We’re rewarding the question askers & reputations are being recalculated! Read more.

Questions tagged [units]

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

Filter by
Sorted by
Tagged with
39
votes
4answers
10k views

Can units be plural? [duplicate]

I was in a conversation with my senior engineer where he kept on insisting that we can use plural when we write down any unit. I argued that it is not the 'common' practice or even throughout my whole ...
-1
votes
1answer
126 views

How to convert a quantity in natural units

Suppose I am working with a system of units where $c = G = \hbar = 1$. I can then write e.g. a distance in units of kg by converting with a factor of $$ \frac{c^2}{G} $$ Now if I have an energy in ...
-1
votes
1answer
259 views

Why is $k$ taken as 1 in the derivation of $F=kma$? [duplicate]

In the derivation of F=ma, when we reach the point F=kma, we take k=1. Why can't we take 'k' as some other value?
0
votes
2answers
87 views

What allows us to treat physical units in algebra?

I have been thinking about this problem: $$Speed = \frac{Distance}{Time}$$ Following this, is makes sense that the units of speed is m/s. However, I do not follow why we are able to divide units to ...
0
votes
0answers
42 views

Scaling atomic units with energy

In Hartree atomic units, $e = m_e = k_e = \hbar = 1$, and \begin{align} 1&\mbox{ a.u. of energy is }E=\frac{m_ee^4}{(4\pi\varepsilon_0 \hbar)^2}=27.21\mbox{ eV.} \\ 1&\mbox{ a.u. of time is }\...
-2
votes
1answer
265 views

Converting illumination to irradiance

I need get intensity of an LED light source, but I cannot convert lux to $W/M^2$ How does one convert illumination to irradiance? And what is the irradiance formula?
-1
votes
1answer
265 views

Are ‘fundamental measures’ a thing?

The question I want to ask is: What measures are needed to describe the physical world and what are the fundamental ones of those, in the proper sense of the word fundamental? But that might be too ...
0
votes
1answer
40 views

Definition of a calorie - confusion

Wikipedia defines a calorie as the (approximate) amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius at a pressure of 1 atmosphere. Is it the case then that, ...
0
votes
1answer
85 views

Entity units from Jackson's Electrodynamics

From Jackson's equation (14.60) $\frac{d^2 I}{d\omega d\Omega}$ is the energy radiated per unit solid angle per frequency interval. From this I deduce that $I$ has energy units, and $\frac{1}{r^2}\...
0
votes
1answer
735 views

Find dimensional formula of $\alpha$ [duplicate]

The power delivered by a force is given by the relation $$P=\frac{\alpha}{\beta}e^{-\beta t},$$ where $t$ is time. Find the dimensional formula for $\alpha$. So, $-\beta t$ doesn't make sense for the ...
1
vote
0answers
49 views

Rewrite equations of physics using Planck units

I was reading this site http://astronomy.swin.edu.au/cosmos/P/Planck+Units and in it is stated that "If we use the Planck Units, we can rewrite all of the equations of physics such that: c = G =$ \...
-2
votes
2answers
76 views

Is there a simple way to represent this concept? [closed]

If something is represented in meters per second, it means the object is changing by x amount of meters every y amount of seconds. But when you have a unit of something similar to, "Meters · Seconds," ...
0
votes
2answers
6k views

How to convert the electron volt into Velocity unit?

About this question,I found this from the Wikipedia:nuclear fission produces neutrons with a mean energy of 2 MeV (200 TJ/kg, i.e. 20,000 km/s), which qualifies as "fast". For my question, I only knew ...
-1
votes
2answers
235 views

Mass-energy equation - The units [closed]

We know that $E=mc^2$ and we use the units from the international system of units instead of the values we get, $\rm J=kg\cdot m/s$. From what I've been taught, I know that momentum is using the $\rm ...
1
vote
2answers
300 views

Units of 1d Poisson equation seem… strange

I'm looking at a problem about the voltage along a 1D line, and the poisson equation comes up: $$ \nabla^2 V(x) = \frac{d^2 V}{dx^2} = \frac{\rho(x)}{\epsilon_0}$$ but if I look at the units of this ...
-1
votes
3answers
311 views

How does anything move through a physical distance? [duplicate]

I was wondering and came across a thought: if an object is moving at a particular speed, say 5 m/s, it is equivalent to say that the object is moving at 500 cm/s, or 5000 mm/s and so on; clearly, you ...
0
votes
2answers
161 views

Uncertainty in multivariable logarithm of a physical quantity

I have a problem in analysis of laboratory data, when I have to take the logarithm of a physical quantity. I know that logarithm is a dimensionless quantity. I have to logarithmize the physical ...
0
votes
1answer
61 views

Relationship between dynes and abamperes?

