Questions tagged [units]
Units are standards of measurement used for different types of quantities.
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Why is sound “loudness” measured in $dB$ instead of $[P]$?
From acoustics we know that sound is an elastic pressure wave. What our human senses perceive are "loudness" and "pitch" which are related to the amplitude and the frequency. While pitch can easily be ...
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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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1answer
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Dimensional analysis of electron-positron spectrum
The theoretical formula for the numbers of particles per energy $\varepsilon$ with colliding photons with energies $\omega_{1}$ and $\omega_{2}$ is given by following expression (reaction $\gamma\...
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1answer
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Confusion on Centigrade/Celsius Scale
There came the centigrade scale. The issue was that many solid/liquid thermometric substances didn't respond to temperature linearly so different thermometers produced different results. This was ...
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1answer
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Is cross-sectional area over length classified separately from length?
The ratio of cross-sectional area to length (or its reciprocal) appears in several formulas, including those for electrical resistance and capacitance in terms of the resistivity and permittivity. ...
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What is the difference between the radian and the steradian? [closed]
In the SI, the radian is a dimensionless unit for the plane angle. The steradian is the unit for a solid angle where 1 sr = 1 rad$^2$, which means the steradian is also dimensionless in the SI.
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1answer
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How can we compute the quantity $hc/e$ vs gauss and centimeter
In some textbook, authors mention that the amount of $hc/e$ equals to $4.136 \,\text{gauss}\times cm^2$. Where, $h$ and $c$ are Plank's constant and the speed of light respectively. Positive $e$ is ...
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67 views
Heaviside Vector Units
Became a little confused over last hour trying to understand the units of the Heaviside vector. So far I have come up with this from reading online:
The Heaviside vector allows us to write the energy ...
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1answer
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Why are angular velocity and angular frequency not measured in Hertz?
Recently, I was doing my homework and I found out that Angular Velocity and Angular Frequency can be calculated using $\omega=v/r$. This means the units of angular velocity and angular frequency are (...
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Do scalar quantities have magnitude only?
I've heard that vector quantities have both magnitude and direction but I've never heard that scalar quantities have magnitude only. Magnitude of vector quantities cannot be negative but what about ...
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1answer
36 views
Virial radius of a collection of point masses
I am experimenting with multi body simulations in astronomy ($N$-body algorithms). It is customary to give the results in dimensionless units.
One of them is a length unit, and the virial radius is ...
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1answer
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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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1answer
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What makes a system of units self-consistent?
What are the rules governing the creation of a self-consistent system of units? To be clear, I'm not asking about making units universal or replicable, I'm asking about the mathematics governing what ...
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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
66 views
Super confused over simple unit conversion from centimeters to cubic centimeters
I'm stuck in a really stupid question. It seems like the meaning of "centi" is chancing meaning. Because if:
$ r = 5 cm = 5 \cdot 10^{-2} m $
so $c = 10^{-2}$
But say i need to take the cubic root ...
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0answers
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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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4answers
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How does natural unit make sense? [duplicate]
Both the fundamental constants $\hbar$ and $c$ have dimensions. In particular, $[\hbar]=ML^2T^{-1}$ and $[c]=LT^{-1}$. But in natural units, we make them dimensionless constants of equal magnitude. ...
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When is the order of magnitude not equal to the exponent of scientific notation?
Explain why the order of magnitude is sometimes not the same as the exponent in scientific notation. It is because of the units?
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0answers
51 views
Self inductance of a circular coil
I have read from my reference that the self inductance of a wire ring is derived in a non-trivial manner. I have copied an image off the result below, let me know if I should translate it to Latex.
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Dimensions of mass in terms of Length and Time
From Maxwell's Treaties on Electricity and Magnetism:
For acceleration due to attraction of a mass m at a distance r is by the Newtonian Law m/(r^2). Suppose this attraction to act fro a very small ...
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2answers
48 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
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Is there no law which includes an addition of different types quantities? [duplicate]
I haerd that we cannot add two different types quantities.
Is there no law which includes an addition of different types quantities?
In the future, maybe we find a law like this $$F = \frac{\...
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2answers
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Is there (or was there) a unit of electric current based on Avogadros number or Coulombs constant?
This has to do with the SI definition of the Ampere. Why the quantity $2*10^{-7} $ Newtons in particular? It would make more sense to define 1 Ampere = 1 mole of electron charge per second. Which ...
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2answers
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Unit of pion-decay constant
In the natural unit system, the pion-decay constant $f_{\pi}$ is $92.4\:\rm MeV$. But I think that a decay constant should have a dimension of $[T]^{-1}$, where $[T]$ is the dimension of time. Then, ...
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1answer
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Units of forcing function in the inhomogeneous wave equation
The units of the d'Alembertian are distance$^{-2}$. It should be the case that the inhomogeneous wave equation describing
$$\square u = f$$
should have matching units on both sides. My understanding ...
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3answers
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Confusion about Unit Systems
I was reading about Deuterium and came to know that its binding energy is $2.22$ MeV. I am curious whether this energy is in the natural unit system where $\hbar = c =1$.
