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Questions tagged [units]

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

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9 views

How to find average uncertainty for a set of results?

I have determined multiple values for the refractive index of a material, each with their own uncertainty. If I find the average refractive index, how do I find its uncertainty? Would I find the ...
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1answer
18 views

Units of magnetic field to obtain energy in MeV

Is it possible to use a unit of magnetic field to obtain the value of kinetic energy in units of MeV? For context, I'm handling beta spectroscopy data and the equation is: $ K = \sqrt{ (eBrc)^2 + m_e^...
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1answer
27 views

Why is there no standardization of indentation hardness measurements? [closed]

The last hundred years have seen a great degree of standardization of units, and the expression of units of measurement in standardized, instrument-agnostic, objective terms. So I wonder why this ...
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2answers
34 views

Which luminous efficacy function does BIPM reference?

The definition of candela given by BIPM is as follows: The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the ...
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2answers
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Power loss due to eddy currents

I am curious about estimating power losses due to eddy currents. Looking on Wikipedia I find an expression for power dissipation under limited circumstances, $$ P = \frac{\pi^2 B^2 d^2 f^2}{6k\rho D} $...
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1answer
131 views

Constants of proportionality in Force Equations/ Physics in General

I was in physics class and we were talking about the gravitational constant G (6.67 x 10^-11 Nm^2/Kg^2). The question came up: "Why does $F= (GMm)/r^2$ have a constant of proportionality and not $...
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1answer
22 views

Quantity, kind of quantity, and dimension

The International Vocabulary of Metrology defines a kind of quantity as the following: However, to me NOTE 1 EXAMPLE 1 and NOTE 1 EXAMPLE 2 are contradictory with NOTE 2 EXAMPLE. NOTE 2 EXAMPLE ...
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Is it possible to convert carbon soot/ black carbon in mg to CO2-equivalent? If so, how?

Since global warming is expressed in terms of carbon emissions or CO2 equivalent, can the short lived carbon soot/ black carbon that is produced from incomplete combustion be expressed in terms of CO2 ...
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1answer
256 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 ...
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15 views

What is the difference between absorbed dose and kerma?

They both have the same unit, $\frac{J}{kg}$. They both deal with ionizing radiation depositing energy in a medium. But they differ in which energy is being measured and the particles that deposited ...
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2k views

Check dimensions of the integral of a function

I and a colleague are arguing about the dimensions of: $$\int_0^x f(x) dx $$ in this particular case $[f(x)]=m^2/s^3$ and $[x]=m$. Does it follow that $[\int_0^x f(x) dx]=m^2/s^3$ or $[\int_0^x f(x)...
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1answer
68 views

Units of Fourier transform

Assume a function $f(t)$ which represents the voltage over time and its Fourier transform $\mathcal{F}(\omega)$. The unit of f(t) is [V] and the unit of $\mathcal{F}(\omega)$ will be [V/Hz]. I am ...
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138 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 ...
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13 views

Converting units for N-body simulation for the solar system

I am implementing a test particle integrator for the solar system, now I've gotten to the phase where I want to test the orbits it integrates. I want to start with the Sun and the Earth (massless), ...
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16k views

How to determine Young's modulus of a wire by Searle's method?

The question is: A student performs an experiment to determine the Young's modulus of a wire, exactly 2m long, by Searle's method. In a particular reading, the student measures the extension in the ...
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1answer
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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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4answers
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Why isn't it $E \approx 27.642 \times mc^2$?

Why is it that many of the most important physical equations don't have ugly numbers (i.e., "arbitrary" irrational factors) to line up both sides? Why can so many equations be expressed so neatly ...
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5answers
320 views

Why is there no constant factor in the equation $E=mc^2$? [duplicate]

As I understand, the metric system starts with an arbitrary weight, and calls it $1\, \mathrm{kg}$, then the volume of this weight in water gives the meter. And then the energy is defined based on the ...
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1answer
57 views

What unit system is used in Feynman's Lectures on Physics, Pt. 2

In part II, lecture 18, of Feynman's Lectures on Physics, on Table 18-1 Feynman writes Ampère's law as $$ c^2 \nabla \times \vec{B} = \frac{\vec{j}}{\epsilon_0} + \frac{\partial \vec{E}}{\partial t}. $...
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6answers
1k views

What examples are there of unrelated physical quantities measured in the same units? [closed]

Torque and energy have the same dimensionality, and so logically are measured in the same units (joules). However, it seems more natural to call the unit for the former a “newton-metre”, because ...
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1answer
164 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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2answers
103 views

Addition of Temperature Units [duplicate]

I would like to know how does the addition of temperatures works. In school, we have been taught that we should convert degrees Celsius to Kelvin unit first and then the sum will be converted back to ...
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Is the calculation of $\mu_0 \cdot M$ = Tesla valid?

in the Paper "Static Stern-Gerlach effect in magnetic force microscopy", they estimate: $$\mu_0 \cdot M= 1 \text{ T}$$ with $\mu_0$ in H/m which is the permeability of the free space, and $M$ is the ...
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Why are the units of angular acceleration the same as that of angular velocity squared?

