Units are standards of measurement used for different types of quantities.
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2answers
46 views
Dividing values with units
I'm reading about the subject of heat in a basic physics book. If I am not mistaken the formula to work out how much energy is required to increase the temperature of water is
...
7
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5answers
134 views
Physical representation of volume to surface area
I was looking at this XKCD what-if question (the gas mileage part), and started to wonder about the concept of unit cancellation. If we have a shape and try to figure out the ratio between the volume ...
-1
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1answer
48 views
Convert units from cal cm^-1 day^-1. to Wm-2
I am trying to calculate the energy budget of a water body and the last term I'm trying to calculate is the energy advected from the water body ($Q_v$). This equation is given by:
$Q_v = V ( (T_1 - ...
0
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1answer
37 views
The units of gain and number of atoms in population inversion in a laser
I am following my university course notes on amplification in laser media, and have come across expressions for the gain of a medium, but the notes are not exactly rigorous... The expression given for ...
3
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1answer
151 views
What are units actually?
This question is about the concept of units in physics.
Firstly - do units have a formal mathematical definition? How are they different from pure numbers? Are pure numbers defined to be ratios of ...
1
vote
2answers
40 views
Base quantities and charges
Is there an unit of color charge? I haven't found it, so I suppose that it doesn't exist, if this is right, why? Isn't it supposed that every measurable quantity can be expressed in terms of base ...
0
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2answers
120 views
Why metric system uses kilogram as a basic SI unit?
SI system uses all (that I know) measurement basic units as 1 (single) instance: meter, second, ampere, etc, except the KILOgram. It already defined with 1000 multiplier (kilo).
It prevents from ...
1
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1answer
89 views
When should angles be expressed in degrees vs. radians?
I am trying to calculate the albedo of a given latitude by following the methods of Brutsaert (1982), I have copied the formula below:
3.6 Shortwave and long-wave radiative fluxes
Albedo ...
0
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1answer
58 views
Magnetic Field and the Speed of Light
Is it just a historical choice that both magnetic field and the Lorentz force equation include the speed of light?
I figure that whoever wrote up the equations (in cgs!) could have put both factors ...
3
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1answer
48 views
CGS Units for Magnetism
Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra $c$ go in SI units?
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1answer
45 views
Rotation System Dynamics - Unit conversion issues
I'm having issues with unit conversions when for rotational systems. All of my components (spring, mass, damper) come in Imperial units, so I did all my calculations in Imperial. When I convert to ...
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0answers
19 views
How to convert lb/MMBTU to concentration?
This might be a stupid question but...
I am looking over this document for allowable emissions for PM particles in biomass furnaces.
They are given in lbs/MBTU, pounds per million British Thermal ...
3
votes
5answers
510 views
Does the unit of a quantity change if you take square root of it?
For example, I have a mass, m = 0.1kg and I square root it, giving me m = 0.316 (3s.f.) does the unit still stay as kg, or does it change?
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2answers
73 views
What Physical Quantity has SI Unit $kg \cdot m$,? [duplicate]
What is the physical quantity that has SI (System International) unit of $kg \cdot m$ (kilogram meters)?
3
votes
1answer
78 views
Units for physical constants
Someone told me that units for $G$ and $\epsilon_0$ (gravitational constant and Coulomb's constant) are placed there simply to make equations work dimensionally and that there is no real physical ...
2
votes
2answers
44 views
What are the units of these virial coefficients?
I'm reading some papers for calculating the vapor pressure of alkali metals as a function of temperature, and I've come across some familiar-looking virial expansions, but when I tried to work out the ...
16
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6answers
1k views
Why isn't temperature measured in Joules?
If we set the Boltzmann constant to $1$, then entropy would just be $\ln \Omega$, temperature would be measured in $\text{joules}$ ($\,\text{J}\,$), and average kinetic energy would be an integer ...
-1
votes
4answers
577 views
Why there is no “Edison” unit in physics? [closed]
In the popular culture the XIX-XX century competition between Thomas Edison and Nikola Tesla is well-known. The example could be the Prestige movie, where there are some "Edison's agents" who sabotage ...
17
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8answers
1k views
Is there a symbol for “unitless”?
