# 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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 ...