Linked Questions

40
votes
3answers
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 ...
3
votes
3answers
188 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 ...
1
vote
0answers
141 views

How is a Kibble balance used in the new definition of the kilogram, and what's the connection between the balance and Planck's constant? [duplicate]

The BBC News article Kilogram gets a new definition says: How does the new system work? Electromagnets generate a force. Scrap-yards use them on cranes to lift and move large metal objects, ...
0
votes
0answers
23 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 ...
70
votes
14answers
138k views

What is the difference between “kinematics” and “dynamics”?

I have noticed that authors in the literature sometimes divide characteristics of some phenomenon into "kinematics" and "dynamics". I first encountered this in Jackson's E&M book, where, in ...
61
votes
5answers
18k views

Why do atomic clocks only use caesium?

Modern atomic clocks only use caesium atoms as oscillators. Why don't we use other atoms for this role?
41
votes
2answers
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 ...
54
votes
5answers
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 ...
16
votes
4answers
4k views

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 ...
9
votes
2answers
2k views

Why do we still not have an exact (constants-based) 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 would seem this should be fairly important. Edit: ...
4
votes
3answers
1k views

Uncertainty of permittivity of vacuum

Question: The value of permittivity of vacuum, $\epsilon_0$, is given with absolutely no uncertainty in NIST Why is this the case? More details: The permeability of vacuum can be given by $$\mu_0=...
6
votes
4answers
891 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 ...
1
vote
1answer
1k views

Magnetic effect on AC circuits?

We know that when currents in two wires move parallel to each other, they attract each other and if they are moving anti-parallel to each other, they repel each other but we cannot observe this in ...
14
votes
1answer
1k views

What is a base unit in the new SI, and why is the ampere one of them?

One question that comes up pretty much always in introductory electromagnetism courses is Why the base unit of electrical measurements is the ampere and not the coulomb, and the usual answer is that ...
7
votes
1answer
2k views

How is the constant of the Biot-Savart Law derived?

In my A-level textbook there is no explanation regarding how the constant in the Biot-Savart law is derived! So how is the constant, $k =\frac {\mu_0}{4\pi}$ derived, and what's the intuition behind ...
3
votes
3answers
242 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 ...
4
votes
1answer
539 views

There are plans to develop a better definition of a “second”. How does the current definition fall short?

The current definition of a second is stated here and I found a presentation on the BIPM site which discusses plans to change to a "better" definition of a second. You can find the presentation here. ...
1
vote
3answers
213 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 ...
2
votes
2answers
143 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 ...
-5
votes
1answer
111 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 ...
1
vote
3answers
586 views

How to distinguish between the spectrum of an atom in motion and the one of a scaled atom?

Galaxies are moving dragged by the space expansion. When atoms are in motion the doppler effect will shift the spectra of the emitted photons. The proton-to-electron mass ratio, $\frac{m_e}{m_p}$ ...
1
vote
1answer
193 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 ...
0
votes
2answers
294 views

SI Base Unit definition of mass - obsolete?

According to the formal definition of the SI Base unit of mass, the kilogram, it is stated that "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the ...
0
votes
2answers
402 views

How accurately can you count electrons?

I read that with modern technology they can now shoot one electron at a time. Can you tell me how accurately it is possible to count charges, how it is made and how they did this in the past? How ...
2
votes
2answers
353 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 ...
0
votes
2answers
338 views

How explain this perturbing equation about the 43 arcseconds?

The planetary orbits have been studied as ellipses but the solar system is in motion in relation to the distant stars. Their path is along the tip of an helix and the ecliptic plane is a convenient ...
0
votes
1answer
111 views

Why is the kilogram the last SI unit which is defined in terms of a physical prototype? [duplicate]

All elementary SI units, except the kilogram, have been redefined depending of a physical constant. However kilogram still depends on the International Prototype Kilogram. Why is this? Is there no ...
1
vote
1answer
200 views

How can a Lego version of a Kibble balance measure the Planck constant?

As the picture shows below [][1] in a Kibble balance, one can drop out the measurement uncertainty of $B$ (magnetic flux intensity) and $L$ (length of coil) by the use of two modes, force mode and ...
0
votes
1answer
81 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 ...
0
votes
0answers
95 views

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 ...