Questions tagged [units]

Units are standards of measurement used for different types of quantities.

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5
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3answers
6k views

Is electronvolt a mass or an energy unit?

I always thought electronvolt was an unit of energy and I knew its definition, but in these days I got some doubts because I saw 2 times it was used as an unit of mass: in my school textbook, in an ...
6
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2answers
976 views

Having trouble weighing the sun

So I gather the way you (and Vera Rubin) calculate a galaxy's mass is by measuring a star's orbital velocity $v$ and its distance $R$ from the galactic center, and then plugging them into this ...
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0answers
29 views

Rulers, Chronometers and Scales [on hold]

Some child questions still makes me wonder why or where is the limit between the classical and regular world, even Einstein's Special and General Relativity, from quantum world, both of them share the ...
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0answers
8 views

Spectral irradiance unit conversion (Jansky)

I'm a bit confused between two units for spectral irradiance, specifically $W/sr/m^2/nm$ and $\mu Jy/"^2$, which is analogous to $W/sr/m^2/Hz$. It seems to me that I need some sort of spectral ...
3
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3answers
4k views

Can anyone explain the unit for rate of expansion of universe?

If you google for 'what is rate of expansion of universe' you get Space itself is pulling apart at the seams, expanding at a rate of 74.3 plus or minus 2.1 kilometers (46.2 plus or minus 1.3 miles) ...
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2answers
137 views

Minimal substitution, four-potential and units

When we make the minimal substitution \begin{equation*} p^\mu\rightarrow p^\mu+\frac{e}{c}A^\mu \end{equation*} the four-potential $A^\mu$ must be proportional to $1/e$ in order to ensure the whole ...
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1answer
59 views

What's the preferred way of writing units per unit? [closed]

Why do we write units such as meters per second as $\mathrm{m\,s} ^{-1}$ instead of $\mathrm{m/s} $. Is the latter still a valid way of expressing units? Why are there two different ways of expressing ...
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2answers
52 views

What is $c$ in Gaussian Units?

Is the speed of light in Gaussian Units just 1 or can it be expressed in metric units? I am getting really confused by all the unit systems. In relativity, for instance, c is often set equal to 1 ...
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5answers
99 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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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 ...
1
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1answer
63 views

Unit conversion in cosmology

In cosmology (Dodelson) , the natural unit is $\hbar=1, c=1, k_B=1 $. There the author writes the critical density of the universe $\rho_{cr}$ as \begin{align*} \rho_{cr}&=1.879 \ h^2\ \times 10^{-...
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38 views

Rescaling the Uehling potential formula

I found that the Uehling potential equation is written in natural units in many books and research papers. But because I’ve to make some calculations with the Uehling correction, the formula must be ...
0
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1answer
58 views

What is the difference between degree celsius and celsius degree? [duplicate]

It seems similar but can you guys please answer it that what is a specific difference between degree celsius and celsius degree?
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49 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/...
7
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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{...
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0answers
11 views

For vibrations in continuous beam, what is the unit of eigenvalue?

I have been solving a fourth order euler bernoulli differential equation to solve for vibrations of a continuous cantilever beam. When I verified for them using Comsol eigenvalue solver, it gives me ...
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1answer
29 views

Adding or subtracting mean absolute error

In an experiment to find the length of a rod, n observations were made. ${a_1,\;a_2,\;.\;.\;.\;a_n}$. The mean (${a_{mean}}$) of those observations will give a value very close to the true length of ...
3
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1answer
93 views

Gauss' law for two point charges and a factor of $4\pi$

When we model electric field as something that represents the "flow" of something that is conserved, we can prove that the flux due to a single point charge, through the following surface, Flux ...
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 ...
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1answer
112 views

Is it possible that “$c = 1$?” [duplicate]

Can c = 1 and if so, what does this mean for Physics? If you were to take any Physics equation, and let c = 1, will it work?
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4answers
625 views

Units in Special Relativity

I have several small questions about units in special relativity: I've read somewhere that $c=1$ can be interpreted as lightyears being out unit of length, and years being our unit of time. But that ...
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0answers
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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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11answers
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What is the logarithm of a kilometer? Is it a dimensionless number?

In log-plots a quantity is plotted on a logarithmic scale. This got me thinking about what the logarithm of a unit actually is. Suppose I have something with length $L = 1 \:\mathrm{km}$. $\log L = \...
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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1answer
242 views

What are the actual electromagnetic units in the foot-pound-second (FPS) system?

So I've been reading this Wikipedia article, and I came across the cited note that someone has derived the electromagnetic units for the FPS system. However, I can't find that publication so does ...
2
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1answer
41 views

Units conversion - quanta/cm$^2$/s and photons/cm$^2$/s

Can you tell me if there is any difference between these 2 units? $\log \mathrm{quanta/cm^2/s}$ vs $\log \mathrm{photons/cm^2/s}$ Thanks
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3answers
105 views

Is there a better definition of magnetic field than this?

