Questions tagged [units]
Units are standards of measurement used for different types of quantities.
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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
178 views
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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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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2answers
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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
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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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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
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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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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
65 views
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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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
41 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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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
42 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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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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3answers
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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
84 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
135 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 ...
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How is a Kibble balance used in the new definition of the kilogram, and what's the connection between the balance and Planck's constant? [duplicate]
The BBC News article Kilogram gets a new definition says:
How does the new system work?
Electromagnets generate a force. Scrap-yards use them on cranes to lift and move large metal objects, ...
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1answer
53 views
What does the Planck-like charge $Q=\sqrt{\frac{\pi}{c\hbar \varepsilon_0}}$ represent?
While experimenting something with equations I got a equation of charge which is not Planck's charge, dimensionally it is correct. I used dimensions method to get this.
$$Q=\sqrt{\frac{\pi}{c\hbar \...
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1answer
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Question on units for voltage equation of a voltage at a distance z from a single line charge
I have a very basic question on the units for the equation of the potential at a distance: $z$ from a single line charge.
The electric potential due to a very long line charge at some distance $z$ is ...
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1answer
53 views
Unit vector in displacement
When we use vectors in physics why does the unit vector (for displacement) equals magnitude of 1 or magnitude of 1m?
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2answers
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How to determine the units of my integral? [closed]
Given that I have some coefficient(i.e. a number) which is to be determined from a radial integral:
$$b_{n00} = \frac{(2\pi)^{1/4}}{\sigma^{3/2}} \frac{1}{\sqrt{3}} [C(000|000)]^2 \int^{\infty}_{r = ...
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4answers
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Why is the speed of light used to define the fourth axis of spacetime?
The four axes of spacetime are $x, y, z$ and $ct$, where $c$ is the speed of light, and $t$ is time. Why is the speed of light (not any other speed) used to define the fourth axis of spacetime? If ...
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1answer
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How is the division of physical quantities into base quantities and derived quantities a matter of convention?
Several physics textbooks and even the SI-The International System of Units Brochure(8th-edition) says that, ''The division of quantities into base quantities and derived quantities is a matter of ...
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1answer
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Coordinates in 1970 Labeyrie article
I'm reading the 1970 article by Labeyrie on speckles in astronomy and I encountered a small problem. The author uses two sets of coordinates, $(\alpha, \beta)$ and $(x, y)$, connected by Fourier ...
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What are the units for state vectors in QM? [duplicate]
Specifically, it seems that a state vector should be dimensionless because:
$| \langle \psi | \psi \rangle |^2$ is a probability which is dimensionless
However, I find myself doing manipulations ...
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1answer
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Meaning of units in the Heisenberg Uncertainty Principle
So we have the inequality $\Delta x\,\Delta p\geq \hbar/2$ Why is the uncertainty of position in meters? Isn't uncertainty measured in probabiliies? Does it make sense to say that the uncertainty in ...
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4answers
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Is a unit vector really unitless and dimensionless?
According to my textbooks, a unit vector has no units and no dimensions, but is only used to specify direction. It only shows the orientation of a corresponding vector in space. I think it's true, or ...
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Are the units of energy the same in higher dimensions?
In 3 spatial dimensions, $$[E] = [ML^2 T^{-2}]$$
Would it change in higher dimensions? If yes, then what would be the dimensions for 4 spatial dimensions?
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1answer
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Significant figures in multiplication with units
Why does it make sense to round to the smallest significant figure instead to the largest unit?
If I have one measurement made to the nearest millimetre and another to the nearest picometre then why ...
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1answer
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In the value of gas constant $R$, what kind of mole is it?
Wikipedia gives $$R=8.3144598(48)\, \rm J\,mol^{−1}\,K^{−1}$$ But I want to ask how is it possible to write $\mathrm{mol}$ without a suffix? I mean, we must determine if we have $1\ \mathrm{mol}$ of ...
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1answer
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What do we mean, concretely, by the unit $\rm N\:m$ (newton $\cdot$ meter)? [duplicate]
What do we mean, concretely, by the unit $\rm N\:m$ (newton $\cdot$ meter)?
For example, $1 \:\rm m/s$ mean that each second, we make one meter. Also, $1\:\rm m/s^2$ mean that each second the speed ...
