Questions tagged [si-units]
A set of internationally accepted units to aid in communication of measurements.
329
questions
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Please check my problem about SI prefixes [on hold]
2.2 A single logic gate in a prototype integrated circuit is found to be
capable of switching from the “on” state to the “off” state in 12 ps. This
corresponds to: (a) 1.2 ns; (b) 120 ns; (c) 1200 ns; ...
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0answers
57 views
How Do I solve Newton’s law of gravity problem? $F = G (M1 * M2) / r^2$ [closed]
I have been give this problem:
F=(6.67∗10−11 (Nm^2/kg^2)) ∗ ((5.64∗1024kg) (1.99∗1031kg)) / (1.51∗1015∗m)^2
I do not understand (Nm^2/kg^2) but I do understand everything else
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5answers
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Units of variable acceleration
If we have a function, which describes, how a displacement in space along a line varies as a function of time: e.g.: $s(t)=vt$, its units are meters because $[v]=\frac{\text{meters}}{\text{seconds}}$ ...
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1answer
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What's the preferred way of writing units per unit? [closed]
Why do we write units such as meters per second as $\mathrm{m\,s} ^{-1}$ instead of $\mathrm{m/s} $. Is the latter still a valid way of expressing units? Why are there two different ways of expressing ...
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Dimension of the candela unit: What does J stand for? [migrated]
The J symbol can represent the unit of energy but it's also the symbol for the dimension of the candela (or luminous intensity).
For the energy unit, it clearly comes from the family name of the ...
3
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1answer
93 views
Gauss' law for two point charges and a factor of $4\pi$
When we model electric field as something that represents the "flow" of something that is conserved, we can prove that the flux due to a single point charge, through the following surface,
Flux ...
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2answers
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Is our physics and system of units based on time (second) or the speed of light?
The speed of light in vacuum is stated in our physics as a universal physical constant, c, when measured locally, in vacuum.
The speed of light in vacuum, commonly denoted c, is a universal ...
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24 views
Units and measurement
I read on the characteristics of units
(i) It should be easily understandable.
(ii) It should be changed with change in physical factors.
(iii) It should not change with place or time.
I want ...
0
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1answer
32 views
What is the unit of the current in a square barrier model?
In Quantum Mechanics textbooks, the equation for a electron tunneling through a barrier is
$$-\frac{\hbar ^{2}}{2m}\frac{d^{2}}{dx^{2}}\psi \left( x\right) +U\psi \left(x\right) =E\psi \left( x\right)...
2
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1answer
63 views
Why does one Coulomb equal that strange number? [duplicate]
Why is one Coulomb equal to 6.24 *10^18 ?
This number is weird . why wasn't it a nice number like 10^20.
I have some guesses about this.
3
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3answers
184 views
How can I explain what a kilogram is using Planck's constant? [duplicate]
I want to understand what 1 kg represents. For example: I know that 1 second is equal to $9\ 192\ 631\ 770$ transitions from the microwave radiation that a cesium-133 atom (at $0$K) emits, if it's ...
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3answers
75 views
Energy Density is expressed in Liters, Liters of What?
When looking at energy density of batteries it is often expressed as Wh/L. For instance, Wikipedia says Lithium Ion batteries have a density of 250–693 W·h/L. Liters is supposed to express the volume ...
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1answer
68 views
New S.I. and the second definition standard
The new SI defines the second as the hyperfine splitting transition time associated to the frequency of that transition of caesium-133, 9192631770 Hz, that is 12 decimals precision, but...I wonder two ...
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1answer
88 views
Why $µ_0$ and $ε_0$ are not considered to be $4π$ and $1/4π$? [closed]
I did some algebra...
In Planck unit, if make $\mu_0 = 4\pi$ and $\epsilon_0 = \frac{1}{4\pi}$ you get:
$$\mu_0 = 4\pi \cdot \frac{m_p l_p}{t_p^2 I_p^2} = 1.2566368452237765 \cdot 10^{-6} N \cdot A^...
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2answers
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Why is the length of the Kelvin unit of temperature equal to that of the Celsius unit? [duplicate]
The Celsius unit is arbitrarily defined, based on the boiling and freezing point of water. Is it a coincidence, then, that the SI unit of temperature Kelvin, which is used in all natural equations, ...
8
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4answers
2k views
The significance of kelvin as a unit of absolute temperature
$$PV=nRT$$
Where $P$ is pressure in pascals ($\text{Pa}$)
$V$ is volume in cubic metres ($\text{m}^3$)
$n$ is amount of substance in moles ($\text{mol}$)
$R$ is the gas constant having units, joules ...
3
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3answers
219 views
Why are there $1 / 1.602176634 \times 10^{-19}$ electrons in a coulomb?
Why that exact number of electrons in one coulomb? who decided it? there is nothing wrong with the number, it just seems slightly messy. Why didn't the scientific community just settle on an easier ...
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0answers
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Units of the Klein-Gordon Propagator in SI Units
What are the SI units of the momentum-space propagator of the Klein-Gordon equation for a free particle?
1
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1answer
39 views
Why is angles given at the end of fundamental units? [closed]
In my study material, there is a chart of the fundamental units. In that solid angle and angle are separated by a line from the other fundamental units. What is the reason for that?
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5answers
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Watts vs. volts amperes
What I understand:
In simple DC circuits, this is a product of the current and voltage, such that 1 watt = 1 ampere x 1 volt
I understand that a watt is a unit of power (change in energy per unit ...
