# Tagged Questions

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

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### 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 ...
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### Why is the candela a base unit of the SI?

The candela is defined as The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency $540\cdot10^{12}$ hertz and that has a radiant ...
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### Relation between Newtons and Kilograms

Work is expressed as $W=Fd$, where the $F$ is in Newton, $d$ is in meters and result $W$ is in Joules. For example, if I take $1N$ on earth and lift it $1m$ up in the air I have done $1J$ of work. ...
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### Units for volume and density

I am a bit confused with using the appropriate units. I have two values for density and volume which contain units of: Density $\rho$ = kg m^-3 Volume v = m^3 One thing i am confused here is why ...
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### Dimension of an angle [closed]

Usually angles are described as dimensionless, justifying this by saying that they can be viewed as length divided by length. As a student of mathematics I'm asking myself wether this is a convention ...
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### How are the SI units “generalised”?

How exactly are the SI units generalised from their definitions? E.g. the kilogram is a weight of an object of cylindrical form, with diameter and height of about 39 mm, and is made of an alloy of ...
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### Pressure at ground level and at sea level

1atm = 101325pa 1bar = 100000 pa 1atm = 1.013 bar 1bar = 0.987atm From wiki: The bar is a metric (but not SI) unit of pressure exactly equal to 100000 Pa.[1] It is about equal to the atmospheric ...
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### Coulomb force in SI and cgs

Coulomb force in SI is $F = \frac{Q1*Q2}{4\pi\varepsilon R^{2}}$ while in CGS $F = \frac{Q1*Q2}{R^{2}}$ why is it? I mean doesn't it any make difference in dimension? since $\varepsilon$ ...
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### Why does Coulomb's constant have units?

I think of Coulomb's constant as a conversion factor (not sure if this is correct). Kind of like how you would do calculations in kg and then times it by the conversion constant to convert your answer ...
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### Why isn't a meter defined from a kilogram of water?

Why are there different official definitions for a kilogram and for a meter when a meter can be defined by the volume of a kilogram of water? For instance, using the triple point or some other state ...
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### Why are measurements standardized the way they are?

Using meters as a base length, squaring or cubing lengths smaller than 0.67m makes the square term larger than the cubed term. This fact causes certain properties of physics (how rain needs to form?) ...
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### Why does the vacuum permeability have the value of $\pi$ in it?

The vacuum permeability, or the capability of the vacuum to permit magnetic field lines, contains the value of $\pi$. Why? What does this have to do with the ratio of a circle's circumference to its ...
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### Are the 7 base quantities in SI system really independent?

In a typical description of the 7 base quantities of the SI system we see the following two points: All other quantities can be derived from them. They are "independent". My question is about ...
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### What is the use of this formula 1 Tesla = 1 Newton/Ampere/Meter?

What does Newtons/Ampere/Meter stand for? From this formula: 1 Tesla = 1 Newtons/Ampere/meter what can it be used for? To do what? Ampere/meter Is the same unit as a field's intensity H? Or what is ...
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### The origin of femto, atto and zepto SI prefixes

Do you know why the SI prefixes: femto, atto, zepto have been accepted by Scientific Community, if this triad of metric units, is neither greek nor latin?
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### What is time measured against? [duplicate]

Today I was observing a clock and its movement, every second is an exact second on every clock. I was making a comparison between a second and a meter. I know in France there is a metal stick one ...
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### Why is the second the SI base unit for time?

Specifically, during the moves towards Le SystĂ¨me international d'unitĂ©s in the 18th and 19th centuries, why didn't anyone attempt to move us away from the definition of there being 24 hours in a day? ...
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### Calculating accuracy, error, full scale error, relative error and more in a specific case

I recently bought a O2(oxygen) sensor and I am trying to figure out the error in the percentage O2 (O2pct, unitless but in %) output value that it gives out. This error is not stated in its data ...
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### System of Units where Coulomb's constant $k_e = 1$?

I am reading (and trying to replicate the results of) a paper in the field of condensed matter physics. The system consists of a spatially separated electron and hole whose Coulomb interaction is the ...
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### 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 $L = 1 km$. $\log L = \log km$ ...
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### Why does physical quantity misinterpretation result in near-identical torque results?

I am going to use an equation $$\text{torque} = \frac{\text{power}\times 5252}{\text{RPM}}$$ derived on Wikipedia. Suppose that $\text{power} = 100\ \mathrm{hp}$ and $\text{RPM} = 5252\ \mathrm{rpm}$ ...
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### How the standard units for length force and time are defined in US Customary Units as standard units are defined in SI?

