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### 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.
182 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 ...
64 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 ...
213 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 ...
62 views

### 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 ...
22 views

### 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 ...
12k views

### 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 ...
203 views

### 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 ...
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 ...
62 views

### 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 $7$ ( ...
187 views

### 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 ...
217 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 : ...
108 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 ...
42 views

### 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 ...
131 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 ...
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 ...
191 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 ...
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 ...
179 views

### Is absolute zero still 0 Kelvin?

Following the recent decision to change the definition of SI units, I understand that Kelvin is no longer defined in terms of the number 1/273.16. Does that mean that absolute zero is no longer ...
103 views

### What will happen to the International Prototype Kilogram? [closed]

The kilogram has been redefined in a way that does not refer to the International Prototype Kilogram. That kind of makes the International Prototype Kilogram useless. So what will happen to it? Can I ...
285 views

### A new definition of mass using Planck constant

Tonight in a italian television news channel well known internationally I have heard that almost certainly the definition of mass that we currently know will be obtained by means of the constant ...
146 views

### What will be the uncertainty in $\mu_0$ under the new SI scheme?

As you may be aware, a new SI system is likely to be adopted in November 2018 (see https://www.nist.gov/si-redefinition/kilogram-introduction). Whilst the speed of light remains a fixed quantity and ...
137 views

### How is the division of physical quantities into base quantities and derived quantities a matter of convention?

Several physics textbooks and even the SI-The International System of Units Brochure(8th-edition) says that, ''The division of quantities into base quantities and derived quantities is a matter of ...
63 views

### Is quantum physics a reason for the measurement differences of the International Prototype Kilogram

I've recently learned that there is an official Kilogram (IPK) kept in Paris which is used to calibrate, directly or indirectly, all weight measurement tools. I've also learned that each time the IPK ...
106 views

### Is the International System of Units complete?

Are there any known (measurable continuous) physical quantities, which are neither base quantities of SI, nor are derivable from the base quantities? In other words, are there any quantities which ...
1k views

### What actually is 1 coulomb? Is it number of electrons or the amount of force?

I've just started my highschool, only to land in the beautiful world of electricity and magnetism, I have many queries and dilemmas, so I want some guidance. Now on to the question. I've just read ...
830 views

### Why can't we define fundamental unit of mass? [duplicate]

In my physics textbook of class $11^{th}$ The kilogram was defined as :- mass of the platinum-iridium standard cylinder kept at Sevre's France But this isn't a proper and scientific definition ...
72 views

### How is the Length of a Meter Physically Measured

I have two parts to this question. First, I understand that the meter is defined as the distance light travels in 1/299,792,458 seconds. But how is this distance actually measured? The second is ...
195 views

### Why was the fraction 1/31,556,925.9747, in the 1956-1968 definition of the second in terms of ephemeris time, chosen?

The recent question Why are leap seconds needed so often? pulled up some interesting details about the definition of the second, and I'd like to have some of them confirmed explicitly. I'm ...
148 views

### Why are the international prototype kilogram and its copies kept under nested bell jars?

I'm watching a YouTube video about the American standard kilogram by the Veritasium channel and noticed something that I'd seen before but never questioned. The standards are kept under two, nested ...
606 views

### Complete list of fundamental properties [closed]

What are all of the fundamental properties, that is all measurable quantities which are not derived from anything else? Many quantities are derived e.g. area is length squared, velocity is length per ...
706 views

### Definition of Joule?

The Wikipedia definition of Joule is: the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of ...
107 views

### Why is the kilogram defined using Earth's gravity? [closed]

Since there are variations of $g$ depending on location on Earth's surface, why not use a reproducible lab experiment using a vertical axis centrifugal balance, and say that one kg is defined by ...
337 views

### Why must the kilogram standard be based on a kilogram mass object?

Inspired by the accepted answer to a question about the Avogadro Project, why must an object used to define a new standard for the kilogram have a mass of one full kilogram? If a smaller mass were ...
5k views

### Why are scientists involved in the Avogadro Project using silicon-28 atoms instead of carbon-12?

My question is, why use silicon-28 atoms to calculate the kilogram when you already have carbon-12 atoms defining the constant? Does the Avogadro Project intend to define the constant by replacing ...
103 views

### Why aren't SI units squared or cubed, e.g. cubic centimetres?

For example when we look at cm^3: the multiplier is then (10^-2)^3 so why don't we write (cm)^3 instead?
655 views

### Why frequency is a SI-derived unit?

Frequency is a derived SI unit but its unit is 1/s. It uses only time once and no other fundamental quantities. So why is not included in SI base unit?
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 ...
975 views

### Proper reference for the International System of Units

I would like to cite a relevant document that guides usage of SI units. It seems like currently there are two nearly identical literature sources: 8th edition of SI Brochure issued by BIPM, 2006; ...
659 views

### What is the name and symbol of the derived unit for one thousand metres? [closed]

I'm still getting confused of what a derived unit is. I used the internet to find an answer but I'm still getting confused.
7k views

### Is the fact that 100 kPa equals about 1 atmosphere accidental?

Typical atmosphere near sea level, in ambient conditions is around 100,000 pascals. But the pascal, as the unit, is not defined through Earth atmospheric pressure. It's defined as one newton per ...
619 views

### Why is a Coulomb not a fundamental unit? [duplicate]

Why is a coulomb not a fundamental unit but an ampere is considering that a coulomb is more ' fundamental '?
2k views

### How can we define the kilogram in terms of Planck's constant?

Scientists are going to redefine the kilogram by using the value of Planck's constant. What is the physics behind this would-be definition of a kilogram? How is Planck's constant related to kilogram? ...
370 views

### Redefining the kilogram using Planck's constant instead of the density of water among other examples

The kilogram is in the process of being redefined in terms of Planck's constant so as to eliminate its dependence on a physical artefact. Since the length and temperature units are already precisely ...
194 views

### Why are the electric and magnetic constants where they are?

$ε_0$, the electric permittivity and $μ_0$, the magnetic magnetic permeability were introduced in Coulomb's Constant and Ampere's Constant in order to make units and magnitudes match, in Coulomb's ...
577 views

### Watt (Kibble) balance and the kilogram - how does the dependence on $g$ get eliminated?

The standard ${kg}$ is now in the process of being redefined by the watt balance (rather than the lump of metal in Paris) A watt balance is very simple, you measure the force needed to support a mass ...
167 views

### Base unit definition and realization under relativistic conditions

Question Section 1.5 on page 107 of the full SI brochure, Le Système international d’unités, 8ᵉ édition, comments on physically realizing unit definitions when accounting for relativistic effects (I ...
836 views

### Why are electrical units (specifically, electrical current) considered a base unit?

Note: this is NOT a question why current is the base unit as opposed to charge—that’s because measuring $1 \ \mathrm{ A }$ through a wire is easier to measure in a lab than is $1 \ \mathrm{ C }$ in ...