For instance, the energy ...
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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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2answers
60 views
Is there any constant with unit meter-second?
I wanted to know if there exists a constant with a unit meter-time or say length-time. Dimensionally [LT].
I have searched browsed a lot. Is there any quantity arising with such a unit?
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2answers
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Is length an extensive property?
From my experience, volume, surface and length are extensive properties. Indeed :
the reunion of two cubes of 1 $m^3$ leads to a cube of 2 $m^3$
the reunion of two tiles of 1 $m^2$ leads to a tile ...
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2answers
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Is gravitational constant a rational number? [duplicate]
The question is the title. But I'm quite doubtful if this question is meaningful or not. Since this constant is obtained by experiment, we can never know its exact value, unlike $π$ or $e$. Is it ...
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1answer
67 views
Correct form of Lorentz force equation [duplicate]
When I was reading the first volume of the theoretical minimum by Leonard Susskind, I encountered upon this:
But when I checked Griffiths and all my other electricity and magnetism books, it showed $...
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2answers
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Scientific notation and usage thereof [closed]
I'm taking my first physics exam in university this Saturday, and I'm currently working on a practice exam.
I got the answer 3.1MeV for a alpha-decay question, but the answer was 3100keV. I ...
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Why is torque sometimes reported in kg m, instead of the usual N m?
On various websites I see torque expressed as $\rm kg\: m$, but I was always thought that torque is $\rm N\:m$ or $\rm kg\: m^2/s^2$. These are clearly not the same, so why are they called the same, ...
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2answers
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What does it mean for a unit vector to have a magnitude of 1?
Imagine a Cartesian coordinate system whose origin is associated with two unit vectors, ê and â, in a 2D-space. Now, let 0.5 cm be the unit of length in this coordinate system.
The magnitude of a ...
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1answer
44 views
Acceleration: $G$ and $G$s difference
In some literature the acceleration is stated in Gs units. For example, "All structures shall be built to survive 15 Gs of launch acceleration".
However, g-force sometime also stated in G units ...
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What are the starting units to perform the Gauss method?
To calculate the Gauss method, we need :
the respective geodetic latitude
the equatorial radius of the body (here earth)
the respective local sidereal time
the respective declination
the respective ...
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1answer
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Are there any physical quantities that are loglog?
Most units, e.g. for length, time, mass, etc, are linear in relation to the underlying property.
But there are some special quantities, where the units can vary several orders of magnitude within the ...
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1answer
31 views
Name of measurement expressed as the sum of different units
In a paper I'm writing, I want to make a comment about errors in an old paper made when doing arithmetic with measurements expressed as as the sum of quantities with two or more different units, for ...
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1answer
47 views
Profile of a platinum-iridium meter bar
The former meter standard, platinum-iridium meter bar, had a specific cross section somewhat resembling mixed variant of letters "X" and "H" with serifs (Image source):
What was the reasoning behind ...
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1answer
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Unit of $eV^4$ for Density?
I'm going through this paper on electroweak burning, and I ran across the figure $100 GeV^4$ to describe energy density. I'd like an explanation as to how the units work out, and why this figure isn'...
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2answers
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Thermodynamics: Units in calculations of heat (Q)
When calculating an unknown variable in the quantity of heat equation for a system, let's say mixing water of 80 degrees C and ice of -20 degrees C that achieves thermal equilibrium at 10 degrees C (...
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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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What is the sound absorbing characteristic of a material called?
I understand that lead is one of the best acoustic barriers because it absorbs sound very well; in other words, it converts sound to heat. By comparison, other materials tend to either transmit the ...
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1answer
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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
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Definition of a meter and Newtonian law of Gravity
Newtonian law of Gravity:
$$F_g = \frac{m_1 m_2}{l^2} G$$
$$G = 6.7 * 10^{-11} \frac{m^3}{kg * s^2}$$
A meter is defined as:
the length of the path traveled by light in a
vacuum in $1/...
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1answer
216 views
Can a physical quantity be of different dimensions depending of the system of measurement?
When comparing the Wikipedia articles on the International System of Units, the Planck unit system, and the geometrized unit system one question arises: can a physical quantity be of different ...
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Write the dimension of 1D wave function? [closed]
I want to know how to find the dimension or unit of one-dimensional wave function
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Finding the unit of Pressure/Volume
Me and my friend are trying to find the exact unit of Power=Pressure/Volume. We know that the unit has to be W(J/s) so we try from the equation above:
(Kgm²/s³)=(N/A)/V
The right side will be:
(Kgm/s²)...
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2answers
183 views
What are the units of probability density?
Units of probability density? If bound electron is thought of as a cloud of charge, and it's charge density is proportional to the probability density. Then coulombs /m3 proportional to?
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1answer
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What is the mass of $N_A$ atoms of carbon-12?
With the recent redefinition of the kilogram, what is the mass of $N_A$ (Avogadro's constant) of carbon-12 atoms? $N_A$ was defined as exactly 6.02214076×$10^{23}$ atoms. Then how close would the ...
