Questions tagged [units]

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

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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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48 views

Can we get the values of $G$, $h$ and $c$ to be numerically equal if we use a convenient system of measurement?

The speed of light in vacuum is approximated to be $3×10^8\ ms^{-1}.$ But, if we change the units, we can get a different number. For example, it won't be $3×10^8$ if we used $ft\,s^{-1}$ instead of $...
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1answer
77 views

Units conversion - quanta/cm$^2$/s and photons/cm$^2$/s

Can you tell me if there is any difference between these 2 units? $\log \mathrm{quanta/cm^2/s}$ vs $\log \mathrm{photons/cm^2/s}$ Thanks
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36 views

Empirical determination of masses and charges of a set of objects using only kinematics experiments

Suppose you have $n$ physical objects that you want to determine the mass and charge of. You do not have access to any reference object with known mass and charge (that also includes things like the ...
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92 views

Geometrized units in Wald

I am trying to follow a derivation in this paper from Wald Specifically, at the end of the paper, just under Eq. 4.7 there is given the equation $$ Q/m \leq 2 B_0 m$$ where $Q$ is the system ...
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184 views

Using the boltzmann equation to find neutrino decoupling temperature

I'm an undergraduate student currently following a particle astrophysics lecture. I was reading some stuff about the neutrinos decoupling before the $e^+ e^-$ annihilations. It's possible to quickly ...
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0answers
79 views

Units and missing constants in quintessence expressions?

In cosmology, quintessence is an alternative to the cosmological constant. In this approach (described here), we consider a scalar field $\phi$ and its self-interacting potential $V\left(\phi\right)$ ...
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56 views

What is the constant appearing in the low energy action?

Usually one finds this expression for the low energy action $$S = \frac{1}{2\kappa_0^2}\int d^D X\; \sqrt{-G}\; \mathrm{e}^{-2\Phi}\,(R-\frac{1}{12}H_{\mu\nu\lambda}H^{\mu\nu\lambda}+4 \partial_{\mu}\...
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329 views

Scale-invariant differential operator

For example, the differential operator Laplacian is $$\nabla^2 = \frac{\partial^2}{\partial x^2}+\frac{\partial^2}{\partial y^2}.$$ My questions are: Is it scale-invariant? what is scale-...
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32 views

In addition to $K_e/K_m = c^2$ (due to James Clerk Maxwell), are the constants that arise from $G/K_m$ and $G/K_e$ also limits?

Prior to James Clerk Maxwell we understood there to be three laws: $$\text{Gravity: }\;\;\; F_g= G \frac{m_1m_2}{r^2}\hat{r}$$ $$\text{Electricity: } F_e= K_e \frac{q_1q_2}{r^2}\hat{r}$$ $$\text{...
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21 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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86 views

Dimensionally inconsistent!

This equation doesn't seem to be correct. Dimensionally inconsistent, in fact! How then, is it established? $$s_{t}=v_{0}+\frac{1}{2}a(2t-1)$$ (In case you don't know, this is the equation used to ...
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21 views

How to Measure Electric-Field Probe Sensitivity?

I am doing some reading on current methods for detecting electric fields. A figure of merit for the systems is, naturally, the minimum strength of field that can be measured, in units of Volts per ...
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32 views

Grasping systems of units

Perhaps it's that I've grown up with the SI system of units, but there's something about other unit systems that doesn't sit well with me. For example, Coulomb's law in Gaussian units can be written ...
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2answers
58 views

Factors of $c$ when giving masses in natural units?

I am starting a course on particle physics, and have been introduced to natural units. I am slightly confused, because we are using 'natural units', and yet masses are stated as, for instance, $139....
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1answer
72 views

What does pc(eV) = 300 Br(G cm) mean?

In the January 1983 issue of the American Journal of Physics, Edward Purcell published a one-page "Round-Number Handbook of Physics" (see e.g. http://lemeshko.blogspot.com/2008/06/one-page-round-...
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72 views

How do you properly derive atomic units from SI units?

I'm learning atomic units and I'm having trouble figuring out how to derive the atomic units version of energy, time, electric field, magnetic field, etc. For example the Wikipedia page has a list of ...
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150 views

How is thrust per unit power measured?

Typically, thrust efficiency is measured in specific impulse, as thrust per fuel consumption rate: $$I_\text{sp} = \frac{F_\text{thrust}}{\dot{m}_\text{fuel}} \quad \text{or} \quad I_\text{sp} = \...
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23 views

Mixed unit conversion

I have a silly question. I am not used to different unit systems and I am stuck with trivial problem. I have expression for energy of harmonic oscillator: $$ \Delta E = \frac{1}{2}\Omega^2\Delta Q^2. ...
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315 views

Are Heaviside-Lorentz unit and Gaussian unit natural units?