According to this answer, the units for angular velocity squared are $\mathrm{rad}/s^2$. The units for angular acceleration are also $\mathrm{rad}/s^2$. Why is this the case?
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Conversion formula between attenuation coefficient (cm^-1) and (dBcm^-1MHz^-1)

So, I was presented in class the fact that (at least in Ultrasound), attenuation coefficients $\mu$ can be expressed in units $\text{dBcm}^{-1}\text{MHz}^{-1}$ as opposed to the form I was used to; ...
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3answers
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What is the dimensional formula of angular velocity?

I have problem to determine the dimensional formula of angular velocity. My friend said that the dimensional formula of angular velocity is $T^{-1}$. It's come from rad/s, rad is dimensionless, the ...
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1answer
35 views

How is the Time Derivative of the the Electric Field Equal to the Current Density in Gaussian Units?

The microscopic form of Ampere's law with the Maxwell addition in Gaussian units states, \begin{equation} \nabla \times \vec{B} = \frac{1}{c} \left ( 4 \pi \vec{J} + \frac{\partial \vec{E}}{\partial ...
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1answer
55 views

Which unit system is used in the paper: “Multiphoton absorption coefficients in solids”?

I am trying to follow the equations in the paper "Multiphoton absorption coefficients in solids: a universal curve". Here the authors state that they use the following equation for calculating the ...
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3answers
179 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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3answers
68 views

Unit of Pressure - $\frac{N}{m^2}$ or $\frac{kg}{cm^2}$

I am came across Pressure being given with unit $\frac{kg}{m^2}$ at a lot of technical papers I am reading. However, as far as I understand, Pressure is defined as $\frac{N}{m^2}$ which is not same ...
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37 views

What does Planck's Length signify about the reality? How do we know that such a length exists? What is the history of its development? [duplicate]

Wikipedia Says - In 1899 Max Planck suggested that there existed some fundamental natural units for length, mass, time and energy.[5][6] These he derived using dimensional analysis, using only the ...
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2answers
1k views

Are the Amp and coulomb equivalent?

1 coulomb is 1 ampere per second and 1 ampere is 1 coulomb per second therefore they are always equivalent. They would only be different if you changed the time but the units themselves are based only ...
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1answer
111 views

Confusion on the cgs, Heavside-Lorentz and SI system

I am very confused about three measuring systems: cgs, Heavside-Lorentz (never used) and SI ...
0
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2answers
90 views

Unit confusion - How Pressure and Volume combines to Joule

Imagine some helium in a cylinder with an initial volume of $1$ liter ant initial pressure of $1$ atm. Somehow the helium is made to expand to a final volume of $3$ liters, in such a way that its ...
0
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1answer
277 views

Convert units of Empirical Orthogonal Function (EOF) for sea surface temperature data

I have a dataset of daily averaged sea surface temperatures for an entire year. I wish to use an empirical orthogonal function to find the dominant patterns within the data. Consider that my ...
2
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1answer
58 views

How to reintroduce $\hbar$ and $c$ into a formula written in natural units? [duplicate]

I am looking for a way to translate formulas written in natural units into either HLU units or SI units. Seeing the Planck constant and the speed of light would help me understand what is going on. ...
0
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1answer
37 views

Differences charge quantity and electric charge

First of all,my english is not well,so sorry for reading. As a senior middle school from China mainland,I am teaching physics about electri field.I with my workmates,get a problem now.We can not get a ...
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2answers
537 views

Convert cubic feet per second to feet per second

Is there a way to convert cubic feet per second to feet per second. Or in general volumetric flow to velocity? I want to know the time taken by water to travel from point A to point B. I have the ...
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3answers
281 views

Can a quantity have two units?

We know that Force has unit of newton and torque has unit of newton meter. Then if you define the energy, which has same magnitude of work then, $W=Fx$ has unit of Joule ( $J$ ) (or $Nm$ ) while $W=\...
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1answer
104 views

Clarification for the formulae for Differential Cross Section in Scattering theory [duplicate]

I am trying to study scattering theory using "Quantum Mechanics Concepts and Applications" by "Nouredine Zettili" . He starts from the formula $${d \sigma( \theta,\phi) \over d \Omega } = {1 \over ...
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2answers
453 views

Units in general relativity

My question is pretty straight-forward: what are the units of the tensors in General Relativity? This should sound easy, but I always studied those in natural units ($c=1$) so I can't figure it out. ...
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4answers
10k views

Can units be plural? [duplicate]

I was in a conversation with my senior engineer where he kept on insisting that we can use plural when we write down any unit. I argued that it is not the 'common' practice or even throughout my whole ...
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1answer
62 views

Is there a library of physical units?

There are some standard units we all understand intuitively such as: Mass (kg) Length (metres) Time (seconds) From these there are some familiar composite units which are also well understood: ...
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3answers
5k views

What is the length of 1 second in meters

If time is treated as a fourth dimension of spacetime, what is relation between length and time units? Or in other words, how can I convert time units to length units, for instance seconds to meters?
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9answers
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What exactly are we doing when we set $c=1$?

I understand the idea of swapping from unit systems, say from $\mathrm{m\ s^{-1}}$ to $\mathrm{km\ s^{-1}}$, but why can we just delete the units altogether? My question is: what exactly are we doing ...
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3answers
7k 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
979 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
9 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 ...
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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
151 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 ...