# Questions tagged [units]

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

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### 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 ...
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### Different Bekenstein bound equations – what’s the difference?

Can someone help me understand the difference between the Beckenstein bound equations that I’ve come across? They all appear to have different dimensions. I’ve been told that if you include the ...
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### Absorption and emission spectrum terminology

Hi everyone recently i was looking for laser crystals and since I am an autodidact i was confused by the terminology and units of the optics domain. And specialy by the absorption and emission ...
1 vote
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### Intensity of light - questions about the particle standard definition and concerning the units [duplicate]

In the $5.111$ MIT class, lecture $3$, it is written on the last slide "intensity of light = photons per second". Intensity is previously defined in the lecture as the square of the ...
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### 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 ...
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### 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 ...
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### Why different references express mass densities as $\mathrm{MeV}/\mathrm{fm}^3$ or $1/\mathrm{fm}^3$?

I was reading some papers about equations of state and have found that some of them express the mass density in terms of $\mathrm{MeV}/\mathrm{fm}^3$ and others in terms of $1/\mathrm{fm}^3$. Why is ...
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### Gravitational Constant with ENM Units?

To give some context, there's a conspiracy 'theory' that I saw called Electric Universe that says that gravity is not a fundamental force and instead is a "incoherent dielectric acceleration"...
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### 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 ...
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### Does changing units affect Significant Figures

Does changing the unit of a physical quantity has no influence whatsoever on the number of significant figures? Suppose the mass of a particle is given as 0.060kg 0.060 has 2 significant figures as ...
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1 vote
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### Relation between Coulomb's law and Fine-structure constant

There should be some relationship between them due to the nature of FSC, but I could not find anything about it.
1 vote
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### Are we allowed to cancel the units of a derivative?

Since the volume of a sphere $v(r)=\frac{4}{3} \pi r^{3} \left[m^{3}\right]$, its derivative relative to the radius is: $$\frac{dv}{dr} =4\pi r^{2} \left[\frac{m^{3}}{m}\right]$$ Which is also a ...
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### Can there be two different physical units (or dimensions) for a same physical quantity?

I was going through this book "Nonlinear optics" by Robert W. Boyd for my postgraduate subject in Nonlinear optics and I came across the different orders of nonlinear susceptibilities. From ...
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### When we apply these concepts to physics, where do we put the UNITS in vector spaces and manifolds? Do units have a clear mathematical meaning?

We know that the space of all displacements is a vector space. The vector space is defined as a mathematical object $(V,k,+,\cdot)$ such that it satisfies the 8 properties, where $k$ is a field. We ...
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1 vote
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### Why is $G*M_{sun}=4\pi^2$ when using AU/year units?

So, when using AU/year units, it turns out that 3rd Kepler Law: $\frac{r^3}{T^2}=\frac{G*M_{sun}}{4\pi^2}=1$, meaning $G*M_{sun}=4\pi^2$, any easy explanation for this? Cheers.
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### 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
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### 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 ...
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1 vote
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### 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: ...
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### Unit in unit area

Pressure is defined as force per unit area. What is the meaning of unit and why is the term unit used with area?
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### 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 ...
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### The relationship betwen Equivalent Megatons and Yield

This article defines a concept called Equivalent Megatons (EMT) and gives a formula of calculating it in terms of Yield (measured in megatons). In evaluating the destructive power of a weapons system,...
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1 vote
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### Does the unit "volt-seconds" have any interesting interpretation?

I was contemplating the units of capacitance and inductance in electrical circuit theory and noticed something that bugs me. Although there are many equivalent ways to write the farad (seconds per ohm,...
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### Should power of lens be dimensionless quantity?

My book defines the power of lens using this way: The power P of a lens is defined as the tangent of the angle by which it converges or diverges a beam of light falling at unit distant from the ...
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### Apparatus to measure one ampere

The following definition of the Ampere was codified in 1948 at the 9th CGPM (Conference Général des Poids et Mésures) The ampere is that constant current which, if maintained in two straight parallel ...
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### 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 ...
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### Mass, time and length are measured with 5% accuracy each, what will be the total accuracy in torque? [closed]

I don't understand the solution of the above question, the solution i know is :- Accuracy in torque = [M%] + [2 × L%] + [2 × T%] Which will be equal to 25% I dont understand why 2 was multiplied to L% ...
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### When using the slope or area of a graph to calculate things, do the scales of the units on the axes of a graph need to be the same?

If I want to find the resistance(R) from a voltage(V) vs current(I) graph for a given wire using the relation tan Θ = ΔV/ΔI = R, do the scales on the axes need to be the same, independent of the units?...
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### 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 ...
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