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

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-2
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2answers
49 views

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 ...
3
votes
1answer
944 views

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 ...
5
votes
3answers
176 views

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 ...
4
votes
8answers
2k views

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 ...
0
votes
0answers
19 views

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 ...
1
vote
2answers
232 views

Numerically solving 2D poisson equation by FFT, proper units

The 2D Poisson equation is: (1)$$\frac{d^2\varphi(x,y)}{dx^2}+\frac{d^2\varphi(x,y)}{dy^2}=-\frac{\varrho(x,y)}{\epsilon_0\epsilon}$$ And in $k$-space it is in form of: (2)$$(k_x^2+k_y^2) ...
3
votes
1answer
200 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?
-2
votes
2answers
69 views

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 ...
1
vote
3answers
127 views

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. ...
1
vote
1answer
124 views

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 ...
0
votes
2answers
85 views

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 ...
5
votes
3answers
245 views

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 ...
0
votes
1answer
35 views

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 ...
1
vote
3answers
79 views

Why is candela a base SI unit?

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 ...
1
vote
2answers
399 views

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 ...
-1
votes
2answers
290 views

What is the fundamental units for coulomb?

Everything I saw suggests coulomb has no fundamental units. So then how is Newton/Coulomb equivalent to Volt/meter?
25
votes
3answers
1k 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 ...
42
votes
5answers
6k views

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 ...
0
votes
2answers
58 views

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 ...
0
votes
0answers
28 views

How is the Joule normalised?

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: ...
20
votes
5answers
3k views

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 ...
0
votes
2answers
53 views

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 ...
0
votes
1answer
941 views

Is sound considered a subtopic of physics? What are the SI units of sound?

Does physics cover sound? If so, why does sound not have units in the SI system, or how would we measure sounds and frequency? I guess it's debatable, but why isn't it standard practice in physics?
8
votes
3answers
1k views

Why is the second the SI base unit for time?

Specifically, during the moves towards Le Système international d'unités 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? ...
21
votes
1answer
455 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 ...
6
votes
3answers
945 views

Why do we still not have an exact definition for a kilogram?

I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It seems this would be fairly important. Edit Today I ...
3
votes
0answers
46 views

Why is an Ampere an SI unit? [duplicate]

It has always annoyed me that an Ampere is an SI unit, rather than a Coulomb. Why is this the case? Was current discovered first historically? I believe that the standards were published in the 1960s, ...
8
votes
2answers
588 views

Where does the joules come from (ideal gas constant)?

I have not done any physics in ages and have recently started studying it. The first chapter in my book deals with the ideal gas constant: $$pV=nRT$$ It is rewritten as: $$R=\frac{pV}{nT}$$ When I ...
18
votes
4answers
4k views

Why are grams usually only expressed as milligrams, grams or kilograms?

I'm a physics (and electronics and astronomy, etc.) enthusiast. As I learn and research topics, I notice that many SI units are often expressed using a variety of prefixes, such as in electronics ...
0
votes
3answers
132 views

Why was the original definition of meter abandoned?

The meter was originally intended to represent $1$ ten-millionth($10^{-7}$) of the distance from pole to equator of the Earth along a meridian of longitude. The definition was later discarded. ...
0
votes
1answer
64 views

A query regarding constant $\frac{1}{4\pi \epsilon_{0}}$ in Coulomb's Law [duplicate]

In our physics class we were discussing about Coulomb's Law and equation for the electrostatic force between two points: $$F_{e}=\frac{Q_{1}Q_{2}}{4\pi \epsilon_0 r^2}$$ From the equation a query ...
6
votes
1answer
797 views

“Natural units” of mass

Gravitational attraction is given by $\frac{GMm}{r^2}$ while attraction due to electric charge is given by $\frac{q_1 q_2}{r^2}$. Why does gravity need a constant while electric charge doesn't? ...
1
vote
3answers
354 views

Speed of light definition

I understand the speed of light to be an EXACT number. If permeability of free space has a factor of pi in it, how can this be?
-1
votes
1answer
78 views

In the expresion $4\sin (\theta) \frac{d \theta}{d t}=\frac{dy}{dt}$ Which is the correct unit of $\theta$ ¿ radian or degrees? [closed]

If I change units of an angle for radians to degrees in the next expresion $$4\sin (\theta) \frac{d \theta}{d t}=\frac{dy}{dt}$$ the value of $$\frac{dy}{dt}$$ changes. For example at a rate of ...
0
votes
2answers
347 views

Why aren't Faraday's law of induction and Maxwell-Ampere's law (without sources) symmetric?

