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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 ...
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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 ...
Alvin's user avatar
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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 ...
johan's user avatar
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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"...
null_set's user avatar
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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 ...
San26's user avatar
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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 ...
Vignesh's user avatar
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Rescaling time in differential equations

On a scientific paper, I found the following equations about a compass gait (one leg behaves like an inverted pendulum, the other one as a simple pendulum; $\theta$ and $\phi$ are time-dependent): $$ \...
Federica Guidotti's user avatar
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Applicability of the $\frac{1}{4\pi}$ pre-factor for Coulomb Attraction in Nanocrystals

I am currently working on a thesis and stumbled upon a seemingly formal issue which I was not able to resolve myself. In short The classical coulomb energy between two charges (let's say, electrons ...
Alex Schmitz's user avatar
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What are natural units?

I need to make a presentation on natural units. My professor asked me to visualize a world where $c$ and $\hbar$ are actually equal to unity. Like, what are the consequences? I also want to know the ...
Shrishti's user avatar
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Convert Coulomb's law in CGS units to SI units

I recently translated the appendix to an electromagnetics text from 1945 into English. Now the client is asking me to update the formulas (I studied electrical engineering). The formulas use CGS units ...
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How were angles measured in ancient times?

What is one degree (angle measurement)? I know that we calculate one second using atomic clock, one kilogram using Planck's constant, one meter using speed of light but how do we define one degree? If ...
Payal Payal's user avatar
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Why can't we have accelaration in the form of meters per minutes*seconds?

I have been pondering over a concept regarding the general formula for acceleration, which is widely accepted as $\frac{m}{s^2}$. This formula elegantly illustrates that a given mass or body is ...
Sambhav Khandelwal's user avatar
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The Principle of Homogeneity of dimensions states that you can add,subtract quantities with same dimensions but we cannot add a constant with an angle

Both a constant and a plane and solid angle are dimensionless ie they have the same dimensions , so according to principle of homogeneity should you not be able to equate them ? But it would be absurd ...
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What are dimensions and how are they defined? [duplicate]

We all study dimensions as a topic in physics in which we are taught the dimensions of different physical quantities but I don't understand what is the connection between the things that we study ...
ANSH TANDON's user avatar
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Dimensions of constants

We know that pure numbers are dimensionless then how come universal constants like the gravitational constant have a dimension cause they are also equal to some numerical value and if the numerical ...
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Confusion regarding magnitude of units in Electronics

I'm currently studying electronics by myself using the book of Introduction to Electronics, fifth edition, by Earl D. Gates and have reached the topic of Power, in it there's a problem where I have to ...
nooneofinterest I's user avatar
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Units of Poynting vector (or flux of energy) and field intensity

In optics, the Poynting vector of an EM wave is $$\boldsymbol{\mathcal S}=\frac{c}{4\pi}\,\mathbf E\wedge\mathbf H$$ and if I am not mistaken its units in the SI system should be $\mathrm{W/m^2}$, or ...
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Why is the general unit for energy (in terms of energy bills) $\rm kWh$? [closed]

Why is the unit for energy (in terms of energy bills) expressed as $$\text{time} \cdot \frac{\text{energy}}{\text{time}}$$ rather than just energy? Wouldn't it be better to express it in megajoules (...
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Are there any non-SI base units? [closed]

Are there any base units that are not in the SI system? If so why aren't they? Edit: By this I don’t mean other systems that you can convert to (ie. Feet, Ounces, Hours, etc) I mean new base units ...
Gabriel Tellez's user avatar
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Logistic Growth and the Use of Changing Units

I am having a hard time understanding how this unit change is used to get rid of the need to use specific values for $ N $ and $ \mu $. Could somebody explain? I just can't figure it out. Help would ...
j.primus's user avatar
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Why is the term 'Accuracy of 1 part in $x$' used?

My question stems from how we measure the accuracy of Caesium clocks. Most Caesium clocks are said to have an accuracy of '$1$ part in $10^{14}$.' I understand that the terminology means to convey ...
Smarika Singh's user avatar
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Scale dependent density growth in sub-horizon scales

In standard cosmology, we use \begin{equation} \ddot{\delta} + 2 H \dot{\delta} - 4 \pi G_N \rho \delta=0 \tag{1} \end{equation} to study the structure growth in sub-horizon scales. However, at the ...
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Interseting question regarding a variant of vernier calipers

So our professor asked us to come up with interesting questions on vernier calipers as he taught us about them today. Obviously I could have Googled some up and gave it to him however for my own sake ...
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How to think of the unit $\rm eV$?

How to get a sense of $\rm eV$? I mean when I know how much a metre is or a second is, but how to "visualize" when it is said atomic reactions are in order of $\rm eV$ and nuclear reactions ...
Questioningmind's user avatar
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Schrodinger equation with $\hbar =1$

The Schrodinger equation is given by: $$i \hbar \frac{d}{dt}|\psi(t)\rangle = H(t)|\psi(t)\rangle.$$ Sometimes, physicists set $\hbar=1$. I suppose that they achieve this by changing the scaling and ...
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What are the values of the (photopic) CIE 1924 luminous efficiency function?

Apparently, the luminous flux and therefore indirectly the lumen (lm), one of the 7 basic SI units, is defined in terms of "the" 1924 luminous effiency function $\overline{y}(\lambda)$ of ...
BlenderBender's user avatar
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4 answers
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How do units work out in logarithms? [duplicate]

In a problem I am doing, it boiled down to an integral that resulted in $$\ln(x+1\text{ m})\Big|_{x=0\text{ m}}^{x=3\text{ m}}$$ this is basically just $${\color{red}{\ln(4\text{ m}) - \ln(1\text{ m})}...
Captain Chicky's user avatar
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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.
AlexGenesis's user avatar
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217 views

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 ...
Stanislav Bashkyrtsev's user avatar
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4 answers
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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 ...
soulfourier's user avatar
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3 answers
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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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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 ...
Paweł J's user avatar
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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: ...
Aadarsh's user avatar
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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 ...
Markoul11's user avatar
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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,...
Yif's user avatar
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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,...
Adam Haun's user avatar
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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 ...
Razz's user avatar
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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 ...
fmg's user avatar
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8 answers
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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 ...
Dinesh Katoch's user avatar
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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% ...
nevedhya's user avatar
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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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When a laser pulse rate is stated, does that include the time that it is off? If pulse rate is 1,000 per second, is it on for half a millisecond?

Sorry if this seems a stupid question to some, but I am having trouble finding a consistent answer.... The European XFEL hard X-ray headline proudly states that it is currently the fastest of its kind ...
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