I am studying electromagnetism and have encountered the Heaviside-Lorentz unit and Gaussian unit. In these unit systems, the constant $c$ keeps occurring. For example, the Maxwell equations are of ...
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52 views

Rewrite equations of physics using Planck units

I was reading this site http://astronomy.swin.edu.au/cosmos/P/Planck+Units and in it is stated that "If we use the Planck Units, we can rewrite all of the equations of physics such that: c = G =$ \...
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71 views

Units of a quantum electromagnetic field

The recalling $\mathbf p \rightarrow \mathbf p -q\mathbf A$, the vector potential has units $$ \langle \mathbf{A} \rangle \leftrightarrow\Big[\frac{\text{momentum} }{\text{charge} }\Big]$$ If we take ...
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42 views

Units of Neuron input current

My question stems from some of the equations used in a paper in Phys Rev E titled "Dynamical response of the Hodgkin-Huxley model in the high-input regime" (located here: http://journals.aps.org/pre/...
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160 views

Why is the cross section's unit Barn particularly equal to $10^{-28} m^2$?

Why is the microscopic cross-section's unit Barn particularly equal to $10^{-28} m^2$. Is there any history or reason behind that?
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213 views

Photon Propagator in QFT

Please forgive my temporary split-brain, but I'm a little thrown off by something when considering units at the moment. In QED (depending on the guage), the photon propogator is written as $<0|\...
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88 views

Is there a 3D analogue of angle?

A one-dimensional angle is a wedge, almost like a slice of pizza. A two-dimensional angle is an angle squared, like the cone of light produced by a flashlight. This is called a solid angle. Is ...
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195 views

Units of angular power spectrum

I am not sure whether this question might be better suited for crossvalidated or stackoverflow, but I will give it a try here: I have a map of the full sky in in the healpix format, units are $\rm ...
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87 views

What unit system puts a bunch of powers of ten in Maxwell's equations?

In what unit system can we get the Maxwell equation in the following form (section 1.2 from Lewin's book Advanced Theory of Waveguide) $$\nabla\times \vec E = -10^8\mu \frac{\partial \vec H} {\...
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31 views

How to understand/derive Eq 5.5 in Geiner's Quantum Mechanics - An Introduction?

Geiner's Quantum Mechanics - An Introduction has an unnumbered equation above Eq. 6 in section 2.4 discussing density -- not sure if it is energy density -- of radiation: ... $$dE/dV = E/V =a T^4$...
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117 views

Do doomsday arguments influence doomsday hypotheses?

The doomsday argument supposes that in the absence of any other knowledge, if we know the age of something now, we may assume that we are seeing it in the middle of its lifetime and then calculate our ...
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1answer
22 views

Balance the units of the following hamiltonian

The following image is taken from an article and shows the hamiltonian of a spin chain model. I knew that the dimensional units in an equation must balance. To ensure this, the author took a procedure ...
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1answer
57 views

When was magnetic field, first, mathematically defined?

The reason I'm asking this is, that we see that there is a constant $\mu$, in biot-Savart's law. If they were the first one to define it why did they use a constant? They could have simply left the ...
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42 views

Constant Values with non SI units

I'm trying to code a program to study the energy levels of Helium using the variational method, but I'm not sure my constant values are correct. I'm using Angstroms as the unit of distance and $eV$ ...
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18 views

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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31 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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17 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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27 views

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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43 views

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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1answer
69 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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10 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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43 views

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 ...
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1answer
16 views

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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27 views

Units and measurement

I read on the characteristics of units (i) It should be easily understandable. (ii) It should be changed with change in physical factors. (iii) It should not change with place or time. I want ...
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26 views

Units after integrating a ratio metric: Point Source Transfer Mobility

I want to calculate the so called Line Source Transfer Mobility (LSTM) from 10 equispaced Point Source Transfer Mobilities (PSTM). The PSTM measures the response to an excitation, and it is defined ...
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87 views

What's does square root of number of atoms mean?

Number of atoms $N$ was counted in 3 dimension. $x,y,z$. However, when calculate it, i.e. in many cases such as refractive index , people take the square root of it, i.e. in reflective index $n=\sqrt{...
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1answer
127 views

Why is contact resistance measured in $\Omega~\mu m$?

In many papers the contact resistance of a metal in contact with a semiconductor is given in units of $\Omega~\mu m$, for example in the paper by Li et al. (Appl. Phys. Lett. 102 (2013), p. 183110): ...
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111 views

After the redefinition of the units of fundamental physical quantities in 2019, will an uncertainty incur in the Universal constants values?

I am specifically interested in the following constants because being a student these are some of the most common constants that I face: $R$ (universal gas constant) Stefan constant Permeability of ...
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1answer
24 views

Units of forcing function in the inhomogeneous wave equation

The units of the d'Alembertian are distance$^{-2}$. It should be the case that the inhomogeneous wave equation describing $$\square u = f$$ should have matching units on both sides. My understanding ...
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393 views

How to convert J/mol-K to BTU/mol-R manually?

How to convert $\frac{J}{mol-K}$ to $\frac{BTU}{mol-R}$ manually? Sorry for such a simple question, I am new to Physics and I'm getting confused when it comes to conversions of units. Please bear ...
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1answer
213 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 $...