Questions tagged [units]
Units are standards of measurement used for different types of quantities.
1,138
questions
3
votes
2
answers
413
views
The relationship betwen Equivalent Megatons and Yield
This article defines a concept called Equivalent Megatons (EMT) and gives a formula of calculating it in terms of Yield (measured in megatons).
In evaluating the destructive power of a weapons system,...
- 33
0
votes
3
answers
134
views
Does the unit "volt-seconds" have any interesting interpretation?
I was contemplating the units of capacitance and inductance in electrical circuit theory and noticed something that bugs me. Although there are many equivalent ways to write the farad (seconds per ohm,...
- 103
2
votes
1
answer
69
views
Should power of lens be dimensionless quantity?
My book defines the power of lens using this way:
The power P of a lens is defined as the
tangent of the angle by which it converges or diverges a
beam of light falling at unit distant from the ...
- 431
0
votes
0
answers
29
views
Apparatus to measure one ampere
The following definition of the Ampere was codified in 1948 at the 9th CGPM (Conference Général des Poids et Mésures)
The ampere is that constant current which, if maintained in two straight parallel ...
- 101
3
votes
5
answers
897
views
Relationship between bel and decibel
Bel is a unit of $log_{10}$ of ratio of two quantities.
1 Bel = $\log_{10}\frac{P_1}{P_2}$
On Wikipedia it says: 1 decibel = $\frac{1}{10}$ bel
According to this definition then,
1dB = $\frac{1}{10}$ $...
- 321
0
votes
1
answer
85
views
Mass, time and length are measured with 5% accuracy each, what will be the total accuracy in torque? [closed]
I don't understand the solution of the above question, the solution i know is :-
Accuracy in torque = [M%] + [2 × L%] + [2 × T%]
Which will be equal to 25%
I dont understand why 2 was multiplied to L% ...
- 3
0
votes
1
answer
37
views
When using the slope or area of a graph to calculate things, do the scales of the units on the axes of a graph need to be the same?
If I want to find the resistance(R) from a voltage(V) vs current(I) graph for a given wire using the relation tan Θ = ΔV/ΔI = R, do the scales on the axes need to be the same, independent of the units?...
- 3
0
votes
1
answer
46
views
Converting from Planck units to Hartree units
I am trying to include an analytical formula into a numerical program. In Planck units, the expression looks like this: $\frac{A}{B}$, where $A$ is in $m$ while $B$ is in $eV^{-1}$.
If we go to SI ...
- 41
0
votes
1
answer
60
views
When a laser pulse rate is stated, does that include the time that it is off? If pulse rate is 1,000 per second, is it on for half a millisecond?
Sorry if this seems a stupid question to some, but I am having trouble finding a consistent answer....
The European XFEL hard X-ray headline proudly states that it is currently the fastest of its kind ...
- 4,269
0
votes
1
answer
63
views
How do I interpret the unit of [1/dex]?
I am trying to reproduce the mass function graph from a paper. I have calculated the mass function $\frac{\rm{d}f}{\rm{d}\log M}$, where $f$ is the mass fraction with respect to the total mass in the ...
- 1,190
0
votes
1
answer
83
views
Gravitomagnetic Field vs Distance
If we look at what the Wikipedia says about gravitomagnetic fields $B$ generated by rotating planets / stars we have:
https://en.wikipedia.org/wiki/Lense%E2%80%93Thirring_precession#...
- 181
-2
votes
2
answers
51
views
Units of variables within the error function
For the error function, erf$((x-\mu)/\sigma)$, if $x$ has unit of length, what are the units of $\mu$, $\sigma$? Is there a standard for these type of functions?
- 235
1
vote
2
answers
117
views
If fluid flow is in units $m/s$, what units are $div$ in?
Imagine a fluid flow given by the formula $\mathbf{v} = x\mathbf{i} + y\mathbf{j}$, this is a simple radial flow outward and the dimensions would be length/time: At a given point, what is the velocity ...
- 309
0
votes
1
answer
29
views
Is there any difference in dimensionless quantities, $G=1$ and the fine sturcture constant, from prediction prospective?
In the Planck unit, $G=1$ as dimensionless. The fine structure constant is also dimensionless. I had an impression that predicting the speed of light is not meaningful, since it has unit/dimension and ...
2
votes
1
answer
134
views
Calculating Cross Section in terms of barns (help with converting from natural units)
I am trying to calculate the Glashow resonance to get something in terms of barns but I am getting confused switching between natural units and non-natural units. The Glashow Resonance is given by the ...
- 123
0
votes
1
answer
31
views
Triple Integral of Volume Concentration
I am working on a problem involving volume concentrations which are defined as:
$$ \phi_i=\frac{V_i}{V} $$
and are thus dimensionless. My problem is in $3$D and I have assumed that the components in ...