I've been reading about Ampère's law and the historic CGS systems of units, as part of some general reading on the history of electromagnetism. The Wikipedia article states that: The EMU unit of ...
-2
votes
1answer
67 views

Planck units and special relativity

The fundamental relation of special relativity linking time to position is $$ \begin{align} dx=cdt \end{align} $$ I am performing a change of variable to rewrite it in terms of energies as a Force F ...
0
votes
1answer
40 views

How I transfer (ohm*cm)^(-1) units for conductivity into Gaussian units (s^(-1))?

I have tables for conductivity of various material in (ohm*cm)^(-1) units. How I transfer those data into Gaussian units (s^(-1))? Just a coefficient will do....
0
votes
4answers
399 views

Unit of Angular velocity [duplicate]

Why is the angular velocity $\omega$ always written in $rad/sec$? Is there anything wrong if I write it in $degrees/sec$? If no, then why almost all the books have it as $rad/sec$??
0
votes
1answer
214 views

What units for calculating wavenumber of a matter wave?

So I have a problem where I need to calculate the wave number for proton of energy $40~\rm MeV$. I know the formula for wavenumber of a matter wave is $k = \frac{\sqrt{2mE}}{ \hbar}$. But what units ...
2
votes
5answers
175 views

Do the units of $G/c^4$ make intuitive sense?

In classical Newtonian mechanics, gravity is explained by: $$F=G \frac {Mm} {r^2}$$ Where $F$ is the force due to gravitation, $M$ and $m$ are the masses of the two bodies in question, $r$ is the ...
82
votes
6answers
14k views

Why is it “bad taste” to have a dimensional quantity in the argument of a logarithm or exponential function?

I've been told it is never seen in physics, and "bad taste" to have it in cases of being the argument of a logarithmic function or the function raised to $e$. I can't seem to understand why, although ...
2
votes
1answer
285 views

Units and the momentum eigenstate in position basis

Consider $\psi(x)=\langle x|\psi\rangle$. Such a wavefunction must have dimensions of inverse square root length to satisfy the normalization condition. Why then does $\langle x | p \rangle = \frac{1}{...
1
vote
3answers
145 views

Why the time period cannot depend upon the angular velocity and angular acceleration of the pendulum?

To derive the time period $T$ of a pendulum using dimensional analysis it is assumed that it depends upon the mass $m$ of the bob, the length of the string $\ell$, the acceleration due to gravity $g$ ...
1
vote
1answer
277 views

What determines whether a unit is a base unit? [duplicate]

I've had base units explained to me as units that cannot be broken down into a combination of other units. The Ampere is considered a base unit, however it can be written as coulombs per second. Is ...
1
vote
1answer
58 views

Why am I getting two different results in emu and SI unit?

I am computing force between two magnetic poles each of one unit pole (in emu) and situated one centimeter apart. In electromagnetic units: $$F_{dyne}=\dfrac{p^2}{r_{cm}^2}=\dfrac{1^2}{1^2}=1 dyne$$ ...
1
vote
1answer
294 views

Converting Hubble constant to GR Units

I'm working my way through Thomas Moore's General Relativity Workbook and having a problem with Exercise 24.4.1 (p. 288), which asks you to verify Hubble constant values (with errors) for a variety of ...
14
votes
7answers
3k views

Why are work and energy considered different in physics when the units are the same?

There is a question that explains work and energy on stack exchange but I did not see this aspect of my problem. Please just point me to my error and to the correct answer that I missed. What I am ...
1
vote
0answers
70 views

Units of a quantum electromagnetic field

The recalling $\mathbf p \rightarrow \mathbf p -q\mathbf A$, the vector potential has units $$ \langle \mathbf{A} \rangle \leftrightarrow\Big[\frac{\text{momentum} }{\text{charge} }\Big]$$ If we take ...
0
votes
1answer
45 views

$k_e$ (constant of Coulomb force) is sometimes seen in calculations for atomic radii and sometimes not

One of our formula for radius of atom was $\frac{n^2h^2}{4\pi^2me^2}$. And another one was, $\frac{n^2h^2}{4\pi^2me^2K}$. For hydrogen, Why $k_e$ is sometimes included and sometimes excluded ?
0
votes
2answers
111 views

In a force model, $F = ma$, how to understand the units?

