# Questions tagged [units]

Units are standards of measurement used for different types of quantities.

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### 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,...
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### 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,...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
1 vote
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### 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 ...
1 vote
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### 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 ...
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### 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 ...
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### 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, ...
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### 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 ...
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### Per unit quantity meaning

What does per unit quantity mean? like distance covered per unit time? Or mass per unit volume?
1 vote
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### 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{...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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
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### 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 ...
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### 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 ...
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### 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 ...
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### 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?
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### 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. ...
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### 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 ...
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### 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 ...
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### 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 ...
1 vote
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### 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 ...
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### 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 ...
1 vote
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### 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 ...
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### 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 $\rm eV$.
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### 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?
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 ...