- 133
0
votes
1
answer
53
views
Correct way to reference to a percentage value's category/label
When a percentage is written as, for example: 20% of sold products were of product A and 80% product B. What is the correct concept / word to use to describe "product A"? Is it units?
As ...
- 277
0
votes
2
answers
139
views
Notation: units with negative exponents
I'm not sure if this belongs on Meta or here but:
In many scientific journals, books, and posts on this site and others, I see the negative exponent convention used for units, e.g.
$\mathrm{N} \,\...
- 8,458
0
votes
5
answers
190
views
Is the magnitude of a vector equal to its length
Suppose a body has a velocity $V =2\hat i$ metres per second. The magnitude of velocity is 2 meters per second . We can draw a vector of length 2 cm to represent this velocity.
According to the below ...
- 1,200
4
votes
3
answers
1k
views
Defining the second by an alien civilization [closed]
(The above question could be phrased better, so feel free to suggest.)
Like many, I was imprecisely told that the second is 9192631770 oscillations of caesium valence electron (or smth along those ...
- 151
0
votes
2
answers
70
views
Why do we measure plane angle in radians and solid angle in radians and steradians respectively rather than degrees? [duplicate]
Recently, I learnt about physical quantities. When i got to know about plane angle and solid angle, i had a doubt that even though they are just angles, why do we measure it in radians or steradians ...
- 15
1
vote
1
answer
39
views
Value of Inverse of Thermal Voltage
I am trying to solve the Poisson-Boltzmann Equations and have come about the constant
$$
\beta = \frac{e}{K_BT}
$$
which Wikipedia denotes as the inverse of Thermal voltage. Now this constant is ...
- 111
1
vote
3
answers
144
views
A unit of time $\frac{GM}{c^3}$ [closed]
I have come across a time unit $M$ that should be equal to $\frac{GM}{c^3}$. I cannot find anything at all about this anywhere.
I came across it while doing flux analysis on the flux from black hole ...
0
votes
0
answers
41
views
Why do we not use SI units for optical rotation of substances in solution?
This question is with regards to a polarimetric approach in analysing the concentration of a substance in solution such as sugar in water.
I have been trying to find an answer to it through a little ...
-1
votes
4
answers
295
views
Thermal expansion: why in Celsius and not in Kelvin?
When dealing with thermal expansion problems I've encountered a situation where I must calculate the change in length of an iron bridge 2km long if the amplitude of temperature changes by 40ºC, ...
- 147
0
votes
2
answers
93
views
When to convert $x/y$ units in physics questions?
I've searched the internet for this question but found no answer. Sometimes when I plot graphs where the units are not SI units, the units are converted during gradient calculation, and sometimes the ...
-1
votes
1
answer
130
views
Per unit quantity meaning
What does per unit quantity mean? like distance covered per unit time? Or mass per unit volume?
- 11
1
vote
1
answer
107
views
Definition of Planck area and Planck volume
According to https://en.wikipedia.org/wiki/Planck_units, the Planck area is defined as the square of the Planck length, and the Planck volume as the cube of the Planck length.
\begin{equation} l_\text{...
- 195
0
votes
2
answers
220
views
Why dimension units of radius is not $\rm m/rad$ or $\rm cm/rad$? [duplicate]
Radius is not a just simple size or length between the two points. The radius shows the connection of linear and angular values. Something must indicate the information about a perpendicularity of the ...
- 79
2
votes
2
answers
96
views
Parsec confusion
parsec is defined as the distance at which average radius of earth's orbit around sun would subtend an angle of 1''(second of arc). But suppose im looking at this star when earth is on one side of the ...
- 83
2
votes
1
answer
85
views
6-day curie to grams?
I am a pre-med interested in nuclear medicine. I recently came upon this wonderous radioisotope called Mo-99 (which decays into Technetium-99m), but in their industry, they measure the quantity of ...
- 291
0
votes
0
answers
57
views
CGS Units of surface current density
The exercise is to calculate the surface current density of Pb at $T = 4.2K$. The critical temperature $T_c$ and the critical field at zero temperature is given: $T_c = 7.2K$ and $H_c(0) = 803Oe$. I ...
- 72
0
votes
0
answers
61
views
Why do the Stoney units of time, length, and mass differ with the Planck units by the squareroot of the fine structure constant?
The natural units of Planck and Stoney differ by the inverse of the finestructure constant.
For example, the Stoney-mass is:
$$m_s=\sqrt{\frac{k_e e^2}{G}}= 1.85921×10^{-9} kg,$$
where $k_e$ is ...
