Questions tagged [units]
Units are standards of measurement used for different types of quantities.
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Is there a name for the unit “Ampere meter”?
Motivation: I'm doing a homework problem involving a rod sliding freely down a pair of parallel conducting rails. I've got a quantity of unit $A \cdot m$ and want to know what I should name and call ...
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1answer
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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
$$
c^2 \nabla \times \vec{B} = \frac{\vec{j}}{\epsilon_0} + \frac{\partial \vec{E}}{\partial t}.
$...
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5answers
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Why is there no constant factor in the equation $E=mc^2$? [duplicate]
As I understand, the metric system starts with an arbitrary weight, and calls it $1\, \mathrm{kg}$, then the volume of this weight in water gives the meter. And then the energy is defined based on the ...
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6answers
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What examples are there of unrelated physical quantities measured in the same units? [on hold]
Torque and energy have the same dimensionality, and so logically are measured in the same units (joules). However, it seems more natural to call the unit for the former a “newton-metre”, because ...
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2answers
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Addition of Temperature Units [duplicate]
I would like to know how does the addition of temperatures works. In school, we have been taught that we should convert degrees Celsius to Kelvin unit first and then the sum will be converted back to ...
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Is the calculation of $\mu_0 \cdot M$ = Tesla valid?
in the Paper "Static Stern-Gerlach effect in magnetic force microscopy", they estimate:
$$\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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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
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Significant figures in complex calculations
In my physics book it is written that ' In any involved or complex multi-step calculation, you should retain, in intermediate steps, one digit more than the significant figures and round off to proper ...
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1answer
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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
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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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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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3answers
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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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0answers
35 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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1answer
107 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 ...
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2answers
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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 ...
2
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1answer
55 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.
...
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1answer
36 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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1answer
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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
447 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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1answer
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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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0answers
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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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2answers
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What is $c$ in Gaussian Units?
Is the speed of light in Gaussian Units just 1 or can it be expressed in metric units?
I am getting really confused by all the unit systems. In relativity, for instance, c is often set equal to 1 ...
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5answers
106 views
Units of variable acceleration
If we have a function, which describes, how a displacement in space along a line varies as a function of time: e.g.: $s(t)=vt$, its units are meters because $[v]=\frac{\text{meters}}{\text{seconds}}$ ...
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1answer
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What is the difference between degree celsius and celsius degree? [duplicate]
It seems similar but can you guys please answer it that what is a specific difference between degree celsius and celsius degree?
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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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1answer
60 views
What's the preferred way of writing units per unit? [closed]
Why do we write units such as meters per second as $\mathrm{m\,s} ^{-1}$ instead of $\mathrm{m/s} $. Is the latter still a valid way of expressing units? Why are there two different ways of expressing ...
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1answer
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Why revolutions (or turns) are dimensionless?
I think that the reason is because one revolution or one turn is equal to $2 \pi$ rad or to $360$ degrees.
We can relate rads and degrees to two units of length that cancel each other.
rad $= \frac{...
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0answers
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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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1answer
35 views
Adding or subtracting mean absolute error
In an experiment to find the length of a rod, n observations were made. ${a_1,\;a_2,\;.\;.\;.\;a_n}$. The mean (${a_{mean}}$) of those observations will give a value very close to the true length of ...
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1answer
107 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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1answer
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Undoing problems caused by setting $c = 1$ { or “Undoing $c = 1$” }
In the mathematical derivation of equations for physics, and involving wave propagation in particular, the propagation speed at the start of the derivation is often set to one (c = 1).
I am working ...
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2answers
192 views
Why is the equation $E=mc^2$?
The equation $E=mc^2$ never made any sense to me.
c is a constant (speed of light), therefore c squared is also a constant. We're not specifying any units so surely the equation should be reduced to ...
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1answer
120 views
Is it possible that “$c = 1$?” [duplicate]
Can
c = 1
and if so, what does this mean for Physics?
If you were to take any Physics equation, and let c = 1, will it work?
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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 ...
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0answers
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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 $...
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1answer
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Units conversion - quanta/cm$^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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3answers
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Is there a better definition of magnetic field than this?
It may seem a trivial question but the definition of the magnetic field in everyday books is misleading.
"It is the region or area around a magnetic material in which its magnetic force can be felt."
...
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0answers
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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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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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0answers
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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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1answer
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Question about code units and physical units (hydrodynamics simulations)
I'm working on a code that implements smoothed particle hydrodynamics (SPH) method for solving the equations of magnetohydrodynamics (MHD) with self-gravity.
In research papers regarding existing ...
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2answers
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Can you explain the meaning of base units behind the unit gray?
The gray is a unit that measures energy absorbed per unit of mass. It's defined as one Joule per one kilogram. Which can be simplified to $m^2/s^2$.
Can you explain the meaning (interpretation) of ...
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1answer
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Why $µ_0$ and $ε_0$ are not considered to be $4π$ and $1/4π$? [closed]
I did some algebra...
In Planck unit, if make $\mu_0 = 4\pi$ and $\epsilon_0 = \frac{1}{4\pi}$ you get:
$$\mu_0 = 4\pi \cdot \frac{m_p l_p}{t_p^2 I_p^2} = 1.2566368452237765 \cdot 10^{-6} N \cdot A^...
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2answers
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Would setting the ideal gas constant to $1$ yield an attractive natural temperature scale? [closed]
In this recent question, there was a comment 'The "zero point" of Kelvin is natural, but the scale is not'. This led me to wonder whether setting $R = 1$ in the ideal gas law would be an attractive ...
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2answers
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Why is the length of the Kelvin unit of temperature equal to that of the Celsius unit? [duplicate]
The Celsius unit is arbitrarily defined, based on the boiling and freezing point of water. Is it a coincidence, then, that the SI unit of temperature Kelvin, which is used in all natural equations, ...
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4answers
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The significance of kelvin as a unit of absolute temperature
$$PV=nRT$$
Where $P$ is pressure in pascals ($\text{Pa}$)
$V$ is volume in cubic metres ($\text{m}^3$)
$n$ is amount of substance in moles ($\text{mol}$)
$R$ is the gas constant having units, joules ...
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1answer
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What units are represented by pr - micrometer?
In studying column abundances, I often encounter the units $pr-\mu m$. What does this stand for? Part per micrometer? how can this be an abundance unit?
Incredibility I cannot figure out what it ...
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1answer
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Question about change SI unit, what is $h$ and how could they agree with anything? [closed]
I heard a speech about the changing SI unit, but then it got me concerned with a bunch of questions. Basically, they switched from sticks to some "fundamental constants".
What is $h$?
In the Metric ...
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Conversion of $R = 8317\ \mathrm{\frac{J}{kg\cdot mol\cdot K}} $ to $ 8317\ \mathrm{\frac{J}{kg\cdot mol\cdot {^\circ C}}}$
I'm trying to understand the unit conversions when one unit is obtained by shifting the value of another one by some constant. In particular, the unit conversions from Kelvin to Celsius scale:
$$T(\...
2
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1answer
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What is the dimension of the weak gauge field couplin constant NOT in natural units?
What is the dimension of g and g', NOT in natural units, but in terms of mass, length, time, and permittivity?
