# Questions tagged [units]

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

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### 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 ...
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 ...
0answers
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### 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 ...
0answers
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### 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 ...
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) ...
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 ...
1answer
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### 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 ...
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 ...
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 $[v]=\frac{\text{meters}}{\text{seconds}}$ ...
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 ...
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^{-...
0answers
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### 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 ...
1answer
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### 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?
0answers
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/...
1answer
2k views

0answers
46 views