I'm making a table where columns are labelled with the property and the units it's measured in:
Length (m) |||| Force (N) |||| Safety Factor (unitless) ||| etc...
I'd like not to write "unitless" ...
2
votes
3answers
71 views
Curious relation between the dependance in ℏ of Planck units and units dimensions
Looking at Planck units, there seems to be a curious rule between the dependance in $\hbar$ of a Planck unit and the unit dimensions of the corresponding physical quantity.
Let the dimensions of the ...
1
vote
2answers
59 views
Units and Dimensions - Use of proportionality constant
In units and dimensions we learn about Establishing a Formula :
(example) : to establish a relationship between T (Time Period) , m (Mass) , l (length of the string) and g(acc. due to gravity) - ...
0
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0answers
52 views
cgs Gauss' system of units
I had never seen this system until today, and I'm really confused. I've read the wikipedia article about it but I still don't know how to change between this and the international system. For example, ...
-1
votes
2answers
82 views
Which units has the relation $E=mc^2$?
Fast everyone knows the relation $E = mc^2$ but I still ask me, which units the relation has.
$c$ is the velocity of light. It means, a constant, isn't it? We therefore just have the relation $E = ...
1
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1answer
63 views
Difference between nautical and terrestrial miles
Does someone know the historical reason behind the difference in physical units between nautical and terrestrial miles?
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3answers
149 views
Temperature in CGS (Gaussian) units
I've been struggling with conversion from Gaussian to SI units for sometime, trying to figure out how derived units in CGS (current, charge etc) relate to the SI units.
But I couldn't find any ...
1
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2answers
106 views
How to check units?
I've got: $Q=\frac{Er^2}{k}$
how to check the units?
I start with $\left[\frac{\text V}{\text m} \, \text m^2\right]$, tried replacing $[ \text V ]$ with $\left[ \frac{\text J}{\text C} \right]$, but ...
2
votes
1answer
111 views
If we are using [eV] as a unit for energy, what, then, should the unit for mass and distance be?
This seems obvious but it is confusing since I know that in MKS (i.e. SI) system, we use Joule with Meter with KG.
But if we're using electron volt (1.6e-19 J) for energy, should we change the mass ...
0
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1answer
57 views
Buoyancy fluxes in a stratififed fluid and units
I am calculating the buoyancy flux (B) for a stratified fluid as follows:
where g = 9.81 m/s; alpha = 1.6 x10^{-5} + 9.6 x10^{-6} x (20 degC); S = 100 Wm^{-2}, p0 = 1000 kg/m3, and Cpw is the ...
0
votes
1answer
68 views
What is the significance of Planck force?
I have been curious to find what could be the significance of Planck force? It is calculated by the formula $c^4/G$,
where $c$ is the speed of light; $G$ is the gravitational constant. Thus (the speed ...
1
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1answer
80 views
Clarification on factors of $c$ in the Lorentz' force
I was told that Lorentz' force is given by
$${\bf F}= q{\bf v} \times {\bf B}.$$
But I have read that it is given by $${\bf F}= \frac{q}{c}{\bf v} \times {\bf B}.$$
Why have these two relations ...
2
votes
3answers
119 views
Correct expression for D'Alembert operator in $c=1$ units
In QFT texts with $c=1$ units (most of them), D'Alembert operator is written as:
$$\Box ={\partial^2 \over \partial t^2} - \nabla^2$$
For pedagogical purposes, however, some texts don't set $c=1$, ...
1
vote
3answers
190 views
Factors of $c$ in the Hamiltonian for a charged particle in electromagnetic field
I've been looking for the Hamiltonian of a charged particle in an electromagnetic field, and I've found two slightly different expressions, which are as follows:
$$H=\frac{1}{2m}(\vec{p}-q \vec{A})^2 ...
4
votes
7answers
358 views
What exactly are we doing when we set $c=1$?
I understand the idea of swapping from unit systems, say from ms$^{-1}$ to kms$^{-1}$, but why can we just delete the units altogether?
My question is: what exactly are we doing when we say that ...
1
vote
2answers
167 views
Showing that position times momentum and energy times time have the same dimensions
I've been asked to show that both the position-momentum uncertainty principle and the energy-time uncertainty principle have the same units.