I'm much confused about US customary units so I want to ask a lot more questions under single question.First how standard units ,in US customary Units,of force time and length are defined as second or ...
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### 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 ...
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### What is the relationship between keV and kVp?

On X-ray topics we get used to talk about "energy", but what is keV? and what is the relationship between keV and kVp?
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### 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 ...
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### What forms can units take?

They have stated in my physics book that all units can be made by combining SI base units. I have got a few question about this. Can we raise one unit to the power of another unit? For instance: ...
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### Unification: what does it mean for the forces to become equal?

The Coulomb force constant $k_c$ has si-units of $Nm^2C^{-2}$. The gravitational force constant $G$ has si-units of $Nm^2kg^{-2}$. What does it mean that at very high energies these forces become ...
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### How is the Joule normalised acrpss various definitions?

Apologies if this question is a duplicate, I tried searching for this question both on Google and here, but was unable to find an answer. A Joule is defined in various ways, some of them being: ...
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### Intitutive meaning the unit of Force $N$

I infer 1 Newton of force = 1 kg.m/ sec squared means a force if acting continuosly on a body at rest would make it gain an acceleration of 1 m/sec squared each second.So the change in velocity is 1-0 ...
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### Do the trigonometric functions preserve units?

I saw an exercise where you had to calculate the units of $C_i, i=1,2$ from an equation like this: $v^2=2\cdot C_1x$ and $x=C_1\cdot \cos(C_2\cdot t)$ where $x$ means meters, $t$ means seconds ...
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### Is the speed of light in vacuum constant or does the math just happen to work out?

My apologies if my question is really idiotic, but I ask sincerely because I want to learn. Based on this question and lots of other places on the web, this topic seems to be really confusing. Let's ...
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### Why is Ampere not a derived SI unit? [duplicate]

From my understanding, Ampere is amount of elementary charge per time. So, you should be able to derive it from mol, which is amount, and seconds, which is time. My question: Why is Ampere a base ...
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### 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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### What is the meaning of SI unit $Ns$ (NEWTON.TIME)?

for ex: when i say newton per meter it mean that 1 newton is applied per 1 meter. but what i mean by saying N.s (s=time in sec) i.e Impulse or momentum. N.s does not mean that certain force is being ...
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### Messed up units!

In the article, Environment-assisted quantum transport, $\gamma$ is a constant equal to $2\pi kT/\hbar*E_{R}/(\hbar\omega_{c})$ where $T$ is the temperature, $k$ is the Boltzmann constant. ...
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### What is the reason we originally and still use the non-SI unit, the Jansky?

The Jansky is the unit for spectral flux density. It is defined as $$1 {\rm \ Jy} = 10^{-26} {\rm W \ m^{-2} \ Hz^{-1}}$$ in terms of Watts per square meter per Hertz. I've never quite understood ...
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### What is $c$ in the Lorentz force expression?

The usual Lorentz force expression I am familiar with is this: $$\vec F=q(\vec E+\vec v \times \vec B)$$ I have seen some other versions lately that include an extra factor $1/c$: \vec ...
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### Does the definition of the SI unit “second” require that possible perturbation of primary frequency standards should be measured?

The definition of the SI unit "second" is stated as The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground ...
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### Mass-energy-equivalence's effect on SI units?

Maybe I'm misunderstanding mass energy equivalence, but can we use it to get rid of the Joule (or the kilogram) and have a single unit for both? It seems weird that if they're equivalent they'd have ...
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### Is the second defined arbitrarily? [duplicate]

According to the definition a second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of ...
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### Why is the prospective new kilogram standard a sphere?

I can understand the choice of material, silicon 28, but why is it a sphere rather than (say) a cube? Article here I would have thought that a sphere would have been the hardest shape to machine ...
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### Usage of singular or plural SI base units when written in both symbol as well as name [closed]

I have multiple doubts related to the usage of singular or plural SI base units when written in both symbol as well as name. I have framed this question under two parts, namely, Part (a) and Part ...
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### Why is energy not an SI base unit?

According to a textbook I have begun to read, there are seven base SI units: Length Mass Time Temperature Amount of a substance Electric current Luminous intensity What I do not understand is, why ...
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### Convert cubic feet per sec to feet per sec

Is there a way to convert cubic feet per sec to feet per sec. 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 distance ...