It may seem a trivial question but the definition of the magnetic field in everyday books is misleading. "It is the region or area around a magnetic material in which its magnetic force can be felt." ...
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3answers
20k views

273 + degree Celsius = Kelvin. Why 273?

Temperature conversion: $$273 + \text{degree Celsius} = \text{Kelvin}$$ Actually why is that $273$? How does one come up with this? My teacher mentioned Gann's law (not sure if this is the one) but ...
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2answers
160 views

Visualizing Physical Units in Phyiscs

I do best in physics when I can make sense of the units that accompany values, and I do this by visualizing in my mind what is happening. Take for instance, $v=\frac{s}{t}$. When I think of velocity I ...
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0answers
34 views

Empirical determination of masses and charges of a set of objects using only kinematics experiments

Suppose you have $n$ physical objects that you want to determine the mass and charge of. You do not have access to any reference object with known mass and charge (that also includes things like the ...
59
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2answers
3k views

What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?

The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the ...
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2answers
11k views

How does the Fourier Transform invert units?

I don't really understand how units work under operations like derivation and integration. In particular, I am interested in understanding how the Fourier transform gives inverse units (i.e. time ...
3
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1answer
150 views

Black Hole Entropy Calculation

I was watching "Leonard Susskind on The World As Hologram" ( youtube ). At one point he describes the way Bekenstein calculates the entropy of a black hole. Paraphrasing: Take a minimally sized black ...
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0answers
20 views

Units after integrating a ratio metric: Point Source Transfer Mobility

I want to calculate the so called Line Source Transfer Mobility (LSTM) from 10 equispaced Point Source Transfer Mobilities (PSTM). The PSTM measures the response to an excitation, and it is defined ...
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0answers
70 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 ...
0
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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 ...
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1answer
88 views

Why $µ_0$ and $ε_0$ are not considered to be $4π$ and $1/4π$? [closed]

I did some algebra... In Planck unit, if make $\mu_0 = 4\pi$ and $\epsilon_0 = \frac{1}{4\pi}$ you get: $$\mu_0 = 4\pi \cdot \frac{m_p l_p}{t_p^2 I_p^2} = 1.2566368452237765 \cdot 10^{-6} N \cdot A^...
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4answers
2k views

The significance of kelvin as a unit of absolute temperature

$$PV=nRT$$ Where $P$ is pressure in pascals ($\text{Pa}$) $V$ is volume in cubic metres ($\text{m}^3$) $n$ is amount of substance in moles ($\text{mol}$) $R$ is the gas constant having units, joules ...
13
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2answers
3k views

Why is the length of the Kelvin unit of temperature equal to that of the Celsius unit? [duplicate]

The Celsius unit is arbitrarily defined, based on the boiling and freezing point of water. Is it a coincidence, then, that the SI unit of temperature Kelvin, which is used in all natural equations, ...
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 ...
0
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1answer
40 views

What units are represented by pr - micrometer?

In studying column abundances, I often encounter the units $pr-\mu m$. What does this stand for? Part per micrometer? how can this be an abundance unit? Incredibility I cannot figure out what it ...
0
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1answer
47 views

Question about change SI unit, what is $h$ and how could they agree with anything? [closed]

I heard a speech about the changing SI unit, but then it got me concerned with a bunch of questions. Basically, they switched from sticks to some "fundamental constants". What is $h$? In the Metric ...
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6answers
167 views

Conversion of $R = 8317\ \mathrm{\frac{J}{kg\cdot mol\cdot K}} $ to $ 8317\ \mathrm{\frac{J}{kg\cdot mol\cdot {^\circ C}}}$

I'm trying to understand the unit conversions when one unit is obtained by shifting the value of another one by some constant. In particular, the unit conversions from Kelvin to Celsius scale: $$T(\...
27
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3answers
20k views

Why does the metric system use “kilogram” as a base SI unit?

SI system uses all (that I know) measurement basic units as 1 (single) instance: meter, second, ampere, etc, except the KILOgram. It already defined with 1000 multiplier (kilo). It prevents from ...
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2answers
2k views

The position of a particle at any time $t$ is given by $S = V0/a [1-e^{-at}]$. What are the dimensions of $a$ and $V_0$? [closed]

To find the dimensions of and V0, I must know the dimension of S and e. So I want to know it.
0
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1answer
180 views

What units for calculating wavenumber of a matter wave?

So I have a problem where I need to calculate the wave number for proton of energy $40~\rm MeV$. I know the formula for wavenumber of a matter wave is $k = \frac{\sqrt{2mE}}{ \hbar}$. But what units ...
5
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3answers
2k views

Why didn't we replace our SI units with a better system? [closed]

Intro It seems to me that the SI units we use today are nothing but the result of a historical 'coincidence'. I recently began researching about natural (absolute) systems of units, which are ...
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?
0
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3answers
392 views

Why does the angular speed formula end up in radians per second?

So, in my homework I am given the radius and also the tangential speed $v$, the measurement for radius is meters; the measurement for $v$ is $m/s$. I don't understand how by after calculating the RPM ...