I was wondering why Faraday's law of induction and Maxwell-Ampere's law (without sources) are not totally symmetric in the sense that Maxwell-Ampere's law has a $\epsilon_0 \mu_0$ term on the right ...
6
votes
2answers
3k 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 ...
0
votes
1answer
144 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"): p = M v + q A /c it is plain enough that M v is momentum, but if we ...
2
votes
2answers
128 views

25 cm of vacuum correspond to what pressure?

I the following (american) test Vacuum testing consists of placing samples from the packiging operation into a jar filled with water. A lid is placed over the samples to fully immerse them in the ...
2
votes
5answers
328 views

Could velocity be taken as fundamental instead of time?

In physics time and length are taken as fundamental in the SI system and, as it seems, in the thinking of physicists. Could one instead take velocity, with c as its unit, together with length as ...
6
votes
4answers
514 views

Is there a quantity measured in kilogram seconds?

I'm trying to get a full grasp on the relationship between many of the units that are used in kinetic physics. I've found that it is possible to make a venn-diagram that shows the factors of many of ...
2
votes
1answer
79 views

What was a second in the early universe?

I have read some popsci articles and documentaries about the early universe and they often explain how various features of the universe came about and at what time. For example hydrogen atoms came ...
11
votes
3answers
246 views

$1.7\cdot 10^{-24}$ mole apples a day

As the title suggests I was wondering why the International Bureau of Weights and Measures decided a mole to be a standard (SI-)unit. After some research I found I was not alone with this problem. ...
2
votes
0answers
46 views

Different factors of $4\pi$ and $\epsilon_0$ in Poisson equation [duplicate]

Some authors claim the Poisson equation is $$\nabla^2 \psi = -\dfrac{\rho}{\epsilon\epsilon_0}$$ (e.g. Wikipedia) whereas other ones (e.g. Andelmann) claim it is $$\nabla^2 \psi = ...
2
votes
3answers
113 views

How units were defined?

I was wondering how we humans can be sure that one meter is one meter and that one second is one second. Nowadays, except for the Kilogram, all other units are well defined using highly accurate ...
3
votes
1answer
540 views

Why is charge not taken as a fundamental unit? [duplicate]

According to the definition of electric current, it appears to be a derived quantity. Charge on the other hand seems more fundamental than electric current. Then why is current taken as fundamental ...
2
votes
2answers
376 views

How do we convert between the maximum value of the adhesive force (in kilograms or in Newton) of a magnet and its magnetic field strength (in tesla)?

On the following website they indicate how powerful is a magnet by giving the theoretical maximum value of its adhesive force in kilograms or in Newton (its adhesive force at a distance of zero). ...
2
votes
1answer
262 views

Are there reasons for the discrepancies in absolute temp units - Kelvin vs. kelvins vs. degrees Kelvin?

Before 1968, the units for absolute temperature were described as "degrees Kelvin" or "degrees absolute." After that, the SI system got rid of the idea of "degree" for absolute temperature, so the ...
12
votes
3answers
5k views

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 $ ...
9
votes
1answer
107 views

How is a standard unit divided into equally smaller or fractional units physically/experimentally?

Consider the standard unit of length: meter. How was it divided into decimeter, centimeter, millimeter, etc. when there were no shorter lengths than the standard? What is the physical/experimental ...
7
votes
5answers
1k views

Is anything actually 1 meter long (or 1kg of weight)?

I believe that no real objects are actually (exactly) 1 meter long, since for something to be 1.00000000... meters long, we would have to have the ability to measure with infinite precision. ...