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0answers
50 views
How do we move from formula of coupling constant alpha_strong(energy) to alpha_strong(distance). Just using energy=hc/distance?
How do we move from QCD strong interaction formula of coupling constant alpha_strong(energy) to the expression as a function of distance : alpha_strong(distance). Is it just by using energy=hc/...
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1answer
2k views
Why revolutions (or turns) are dimensionless?
I think that the reason is because one revolution or one turn is equal to $2 \pi$ rad or to $360$ degrees.
We can relate rads and degrees to two units of length that cancel each other.
rad $= \frac{...
2
votes
1answer
69 views
Reynolds number in a wind tunnel
I would like to understand how to calculate the Reynolds number in the test chamber of a wind tunnel.
It is known that the Reynolds number for a pipe is:
$$Re = \frac{UL}{\nu}$$
where
$U$ is the ...
0
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1answer
112 views
Undoing problems caused by setting $c = 1$ { or “Undoing $c = 1$” }
In the mathematical derivation of equations for physics, and involving wave propagation in particular, the propagation speed at the start of the derivation is often set to one (c = 1).
I am working ...
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2answers
186 views
Why is the equation $E=mc^2$?
The equation $E=mc^2$ never made any sense to me.
c is a constant (speed of light), therefore c squared is also a constant. We're not specifying any units so surely the equation should be reduced to ...
0
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1answer
34 views
Dimensions in the logarithmic form of the Arrhenius equation [duplicate]
The logarithmic form of the Arrhenius equation is:
$\displaystyle\ln k=\ln A-\frac{E_a}{RT}$
Here $k$ and $A$ have dimensions whereas $\displaystyle\frac{E_a}{RT}$ is dimensionless. In other words, $...
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0answers
24 views
Units and measurement
I read on the characteristics of units
(i) It should be easily understandable.
(ii) It should be changed with change in physical factors.
(iii) It should not change with place or time.
I want ...
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1answer
53 views
Reference request: Oldest dimensional analysis books with exercises?
Per the title, what are some of the oldest dimensional analysis books out there with unsolved exercises? Maybe there are some hidden gems from a long time ago out there.
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1answer
32 views
What is the unit of the current in a square barrier model?
In Quantum Mechanics textbooks, the equation for a electron tunneling through a barrier is
$$-\frac{\hbar ^{2}}{2m}\frac{d^{2}}{dx^{2}}\psi \left( x\right) +U\psi \left(x\right) =E\psi \left( x\right)...
2
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0answers
46 views
Can we get the values of $G$, $h$ and $c$ to be numerically equal if we use a convenient system of measurement?
The speed of light in vacuum is approximated to be $3×10^8\ ms^{-1}.$ But, if we change the units, we can get a different number. For example, it won't be $3×10^8$ if we used $ft\,s^{-1}$ instead of $...
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0answers
71 views
Dimensionally inconsistent!
This equation doesn't seem to be correct. Dimensionally inconsistent, in fact! How then, is it established? $$s_{t}=v_{0}+\frac{1}{2}a(2t-1)$$ (In case you don't know, this is the equation used to ...
1
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1answer
76 views
Question about code units and physical units (hydrodynamics simulations)
I'm working on a code that implements smoothed particle hydrodynamics (SPH) method for solving the equations of magnetohydrodynamics (MHD) with self-gravity.
In research papers regarding existing ...
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4answers
79 views
Dimensional analysis with derivatives, logs, exponents and trigonometric functions
How should we do dimensional analysis when we have derivatives, logs, exponents and trigonometric functions in an equation. Should we assume that the operands are pure dimensionless numbers? Coming ...
2
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0answers
50 views
Existence of interacting scalar field theory
I saw a comment in Schwartz's introductory text on Quantum Field Theory (cf. Section 14.5) that it is known that $\phi^4$ theory in five dimensions does not exist. In four dimensions it is not known, ...
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2answers
53 views
Can you explain the meaning of base units behind the unit gray?
The gray is a unit that measures energy absorbed per unit of mass. It's defined as one Joule per one kilogram. Which can be simplified to $m^2/s^2$.
Can you explain the meaning (interpretation) of ...
3
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2answers
61 views
Would setting the ideal gas constant to $1$ yield an attractive natural temperature scale? [closed]
In this recent question, there was a comment 'The "zero point" of Kelvin is natural, but the scale is not'. This led me to wonder whether setting $R = 1$ in the ideal gas law would be an attractive ...
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1answer
98 views
How to calculate fractal dimension by fitting on a log-log plot?
I have simulated a DLA pattern by MC method and the data is for fractal dimension. The right column is the number of particle N(r) into radius r and the left column is the radius r. I plotted a log-...
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0answers
17 views
Units of the Klein-Gordon Propagator in SI Units
What are the SI units of the momentum-space propagator of the Klein-Gordon equation for a free particle?
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1answer
39 views
Why is angles given at the end of fundamental units? [closed]
In my study material, there is a chart of the fundamental units. In that solid angle and angle are separated by a line from the other fundamental units. What is the reason for that?
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0answers
8 views
About irrelevance in dimensions & about absorptive and spectral absorptive power
I have read that spectral absorptive power(a) is a dimension less term and total absorptive power(a) is just a ratio hence it is also dimension less. But I have also seen expression in many books that ...
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1answer
106 views
Is there a physical observable with the same units as $c/G$?
Dividing the speed of light $c$ by the gravitational constant $G$
yields the dimension mass*time/area or mass/(length * speed)
Is there a physical quantity used in textbooks with this dimension? I ...
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0answers
19 views
What Dimensionality Reduction method I can follow on a dataset that has Physics Parameter?
I am trying to model data related to Locomotive Train. We have a various set of parameters and we have the possibility to generate a few more parameters from this. Our model is currently using a lot ...