I've never see a question of this type, so am I allowed to ...
0
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0answers
65 views
Units and use of the formula of the angular acceleration? [closed]
Question: On my document, it is written:
Angular acceleration:
$$ \frac{d\omega}{dt}=r\times \alpha $$
What are the units please?
What are $d \omega$, $dt$ and $r$?
I already know that $\alpha$ is ...
0
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1answer
169 views
CGS Units to SI Units
This may be a stupid question but I am having trouble getting the same result in CGS units as I would if I used SI units for a unitless calculation.
I have to calculate $E_c=\frac{m_e c\, ...
2
votes
1answer
155 views
Why is the candela dimension J, not W?
According to the table at the bottom of the Wikipedia page for the candela, the dimension for candelas is J (joules). Why is this not W (watts)?
The luminous intensity for light of a particular ...
4
votes
1answer
273 views
$E^2 = (mc^2)^2 + (pc)^2$: What units are used to measure $E$, $m$, $c$ and $p$?
\begin{equation}
E^2 = (mc^2)^2 + (pc)^2
\end{equation}
If I am using this equation to figure out the energy of something, what units would I use? Would it be the metric system? I.e. kilograms ...
1
vote
2answers
147 views
Special relativity, spacetime, velocity and units
We all know that space and time are the fundamental units, means no mathematical expression can express their relation to other variable fundamentally. But as we know that moving rod has a contracted ...
2
votes
1answer
213 views
What really is Planck's constant and what are its origins?
In the physics texts I have read and other info online, they says Planck's constant is the quantum of action or that it is a constant of the ratio of the energy of a particle to its frequency. Im ...
2
votes
6answers
344 views
Confused about unit of kilowatt hours
So I am a little confused on how to deal with the Kilowatt hours unit of power, I have only ever used Kilowatts and I have to design a residential fuel cell used as a backup generator for one day.
...
0
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0answers
67 views
I'm having trouble understanding the dimensions of this falling body problem
I'm working with this:
A 50 kg mass is shot from a cannon straight up with an initial
velocity of 10m/s off a bridge that is 100 meters above the ground.
If air resistance is given by 5v ...
0
votes
1answer
56 views
Is $kg_f$ the same everywhere in the universe?
A textbook question says that a vehicle weighs $25kg_f$ on Earth, and asks us to consider certain issues related to its behaviour on the moon.
My question is, does the unit kilogram-force $kg_f$ ...
15
votes
3answers
740 views
What is the length of 1 second in meters
If time is treated as a fourth dimension of spacetime, what is relation between length and time units?
Or in other words, how can I convert time units to length units, for instance seconds to meters?
...
5
votes
2answers
198 views
Einstein tensor in Friedmann equations : where is the missing $c^2$?
I would like to demonstrate the several forms of the Friedmann equations WITH the $c^2$ factors. Everything is fine ... apart that I have a missing $c^2$ factor somewhere.
In all the following $\rho$ ...
1
vote
2answers
303 views
What is the physical interpretation of force times area?
I know that $\text{Force} \times \text{Distance = Work}$.
But, what would be the physical meaning of $\text Force \times \text Area?$
Is such a quantity used in physics?
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1answer
131 views
Units in cgs system
How do I find the dimensions of this quantity (in $cgs$)...
$$\frac{4\pi me^2}{h^2n_o^{1/3}}$$
where $m$ is the mass of electron
$e$ is the magnitude of electronic charge
$h$ ...
0
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1answer
178 views
What is the difference between “in-plane shear modulus” and “shear modulus”
In the wiki it is article only about shear modulus and it is written that it's unit is Pascal.
At the same time in all researches about Red Blood Cells shear stresses, authors write "in-plane shear ...
12
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6answers
2k views
Why is torque not measured in Joules?
Recently, I was doing my homework and I found out that Torque can be calculated using $\tau = rF$.
This means the units of torque are Newton meters. Energy is also measured in Newton meters which are ...
8
votes
2answers
920 views
273 + degree Celsius = Kelvin. Why 273?
Temperature conversion:
273 + degree Celsius = Kelvin
Actually why is that 273? How does one come up with this?
My teacher mentioned Gann's law (not sure if this is the one) but I couldn't find ...