1
vote
3
answers
218
views
Is it possible to have non-linear relationship between temperature scales?
Is it possible to have a hypothetical temperature scale that does not follow a linear relationship with another temperature scale? For example, the graph formed between the readings taken by a ...
- 21
-1
votes
1
answer
67
views
Integrating Equations with Units - 2023
I have a worked example from a recorded lecture and lecture notes which I am having difficulty with regarding Rectilinear Motion in Mechanical Dynamics.
When I asked my lecturer about the following, I ...
-2
votes
2
answers
80
views
i dont know anything in physics but what i know is number system now explain me why we need unit and dimensions and what exactly are they? [closed]
imagine I just know number system nothing else I want to intuitively understand development of units and dimension with very very clear definition of both of them what exactly is a unit how is it ...
- 99
-4
votes
4
answers
116
views
What is the connection between watts and electron volts?
I saw that $1$ w/s is equivalent to $6.2 \times 10^{18}$ eV. But if that was true $1$ w/s or less could fully ionize any atom is that true?
0
votes
0
answers
75
views
Galaxy S4G database: convert pixel to kpc
I am referring to S4G database (https://irsa.ipac.caltech.edu/data/SPITZER/S4G/overview.html ).
The information on the length of galaxies, and various length parameters are given in terms of pixels.
...
- 953
0
votes
2
answers
100
views
Why this form of Lorentz force law in Gaussian units?
In Ashcroft and Mermin's first chapter, equation (1.13), they write the Lorentz force law as (due only to a magnetic field)
$$\textbf{F} = \frac{-e}{c} \textbf{v} \times \textbf{H}$$
rather than as
$$\...
- 1,045
0
votes
1
answer
51
views
Elementary calculation of the critical density
In the book Cosmology by Daniel Baumann, the author states that the critical density of the universe at the current time is:
$$\rho_{crit,0}=\dfrac{3H_0^2}{8\pi G}=1.9\cdot 10^{-29}h^2\text{grams}\...
- 297
1
vote
1
answer
158
views
Dimensions of Stress-Energy-Momentum Tensor [duplicate]
The dimension of the stress-energy-momentum tensor is similar to that of pressure, according to wikipedia.
The stress-energy-momentum tensor $T_{μν}$ is defined as $μ$ momentum in a spacetime box of ...
- 858
2
votes
2
answers
107
views
Is there a system of measurement that consistently includes logarithmic units?
Logarithmic scales are not uncommon: magnitudes in astronomy, f-stops in photography, the Richter magnitude scale, etc.
But decibels are weird.
I am not even talking yet about the fact that we use a ...
- 1,325
0
votes
4
answers
74
views
Electric field definition
In my book it says that "electric field" is defined as the "force per unit charge". What does the "unit charge" mean? My initial thoughts was that it meant 1 Coulumb, but ...
- 93
1
vote
2
answers
537
views
What is the full QED Lagrangian with physics units written out?
I wonder what the QED Lagrangian would look like if you carefully write out all units of the terms and make sure they are consistent. I think there is something missing about Coulomb.
Can you write ...
- 1,437
2
votes
3
answers
468
views
What exactly does it mean for a unit to be dimensionless?
For instance, why are moles and decibels considered dimensionless, but kg and meters aren't? Or, in other words, what exactly is a "dimension" in this context? Is just about the system of ...
- 2,183
1
vote
0
answers
85
views
Why are the seconds/minutes/hours units are universally used, while other measures have several units? [closed]
There are several units to measure length like meter, foot, and rod. The same is true for weight. While time seem to have a universal unit which is the second. Was there other measures of time that ...
- 11
0
votes
2
answers
63
views
Why is excitation potential taken in $\rm eV$ when $\rm eV$ is actually the unit of energy and not potential difference? Shouldn’t it be Volt?
The unit of excitation potential should be Volt and not $\rm eV$. My book says it’s $\rm eV$.
- 1
-6
votes
2
answers
113
views
What are light years? like you know how people measure some things in light years? what is light years [closed]
so basically you know how scientists measure how far things are but in space and they measure in light years well what are light years and how long are light years?
0
votes
1
answer
110
views
Notation to give units in legend/axis label/etc
When writing e.g. an axis label for a plot or a header for a table column that contains data that is associated with a unit (e.g. a length in meters), I always used to write it down like this:
...
- 117
1
vote
1
answer
57
views
Why can't I balance the units in the equation for power of a sound wave?
According to this answer (source), the time averaged power $P$ of an acoustic wave is:
$$P = \frac{1}{2} \mu v \omega^2 A^2$$
Where $\mu$ is mass density of the medium, $v$ is speed of sound in the ...
- 1,004