Questions tagged [units]
Units are standards of measurement used for different types of quantities.
1,186 questions
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Light radiation intensity equation [closed]
Is there an equation to convert Light radiation intensity from W/m² to Lux?
Note: The light source is the sun throughout the day (its intensity will vary over the day).
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SI Units for areas and volumes [closed]
Have different units of measurement for areas and volumes ever been seriously proposed to the BIPM?
Something like
$(\text{cm})^2$ instead of $\text{cm}^2$
or
$(\text{mm})^3$ instead of $\text{mm}^3$
...
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What would the unit for the slope of a graph where the $x$-axis is mass and the $y$-axis is the drag coefficient?
I have written a paper on the effect of change in mass on the drag coefficient of a ping pong ball. While making my last calculations, I decided to use the slope of the final graph (where the $x$ axis ...
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Do all the geodesic equation of Schwarzschild metric should have the same unit?
I am plotting the rotation of Earth around the sun by using Schwarzschild Geodesic equations considering that $c$ is different than 1.
My question is: Should the geodesic equations have the same unit? ...
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Operation of Units in Vector Quantities [duplicate]
If we consider torque as a cross product of the force and radius vectors (pointing outwards).
$$\overrightarrow{𝜏} =\overrightarrow{r} \times \overrightarrow{F}$$
Where the force is measured in ...
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Why can't the mutual attraction between two parallel AC currents (in the same phase) be used to define an ampere for AC currents?
I encountered this text while reading about AC currents:
Both alternating current and direct current are measured in amperes.
But how is the ampere defined for an alternating current? It cannot be
...
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Dimensions And dimensional Formula
I was recently studying about dimensions and I am stuck at a question that why Angles and exponents are dimensionless?
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How can I equate two different ways to write $\Delta G$ if they aren't even the same units?
So $\Delta G$ can be written as $-RT\ln(K)$ and another way is $-nFE$. What I don't understand though is how they can be equated to each other when $-RT\ln(K)$ yields J/mol and $-nFE$ yields J.
How ...
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"Kilogram" confusion: If you weigh a stationary object in "kilograms" and divide the result by about $9.8$, does that tell you the mass of the object?
Update: I appreciate all of the answers very much
In a way, I'm delighted to find that such a simple question has generated so much disagreement (most of which can be attributed to issues with wording,...
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Why is amount of substance considered a fundamental quantity? [duplicate]
The unit of amount of substance is mole. 1 mole is defined as the amount of substance that contains Avogadro Number (6.022 x 10^23) no. of entities in a substance. It is a fundamental unit.
However, ...
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Units and dimensional analysis
I was reading a paper about "Radio emissions from pulsar companions" by F. Mottez and P. Zarka (https://arxiv.org/abs/1408.1333) and stepped over equation (16) about the relativistic ...
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Why different versions of the Schwarzschild metric online?
I started studying the Schwarzschild metric and have seen multiple versions of the metric online; specifically different versions of the Schwarzschild radius. In some cases it is $\frac{2GM}{c^2}$, ...
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Can we measure $c=1$?
Usually the speed of light is measured using the S.I. units, yielding a result in $m/s$.
In geometric/Planck/Natural units it is known that we take $c=1$.
Clearly this is just a redefinition of the ...
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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 ...
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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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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):
$$
\...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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.$$
Sometimes, physicists set $\hbar=1$. I suppose that they achieve this by changing the scaling and ...
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What are the values of the (photopic) CIE 1924 luminous efficiency function?
Apparently, the luminous flux and therefore indirectly the lumen (lm), one of the 7 basic SI units, is defined in terms of "the" 1924 luminous effiency function $\overline{y}(\lambda)$ of ...
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How do units work out in logarithms? [duplicate]
In a problem I am doing, it boiled down to an integral that resulted in $$\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})}...
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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.
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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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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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