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Questions tagged [si-units]

A set of internationally accepted units to aid in communication of measurements.

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10
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3answers
179 views

Why was the Planck constant $h$ fixed to be exactly 6.62607015×10−34 and not some other value?

So apparently in May, all of the SI base units were redefined to be relative to the Planck Constant $h$, instead of relying on physical objects like the Kilogram Prototype in a Paris Basement. Planck'...
0
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0answers
18 views

Units in Representation Of Energy Density Spectrum

In most of the texts, I referred to The Axis being labelled with Units as shown above. But I also found few images with different Notation (1/Units) Being Followed as shown Below. The units of the ...
0
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1answer
34 views

Help understanding unit: Micromoles per square meter per second

The context is light, illumnination, photons. The units seem to suggest something different from the definitions I have found: $$\frac{µmol}{m^2 s}$$ This, to me, says I have one millionth of a ...
0
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2answers
34 views

Which luminous efficacy function does BIPM reference?

The definition of candela given by BIPM is as follows: The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the ...
55
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6answers
10k views

Why is the mole/“amount of substance” a dimensional quantity?

According to the BIPM and Wikipedia, "amount of substance" (as measured in moles) is one of the base quantities in our system of weights and measures. Why? I get why the mole is useful as a unit. In ...
53
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3answers
20k views

Why is the ampere a base unit and not the coulomb?

I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the ampere that is the base unit and not the coulomb. Why is this? It seems to ...
0
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1answer
22 views

Quantity, kind of quantity, and dimension

The International Vocabulary of Metrology defines a kind of quantity as the following: However, to me NOTE 1 EXAMPLE 1 and NOTE 1 EXAMPLE 2 are contradictory with NOTE 2 EXAMPLE. NOTE 2 EXAMPLE ...
-1
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0answers
40 views

What physical quantity does Mass Moment of Inertia times Torque squared times Seconds may represent?

In some calculus I performed involving a mass moment of inertia, a torque squared and time, the resulting SI units were (kg.m^2) (kg.m.s^(-2))^2 (s) = (kg^3) (m^6) (s^(-3)) What may this quantity ...
-3
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1answer
53 views

Please help with calculation /conversion

Can someone please tell me how much 100 mw/ m-1 is converted to v /m? In terms of electromagnetic radiation? I have a gaussmeter that reads about 10-20 volts per meter in one area for electric field ...
2
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2answers
138 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 ...
0
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3answers
73 views

Is there a name for the unit “Ampere meter”?

Motivation: I'm doing a homework problem involving a rod sliding freely down a pair of parallel conducting rails. I've got a quantity of unit $\mathrm{A \cdot m}$ and want to know what I should name ...
4
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5answers
320 views

Why is there no constant factor in the equation $E=mc^2$? [duplicate]

As I understand, the metric system starts with an arbitrary weight, and calls it $1\, \mathrm{kg}$, then the volume of this weight in water gives the meter. And then the energy is defined based on the ...
0
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1answer
57 views

What unit system is used in Feynman's Lectures on Physics, Pt. 2

In part II, lecture 18, of Feynman's Lectures on Physics, on Table 18-1 Feynman writes Ampère's law as $$ c^2 \nabla \times \vec{B} = \frac{\vec{j}}{\epsilon_0} + \frac{\partial \vec{E}}{\partial t}. $...
0
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2answers
103 views

Addition of Temperature Units [duplicate]

I would like to know how does the addition of temperatures works. In school, we have been taught that we should convert degrees Celsius to Kelvin unit first and then the sum will be converted back to ...
-4
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2answers
98 views

Why is the ångström not a metric unit? And why is the ångström spelt with the Scandinavian letters “å” and “ö”?

The website here http://unitsofmeasure.org/ucum.html tells us whether every unit is metric or not. Metric units can be multipled by a power of 10 and can be combined with a prefix. 1 ångström is ...
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0answers
15 views

Conversion formula between attenuation coefficient (cm^-1) and (dBcm^-1MHz^-1)

So, I was presented in class the fact that (at least in Ultrasound), attenuation coefficients $\mu$ can be expressed in units $\text{dBcm}^{-1}\text{MHz}^{-1}$ as opposed to the form I was used to; ...
0
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1answer
26 views

Electrostatics, measurement of charge

How did physicists come up with a standard for measuring charge? How an electrical measurement of charge was made?
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0answers
12 views

Planck's Constant to the Kibble Balance

A tad confused on how the Kibble balance actually relates to the Planck Constant. There's most likely a very simple relationship that I'm missing (I feel this is a stupid question ..) If the Kibble ...
0
votes
1answer
35 views

How is the Time Derivative of the the Electric Field Equal to the Current Density in Gaussian Units?