1
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1answer
119 views
Why coupling constants with negative mass dimensions lead to non-renormalizable theories?
can somebody explain or point to the relating mathematics showing Why coupling constants with negative mass dimensions lead to non-renormalizable theories?
1
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1answer
55 views
Problem in the continuum limit of a Kronecker delta
I am having troubles in understanding how to correctly perform the continuum limit of a double sum containing a Kronecker delta.
Imagine to integrate a function depending on $t$ and $t'$, both ...
2
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1answer
73 views
What is the dimension of the weak gauge field couplin constant NOT in natural units?
What is the dimension of g and g', NOT in natural units, but in terms of mass, length, time, and permittivity?
4
votes
2answers
125 views
Why does electrical resistivity have units of $\Omega \cdot \mathrm{m}$ rather than $\Omega \cdot \mathrm{m}^3 ?$
Electrical resistivity has units of $\Omega \cdot \mathrm{m} .$ However, since resistivity can be described as the resistance of a unit cube, shouldn't the units therefore be $\Omega \cdot \mathrm{m}^...
3
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1answer
74 views
What are the units of dark energy?
Popular literature seems to equate dark energy with the cosmological constant of the Einstein field equations. We know however that the dimensions of any constituent of the field equations is ${\rm ...
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1answer
55 views
Why can we multiply different units but not add them? [duplicate]
Like units can be added together or, subtracted from one another. However, multiplication and division of units does not have such boundations.
multiplication is just repeated addition, similarly ...
2
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1answer
56 views
Why does the universe manifest scale?
I'll try outline my question in clear terms, articulating specific aspects that are its primary motivators. I'm just beginning in my exploration of physics as a student, but a persistent question that ...
4
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1answer
70 views
Size of a raindrop
Thinking about the fact that raindrops come with a typical size I was wondering how this can be determined.
I am pretty sure that the friction with air and the quantity of water in the clouds are ...
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0answers
58 views
What's does square root of number of atoms mean?
Number of atoms $N$ was counted in 3 dimension. $x,y,z$.
However, when calculate it, i.e. in many cases such as refractive index , people take the square root of it, i.e. in reflective index $n=\sqrt{...
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0answers
27 views
How to introduce dimensionality in a dimensionless framework?
This question is an extension of this one. I have been told that to introduce dimensionality in a dimensionless quantity I need to multiply with suitable parameters. For instance, for velocity I have ...
3
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2answers
86 views
How to find the corrsponding expression after working with natural units $\hbar=c=1$?
If one does long calculations in natural units how does one find the right expression in let's say SI units in the end?
I know that natural units make the calculations easier and also help to show ...
2
votes
1answer
75 views
Electrostatic in 2D: dimensional analysis
After reading this very interesting post about the electric field and the electric potential of a point charge in 2D and 1D, I've understood that, for the $2D-$case, the following formulas hold:
$$
\...
1
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1answer
23 views
How to deal with motion on a 2-D lattice in terms of dimension?
I am reading a paper titled: Random walks of molecular motors arising from diffusional encounters with immobilized filaments. There the authors consider the molecular motor moving on a 1-D protein ...
6
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3answers
145 views
Why does $\sqrt{\frac km}$ represent angular velocity and not frequency?
When I break down $\omega = \sqrt{\frac km}$ (angular velocity for a simple harmonic oscillator) into its units, I get:
$$\omega = \sqrt{\frac{kg * \frac {m}{s^2}}{kg *m}}$$
which simplifies to:
$$\...
1
vote
1answer
33 views
Is cross-sectional area over length classified separately from length?
The ratio of cross-sectional area to length (or its reciprocal) appears in several formulas, including those for electrical resistance and capacitance in terms of the resistivity and permittivity. ...
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4answers
108 views
Do smaller aircraft have lower take-off speeds?
Assuming there are two aircraft, each of the same density and each the same shape, am I correct in understanding that the smaller aircraft would have a lower take-off speed? I have explained how I ...
3
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2answers
94 views
Are $2$ and $1/2$ universal constants? [closed]
For example, if the equation for energy were:
$$E = mc^{2.713397972993}$$
clearly $2.713397972993$ would be a universal constant.
And in the Einstein field equation:
$$R_{\mu \nu} - \tfrac{1}{2}R \...
1
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1answer
60 views
Are constants derived or calculated?
I am currently writing up a lab report on the determination of Planck's constant using x-ray diffraction and atomic spectra.
In my introduction, I am talking about the history of Planck's constant, ...
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2answers
72 views
Why the quantities are dimensionless in curves plots?
In a lot of plots they use dimensionless quantities, why really we don't let quantities in their physical dimension and plot the curves normally.
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1answer
137 views
Why are angular velocity and angular frequency not measured in Hertz?
Recently, I was doing my homework and I found out that Angular Velocity and Angular Frequency can be calculated using $\omega=v/r$. This means the units of angular velocity and angular frequency are (...
0
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1answer
87 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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3answers
154 views
Do scalar quantities have magnitude only?
I've heard that vector quantities have both magnitude and direction but I've never heard that scalar quantities have magnitude only. Magnitude of vector quantities cannot be negative but what about ...
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1answer
62 views
Why do the units in the period of a mass-spring SHM not work out? [closed]
I am a high school physics teacher having students use the period of a mass-spring system with a known mass to determine the spring constant. We are practicing linearizing functions, so rather than ...
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1answer
23 views
Is (velocity=angular rotation*radius) dimensionally homogenous? [duplicate]
I have been driving myself mad trying to prove it one way or the other, I understand how it is derived and how to use it etc. but it still seems to me to be saying that (m/s)=(rad/s)*(m) which I don't ...