For instance, in an aerodynamic force model, the force terms can be lift terms, drag terms, both of which have translational velocities as factors in their models; but, velocities are in units of, say,...
-1
votes
4answers
1k views

Units in the power equation

I have a question with the formula power = energy / time. I know it's a simple formula, but when substituting into this formula, Does the energy have to be in joules or watts (no kJ or something ...
3
votes
1answer
116 views

Finding a substitution of $\mu_0$ to convert from SI to c.g.s in EM laws [closed]

In c.g.s, we set $$k=\frac 1{4\pi\epsilon_0}=1$$ which gives $$\epsilon_0=\frac 1{4\pi}$$ This conversion works, for example, in Gauss' law: in SI $$\vec{\nabla}\cdot\vec{E}=\frac\rho{\epsilon_0}$...
15
votes
5answers
3k views

Why does a calculation to count objects covering a certain area seem to give nonsensical units?

Suppose you want to estimate the number of atoms in a rectangular sheet of graphene. You might estimate the sheet to have $10^{7}$ atoms along one edge and $2*10^{7}$ atoms along the other edge. ...
10
votes
2answers
1k views

Proper reference for the International System of Units

I would like to cite a relevant document that guides usage of SI units. It seems like currently there are two nearly identical literature sources: 8th edition of SI Brochure issued by BIPM, 2006; ...
1
vote
1answer
98 views

Dimensions/Units and Matrix Inverses

I was going through a homework problem which is essentially a math review in a kinematics frame. This group of problems start as follows, Given $a_x$, constant acceleration, and initial conditions, ...
1
vote
2answers
1k views

Why is unit of pressure (psi) used to determine things like bite force of an animal?

Whether in tv documentaries or journals, whenever they talk about an animal's bite force, it's measured in PSI anytime imperial units are used (ex: National Geographic, NIH Journal). Many even seem to ...
-5
votes
2answers
813 views

What is the name and symbol of the derived unit for one thousand metres? [closed]

I'm still getting confused of what a derived unit is. I used the internet to find an answer but I'm still getting confused.
0
votes
1answer
171 views

Electromagnetic Dirac equation in CGS units

The Hamiltonian of the Dirac equation in with an electromagnetic field in SI units is $$H=\gamma^0\left[ mc^2+c\gamma^k\left( p_k-\frac{q}{c}A_k \right) \right]+qA^0$$ (from https://en.wikipedia.org/...
0
votes
3answers
2k views

Velocity squared in natural units

I encountered a mental block in my understanding of natural units. According to natural units, $c = 1$, which would imply that if we wanted to convert velocity from SI units ($m/s$) to natural units (...
0
votes
2answers
1k views

Are the Amp and coulomb equivalent?

1 coulomb is 1 ampere per second and 1 ampere is 1 coulomb per second therefore they are always equivalent. They would only be different if you changed the time but the units themselves are based only ...
-1
votes
1answer
543 views

Why are ft-lbs used to describe muzzle energy? And how do you work with it?

Whether it's kicks, punches, or bullets, people use ft-lbs or joules to describe kinetic energy. But how is this useful? Why use torque to describe linear motion such as a bullet? And how does one ...
1
vote
1answer
258 views

What are the units for the C parameter in the formula for surface excess concentration?

I came across this with a lecturer while discussing the Gibbs-Adsorption Isotherm. The lecturer insisted that the unit for the concentration parameter in the the formula surface excess concentration ...
1
vote
1answer
203 views

How do the radian have a unit? [duplicate]

The radian is defined as the ratio of the circumference and the radius. Both are measured in meters. So there should not be a unit for that. But we use 'rad' as the unit of the radian value. The ...
1
vote
0answers
71 views

TINKER molecular modeling software package- Boltzmann constant units [closed]

I use the TINKER molecular modeling software package (version 7.1.2) for computational physical chemistry. There is an extra file (extra1.f) included for the user to edit and incorporate in their own ...
2
votes
1answer
133 views

Confusion over the standard units used in special relativity and particle physics? [duplicate]

I am new to relativity and particle physics and I am struggling to understand the conventions for the dimensions that are used in particle physics. I have read this and as I understand it: $$\sqrt{s}=\...
1
vote
1answer
526 views

Does the diffusion coefficient depend on units of concentration?

I'm sure this is an elementary question, but I was struggling to explain the following concept to a (math) student recently and it exposed my own deficiencies in discussing units. In Fick's law(s), ...
6
votes
4answers
11k views

What is the unit (dimension) of the 3-dimensional position space wavefunction $\Psi$ of an electron?

I googled for the above question, and I got the answer to be $$[\Psi]~=~L^{-\frac{3}{2}}.$$ Can anyone give an easy explanation for this?