The microscopic form of Ampere's law with the Maxwell addition in Gaussian units states, \begin{equation} \nabla \times \vec{B} = \frac{1}{c} \left ( 4 \pi \vec{J} + \frac{\partial \vec{E}}{\partial ...
0
votes
1answer
123 views

Why $1 \ \mathrm{fm}$ is often used in modern physics?

In the International System, we all know that a one femto (or one Fermi) is equal to $$1 \ \mathrm{fm}=10^{-15} \ \mathrm{m}$$ What is the historical reason why this unit of measure was adopted as ...
0
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0answers
75 views

Is there a notation alternative to Hertz?

Hertz, as we all know, is a unit of frequency where 1 Hz equals one cycle per second. Conversely ...
0
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1answer
40 views

Basic question about units of velocity and speed of a curve on a smooth manifold

Frederic Schuller says that velocity has units in Hertz in The WE-Heraeus International Winter School on Gravity and Light. He says: \begin{align} [v^a]&=\frac 1 T \\ [g_{ab}]&=L^2 \\ \Big[\...
1
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6answers
54 views

Standard Definition of speed of light and metre

The speed of light is the speed at which lightwaves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per ...
1
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3answers
68 views

Unit of Pressure - $\frac{N}{m^2}$ or $\frac{kg}{cm^2}$

I am came across Pressure being given with unit $\frac{kg}{m^2}$ at a lot of technical papers I am reading. However, as far as I understand, Pressure is defined as $\frac{N}{m^2}$ which is not same ...
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1answer
62 views

How is do you say the “standard acceleration of gravity, $g=9.80 m/s^2$” in words? [closed]

How is do you say 9.80 m/s^2 in words? (I Don't get if only "the second" is squared or the whole thing is squared or just the m/s is squared)
2
votes
1answer
85 views

Why is a Coulomb the charge of $6.24 \times 10^{18}$ electrons? [duplicate]

Where does the $6.24 \times 10^{18}$ number come from? How was it historically derived? I know that $1$ C $=$ $1$ A s but that just pushes the question down another step, and another and another, at ...
2
votes
1answer
111 views

Confusion on the cgs, Heavside-Lorentz and SI system

I am very confused about three measuring systems: cgs, Heavside-Lorentz (never used) and SI ...
0
votes
1answer
602 views

Dimensional Analysis in Electromagnetism (SI vs Gaussian-cgs)

Looking at Konopinski's formula for conjugate momentum (in the comment after equation 3 of "What the Vector Potential Describes"): $\mathbf{p}= M \mathbf{v} + q\mathbf{A} /c$ it is plain enough that ...
0
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2answers
90 views

Unit confusion - How Pressure and Volume combines to Joule

Imagine some helium in a cylinder with an initial volume of $1$ liter ant initial pressure of $1$ atm. Somehow the helium is made to expand to a final volume of $3$ liters, in such a way that its ...
5
votes
3answers
123 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 ...
0
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1answer
39 views

Differences charge quantity and electric charge

First of all,my english is not well,so sorry for reading. As a senior middle school from China mainland,I am teaching physics about electri field.I with my workmates,get a problem now.We can not get a ...
0
votes
2answers
541 views

Convert cubic feet per second to feet per second

Is there a way to convert cubic feet per second to feet per second. Or in general volumetric flow to velocity? I want to know the time taken by water to travel from point A to point B. I have the ...
-4
votes
1answer
62 views

Is there a library of physical units?

There are some standard units we all understand intuitively such as: Mass (kg) Length (metres) Time (seconds) From these there are some familiar composite units which are also well understood: ...
1
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3answers
291 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. 7.8 LC Oscillations We know that a capacitor and an inductor ...
1
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0answers
39 views

Please check my problem about SI prefixes [closed]

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; ...
0
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1answer
61 views

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 ...
0
votes
5answers
108 views

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 $[v]=\frac{\text{meters}}{\text{seconds}}$ ...
-3
votes
2answers
78 views

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 ...
0
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0answers
27 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
votes
1answer
33 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)...
52
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11answers
33k views

What is the logarithm of a kilometer? Is it a dimensionless number?

In log-plots a quantity is plotted on a logarithmic scale. This got me thinking about what the logarithm of a unit actually is. Suppose I have something with length $L = 1 \:\mathrm{km}$. $\log L = \...
2
votes
1answer
66 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.
-1
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1answer
1k views

Is displacement vector fundamental or derived quantity?

We know that we have 7 fundamental quantities (all scalars) and length is one of them. I classify velocity as a derived quantity. What about a position displacement vector? How do I classify ...
3
votes
3answers
191 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 ...
0
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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 ...
59
votes
2answers
3k views

What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?

The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the ...
1
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1answer
88 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 ...
-2
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1answer
91 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^...
8
votes
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 ...
13
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2answers
3k views

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, ...