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0answers
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Does the New SI spoil the nice Coulomb law in Gaussian units?
Now, after the redefinition of SI, the elementary charge $e$ and the reduced Planck constant $\hbar$ (and also $k_B$) are exact quantities in the SI units, as is the velocity of light $c$. The ...
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0answers
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SI redefinition of the kilogram - what is one measuring? [duplicate]
I have been reading about the new SI units and specifically, want to get a better understanding of the definition of a kilogram. It was written that the kilogram will be defined in terms of Planck's ...
40
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3answers
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Will the volt, ampere, ohm or other electrical units change on May 20th, 2019? [duplicate]
When watching a video by Veritasium about the SI units redefinition (5:29), a claim that the volt and unit of resistance (presumably the ohm) will change by about 1 part in 10 million caught my ...
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3answers
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Redefinition of everything on May 20th, 2019 [closed]
A couple of issues:
So after May 20th, 2019, what exactly will be the defined value of $\hbar$?
What will be the defined number of elementary charges in a Coulomb?
Then $\mu_0$ and $\epsilon_0$ will ...
3
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2answers
86 views
How to find the corrsponding expression after working with natural units $\hbar=c=1$?
If one does long calculations in natural units how does one find the right expression in let's say SI units in the end?
I know that natural units make the calculations easier and also help to show ...
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2answers
44 views
Why have a conversion factor of $10 \:\rm mA / 10 \: mV$ instead of $1\:\rm A/1\:V$?
I’m working with a device that measures current by measuring voltage and then giving current with a conversion factor of $10 \:\rm mA / 10 \:mV$. Why write this instead of $1\:\rm A/1\:V$?
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1answer
86 views
In the SHM equation $F= -kx$, $k =mw^2$ why not use $mf^2$ where $f$ is frequency $w$ here comes out to be $1/s$ not $\text{rad}/s$?
The reason I am stating this is because on calculating the units of w(omega) I found is equal to s^-1 not regular the rad/s.
Proof:
...
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2answers
63 views
Has the kilogram changed?
In late 2018 the kilogram's definition switched from a physical object (the International Prototype Kilogram) to Planck's Constant; a grand move in my opinion.
However, I haven't heard of the ...
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0answers
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Grasping systems of units
Perhaps it's that I've grown up with the SI system of units, but there's something about other unit systems that doesn't sit well with me. For example, Coulomb's law in Gaussian units can be written ...
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0answers
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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 ...
4
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4answers
259 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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2answers
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About the dimension of the SI units vector space
We know that the set of fundamental and derived physical units can be structured as a vector space over the rational numbers. In the International System of Units the dimension of this space is $7$ ( ...
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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 ...
0
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1answer
80 views
A question about natural/geometrized units
I had a question about the following document- Natural units
I understand the conversion factors. But if you look at the tables, they take an SI unit, say 1 kg, convert it into geometrized units, say ...
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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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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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0answers
235 views
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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1answer
79 views
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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1answer
281 views
Why is the MKS unit of time the same as the CGS unit? [closed]
There are many system of units used in physics. In the CGS, the units are,
length : centimetre
mass : gram
time : second
And in the MKS system the units are,
length : metre
mass : kilogram
time : ...
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3answers
151 views
Is natural frequency of an LC circuit equal to angular frequency? Why don't the units match?
When I was reading the LC circuit in my textbook I came across the derivation of equations of instantaneous charge and current.
Which is no problem, but when I got to the derivation to current . It ...
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3answers
112 views
What was wrong with the old definition of temperature scale in kelvin?
Wikipedia's article on the recent change to the definition of the SI base units states, as the reason for changing the definition of the kelvin:
A report published in 2007 by the Consultative ...
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3answers
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What is $\text{kg}\cdot \text{m}^2$ concretely? (multiplication of units)
I have problem to understand what $\text{kg}\cdot \text{m}^2$ (moment of inertia) is. So, for example a force does a work of $3\text{J}=3\text{Nm}$ means that the force can displace a weight of $3\...
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1answer
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Is there a “standard” Newton?
Basic SI units have definitions through experiments that seems to imply a pretty obvious setup.
Is there a standard experiment for calibrating Newtons?
The definition is the force needed to cause a ...
1
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1answer
57 views
Units of Klein-Gordon equation
I'm looking at the units of the Klein-Gordon equation
$$u_{tt} - c^2\Delta u = -\frac{m^2c^2}{\hbar^2}u.
$$
Disregarding the units of $u$, which are the same everywhere and so cancel, I get $seconds^{-...
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1answer
141 views
How did Coulomb arrive at value of electron charge?
Charge of one electron is known to be as $1.6$ x $10^{-19}$ C or alternative 1 Coulomb contains charge of $6.24$ x $10^{18}$ electrons. I am just wondering if these numbers are arbitrarily chosen or ...
2
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2answers
142 views
Does the death of Kilogram ($kg$) affect us in any means in our day to day life? [closed]
Recently, the sleek cylinder of platinum-iridium metal has been discarded and the kilogram is set to be redefined along with ampere for electricity and Kelvin for temperature. Hereafter the Kilogram ...
1
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1answer
2k 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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1answer
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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)...
6
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2answers
194 views
Will the SI units need redefining ever again?
Up until recently, there were obvious problems with the SI definitions of fundamental units, like bits rubbing off the kilogram prototypes (or mercury vapour absorption), and the water used for the ...
0
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1answer
79 views
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 ...
